March 2014

Steno's Principles of Stratigraphy

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</span></span> <span style="font-family: inherit;"><span style="background-color: white; line-height: 23px;">In 1669, Niels Stensen (1638-1686), better known then and now by his Latinized nameii Nicolaus Steno, formulated a few basic rules that helped him make sense of the rocks of Tuscany and the various objects contained within them. His short preliminary work, </span><i style="background: rgb(255, 255, 255) none repeat scroll 0px 0px; border: 0px none; box-sizing: border-box; line-height: 23px; margin: 0px; padding: 0px; vertical-align: baseline;">De Solido Intra Solidum Naturaliter Contento — Dissertationis Prodromus</i><span style="background-color: white; line-height: 23px;"> (Provisional report on solid bodies naturally embedded in other solids), included several propositions that have since become fundamental to geologists studying all kinds of rocks. Three of these are known as Steno's principles, and a fourth observation, on crystals, is known as Steno's Law. The quotes given here are from the </span>English translation of 1916<span style="background-color: white; line-height: 23px;">.</span></span></div>
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<span style="font-family: "georgia" , "times new roman" , serif;">The Principle of Superposition</span></h3>
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<span style="font-family: inherit;"> In a sequence of strata, any stratum is younger than the sequence of strata on which it rests, and is older than the strata that rest upon it.</span></div>
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<span style="font-family: inherit;">"...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." Steno, 1669.</span></div>
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<span style="font-family: inherit;"><span style="background-color: white; line-height: 23px;">Today we restrict this principle to sedimentary rocks, which were understood differently in Steno's time. Basically, he deduced that rocks were laid down in vertical order just as sediments are laid down today, under water, with new on top of old. This principle allows us to piece together the succession of fossil life that defines </span>much of the geologic time scale<span style="background-color: white; line-height: 23px;">.</span> </span></div>
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<span style="font-family: "georgia" , "times new roman" , serif;">Principle of Initial Horizontality</span></h3>
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<span style="font-family: inherit;"> Strata are deposited horizontally and then deformed to various attitudes later.</span></div>
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<span style="font-family: inherit;">"Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon." Steno, 1669.</span></div>
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<span style="font-family: inherit;"><span style="background-color: white; line-height: 23px;">Steno reasoned that strongly tilted rocks did not start that way, but were affected by later events—either upheaval by volcanic disturbances or collapse from beneath by cave-ins. Today we know that some strata start out tilted, but nevertheless this principle enables us to easily detect unnatural degrees of tilt and infer that they have been disturbed since their formation. And we know of many more causes, from tectonics to intrusions, that can tilt and fold rocks.</span> </span></div>
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<span style="font-family: "georgia" , "times new roman" , serif;"> Principle of Strata Continuity</span></h3>
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<span style="font-family: inherit;"> Strata can be assumed to have continued laterally far from where they presently end.</span></div>
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<span style="font-family: inherit;">"Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." Steno, 1669</span></div>
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<span style="font-family: inherit;"><span style="background-color: white; line-height: 23px;">This principle allowed Steno to link identical rocks on opposite sides of a river valley and deduce the history of events (mostly erosion) that separated them. Today we apply this principle across the Grand Canyon—even </span>across oceans to link continents that once were adjoined<span style="background-color: white; line-height: 23px;">.</span> </span></div>
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<span style="font-family: "georgia" , "times new roman" , serif;"> Principle of Cross Cutting Relationships</span></h3>
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<span style="font-family: inherit;"><b>*</b> Things that cross-cut layers probably postdate them.</span></div>
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<span style="font-family: inherit;">"If a body or discontinuity cuts across a stratum, it must have formed after that stratum." Steno, 1669</span></div>
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<span style="font-family: inherit;"> <span style="background-color: white; line-height: 23px;">This principle is essential in studying all kinds of rocks, not just sedimentary ones. With it we can untangle intricate sequences of geologic events such as </span><a href="http://www.geologyin.com/2014/11/types-of-faults-geology.html" style="background: rgb(255, 255, 255) none repeat scroll 0px 0px; box-sizing: border-box; line-height: 23px; margin: 0px; padding: 0px; vertical-align: baseline;" target="_blank">faulting</a><span style="background-color: white; line-height: 23px;">, </span>folding<span style="background-color: white; line-height: 23px;">,</span><a href="http://www.geologyin.com/2014/11/terminology-of-brittle-deformation.html" target="_blank">deformation</a><span style="background-color: white; line-height: 23px;">, and emplacement of </span>dikes<span style="background-color: white; line-height: 23px;"> and </span><a href="http://www.geologyin.com/2014/11/veins-and-hydrothermal-deposits.html" style="background: rgb(255, 255, 255) none repeat scroll 0px 0px; box-sizing: border-box; line-height: 23px; margin: 0px; padding: 0px; vertical-align: baseline;" target="_blank">veins</a><span style="background-color: white; line-height: 23px;">.</span></span></div>
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<span style="font-family: "georgia" , "times new roman" , serif;"> Steno's Original Principle I</span></h4>
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<span style="font-family: inherit;">Steno did not call out his Law and his Principles as such. His own ideas of what was important were quite different, but I think they are still well worth considering. He put forth three propositions, the first being this:</span></div>
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<span style="font-family: inherit;">"If a solid body is enclosed on all sides by another solid body, of the two bodies that one first became hard which, in the mutual contact, expresses on its own surface the properties of the other surface."</span></div>
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<span style="font-family: inherit;">(This may be clearer if we change "expresses" to "impresses" and switch "own" with "other.") While the "official" Principles pertain to layers of rock and their shapes and orientations, Steno's own principles were strictly about "solids within solids." Which of two things came first? The one that was not restricted by the other. Thus he could confidently state that fossil shells existed before the rock that enclosed them. And we, for example, can see that the stones in a conglomerate are older than the matrix that encloses them.</span></div>
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<span style="font-family: "georgia" , "times new roman" , serif;">Steno's Original Principle II</span></h4>
<span style="font-family: inherit;">"If a solid substance is in every other way like another solid substance, not only as regards the conditions of surface, but also as regards the inner arrangement of parts and particles, it will also be like it as regards the manner and place of production . . ."</span></section><section class="list-unit group" dir="ltr" style="background: rgb(255, 255, 255) none repeat scroll 0px 0px; border: 0px none; box-sizing: border-box; line-height: 25px; margin: 0px 0px 26px; padding: 0px; text-align: left; vertical-align: baseline;"><div class="expert-content-text" style="background: 0px 0px; border: 0px; box-sizing: border-box; margin: 0px; padding: 0px; position: relative; top: -4px; vertical-align: baseline;">
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<span style="font-family: inherit;">Today we might say, "If it walks like a duck and quacks like a duck, it is a duck." In Steno's day a long-running argument centered around fossil shark's teeth, known as <i style="background: 0px 0px; border: 0px; box-sizing: border-box; margin: 0px; padding: 0px; vertical-align: baseline;">glossopetrae</i>: were they growths that arose inside rocks, remains of once-living things, or just weird things put there by God to challenge us? Steno's answer was straightforward.</span></div>
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<span style="font-family: "georgia" , "times new roman" , serif;">Steno's Original Principle III</span></h4>
<span style="font-family: inherit;">"If a solid body has been produced according to the laws of nature, it has been produced from a fluid."</span></section><section class="list-unit group" dir="ltr" style="background: rgb(255, 255, 255) none repeat scroll 0px 0px; border: 0px none; box-sizing: border-box; line-height: 25px; margin: 0px 0px 26px; padding: 0px; text-align: left; vertical-align: baseline;"><div class="expert-content-text" style="background-attachment: initial; background-clip: initial; background-color: initial; background-image: initial; background-origin: initial; background-position: 0px 0px; background-repeat: initial; background-size: initial; border: 0px; box-sizing: border-box; margin: 0px; padding: 0px; position: relative; top: -4px; vertical-align: baseline;">
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<span style="font-family: inherit;">Steno was speaking very generally here, and he went on to discuss the growth of animals and plants as well as minerals, drawing on his deep knowledge of anatomy. But in the case of minerals, he could assert that crystals accrete from outside rather than grow from inside. This is a profound observation that has ongoing applications for igneous and metamorphic rocks, not just the sedimentary rocks of Tuscany.</span><br />
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Steno's Principles of Stratigraphy

In 1669, Niels Stensen (1638-1686), better known then and now by his Latinized nameii Nicolaus Steno, formulated a few basic rules t...

Growth fault

Structural Geology 3:00 PM

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<b>Growth faults </b>are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply.Their fault plane dips mostly toward the basin and has long-term continuous displacement. Figure one shows a growth fault with a concave upward fault plane that has high updip angle and flattened at its base into zone of detachment or décollement. This angle is continuously changing from nearly vertical in the updip area to nearly horizontal in the downdip area.</div>
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Sedimentary layers have different geometry and thickness across the fault. The footwall – landward of the fault plane – has undisturbed sedimentary strata that dip gently toward the basin while the hanging wall – on the basin side of the fault plane – has folded and faulted sedimentary strata that dip landward close to the fault and basinward away from it.These layers perch on a low density evaporite or over-pressured shale bed that easily flows away from higher pressure into lower pressure zones.Most studies since the 1990s concentrate on the growth faults' driving forces, kinematics and accompanied structures since they are helpful in fossil fuel explorations as they form structural traps for oil.</div>
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Growth faults maturation is a long term process that takes millions of years with slip rate ranges between 0.2-1.2 millimeters per year. It starts when sedimentary sequences are deposited on top of each other above a thick evaporite layer (fig. 2).A growth fault is initiated when the evaporite layer can no longer support the overlying sequences. The thicker and denser portion applies much more pressure on the evaporite layer than the thin portion. As a result, a flow within the evaporite layer is initiated from high pressure areas toward low pressure areas causing growth ridges to form below the thin portion. Also, sinking zones are noticed among these ridges at areas where thicker and denser layers form</div>
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<span style="font-size: x-small;">Figure 2. Sketch showing evolution stages of three growth faults. The black arrow shows the direction of evolution.</span></div>
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Consequently, the passive margin experience unequal subsidence across the continental shelf. Both the new-created accommodation spaces and the thickness of the new-deposited sedimentary layers are greater above the sinking zones than above the growth ridges. The new added layers are thicker within the footwall than within the hanging wall (fig. 2). These variations result in an increasing of differential load intensities - unequal distribution of sediments load - across the shelf with time as more sediment layers are added(fig. 2). Therefore, the rate by which the pressure increases upon the evaporite layer below the sinking zone are much more than the rate of pressure increase upon the same evaporite layer at the growth ridges. So, the flow rate within the evaporite layer is progressively increasing as deferential load intensifies(fig. 2). The growth ridges end up with salt diapir when the sinking zone sequences weld to the base of the evaporite layer.</div>
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Growth fault

Growth faults are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. T...

Contact Metamorphism

Metamorphic, Rocks 10:19 PM

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Contact metamorphism is the name given to the changes that take place when magma is injected in the surrounding solid rock (country rock).<br />
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The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism <b>aureole</b>.<br />
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<b>Aureoles </b>may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away. The formation of important ore minerals may occur by the process of metasomatism at or near the contact zone.<br />
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When a rock is contact altered by an igneous intrusion it very frequently becomes more indurated, and more coarsely crystalline. Many altered rocks of this type were formerly called hornstones, and the term <b>hornfels </b>is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism.<br />
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A shale may become a dark argillaceous hornfels, full of tiny plates of brownish biotite; a marl or impure limestone may change to a grey, yellow or greenish lime-silicate-hornfels or siliceous marble, tough and splintery, with abundant augite, garnet, wollastonite and other minerals in which calcite is an important component.<br />
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A diabase or andesite may become a diabase hornfels or andesite hornfels with development of new hornblende and biotite and a partial recrystallization of the original feldspar. Chert or flint may become a finely crystalline quartz rock; sandstones lose their clastic structure and are converted into a mosaic of small close-fitting grains of quartz in a metamorphic rock called <b>quartzite</b>.</div>
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If the rock was originally banded or foliated (as, for example, a laminated sandstone or a foliated calc-schist) this character may not be obliterated, and a banded hornfels is the product; fossils even may have their shapes preserved, though entirely recrystallized, and in many contact-altered lavas the vesicles are still visible, though their contents have usually entered into new combinations to form minerals that were not originally present.<br />
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The minute structures, however, disappear, often completely, if the thermal alteration is very profound. Thus small grains of quartz in a shale are lost or blend with the surrounding particles of clay, and the fine ground-mass of lavas is entirely reconstructed.<br />
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Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. An extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into <b>metasomatism</b>.<br />
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If the intruded rock is rich in carbonate the result is a <b>skarn</b>. Fluorine-rich magmatic waters which leave a cooling granite may often form <b>greisens </b>within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest.<br />
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A special type of contact metamorphism, associated with fossil fuel fires, is known as <b>pyrometamorphism</b>.<br />
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<span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-size: large;"><a href="http://www.geologyin.com/2014/05/the-formation-of-foliated-metamorphic.html" target="_blank">The formation of Foliated Metamorphic Rock</a></span></span></div>
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Contact Metamorphism

Contact metamorphism is the name given to the changes that take place when magma is injected in the surrounding solid rock (countr...

Volcanic Bombs

Igneous, Rocks, volcano 9:36 PM

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<tr><td style="text-align: center;"><a href="https://3.bp.blogspot.com/-nTYg4fuPMp8/WUiKvx5d3aI/AAAAAAAAMts/SRdEkf90cR0Ccqsi47AWET--kHtpfAB8QCLcBGAs/s1600/Volcanic%2Bbomb.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img alt="Volcanic Bomb" border="0" data-original-height="738" data-original-width="960" height="492" src="https://3.bp.blogspot.com/-nTYg4fuPMp8/WUiKvx5d3aI/AAAAAAAAMts/SRdEkf90cR0Ccqsi47AWET--kHtpfAB8QCLcBGAs/s640/Volcanic%2Bbomb.jpg" title="" width="640" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Fresh Volcanic Bomb photo by J.D. Griggs</td></tr>
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Volcanic bomb is a <b>pyroclast </b>which was semi-molten (viscous) while ejected from a volcanic vent and is therefore shaped while in flight. A mass of molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they are <b>extrusive igneous</b> rocks.<br />
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Volcanic bombs are larger than 64 mm in diameter. Smaller pyroclasts are called <b>lapilli</b>. Pyroclasts (>64 mm) that were already solid when ejected are called blocks. Blocks are typically more angular because unlike bombs they are not aerodynamically shaped by airflow while in flight. </div>
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<tr><td class="tr-caption" style="text-align: center;">"Almond" volcanic bomb found in the Cinder Cones region of the Mojave National Preserve.</td></tr>
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<span style="font-size: small;"><span style="font-family: "georgia" , "times new roman" , serif;"><span style="font-family: "georgia" , "times new roman" , serif;">Bombs are named according to their shape, which is determined by the fluidity of the magma from which they are formed.</span></span></span></h3>
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<b>Ribbon or cylindrical </b>bombs form from highly to moderately fluid magma, ejected as irregular strings and blobs. The strings break up into small segments which fall to the ground intact and look like ribbons. Hence, the name "ribbon bombs". These bombs are circular or flattened in cross section, are fluted along their length, and have tabular vesicles.<br />
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<b>Spherical bombs</b> also form from high to moderately fluid magma. In the case of spherical bombs, surface tension plays a major role in pulling the ejecta into spheres.</div>
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<b>Spindle</b>, fusiform, or almond/rotational bombs are formed by the same processes as spherical bombs, though the major difference being the partial nature of the spherical shape. Spinning during flight leaves these bombs looking elongated or almond shaped; the spinning theory behind these bombs' development has also given them the name 'fusiform bombs'. Spindle bombs are characterised by longitudinal fluting, one side slightly smoother and broader than the other. This smooth side represents the underside of the bomb as it fell through the air.</div>
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<b>Cow pie bombs </b>are formed when highly fluid magma falls from moderate height, so the bombs do not solidify before impact (they are still liquid when they strike the ground). They consequently flatten or splash and form irregular roundish disks, which <br />
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<b>Bread-crust bombs</b> are formed if the outside of the lava bombs solidifies during their flights. They may develop cracked outer surfaces as the interiors continue to expand.</div>
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<b>Cored bombs</b> are bombs that have rinds of lava enclosing a core of previously consolidated lava. The core consists of accessory fragments of an earlier eruption, accidental fragments of country rock or, in rare cases, bits of lava formed earlier during the same eruption.</div>
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<tr><td class="tr-caption" style="text-align: center;">The interior of a sample from a scoria cone in Tenerife, Canary Islands. </td></tr>
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<tr><td class="tr-caption" style="text-align: center;">Fusiform lava bomb. Capelinhos Volcano, Faial Island, Azores.</td></tr>
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Volcanic Bombs

Fresh Volcanic Bomb photo by J.D. Griggs Volcanic bomb is a pyroclast which was semi-molten (viscous) while ejected from a volcan...

Hotspots the Hottest places on Earth

Geology 1:30 PM

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The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be unanimously hot, and provide a great deal of molten magma. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them. One suggests that they are due to hot mantle plumes that rise as thermal diapirs from the core-mantle boundary. The other hypothesis postulates that it is not high temperature that causes the volcanism, but lithospheric extension that permits the passive rising of melt from shallow depths.</div>
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An example of mantle plume locations suggested by one recent group.</div>
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Hotspot volcanoes are considered to have a fundamentally different origin from island arc volcanoes. The latter form over subduction zones, at converging plate boundaries. When one oceanic plate meets another, the denser plate is forced downward into a deep ocean trench. This plate, as it is subducted, releases water into the base of the over-riding plate, and this water mixes with the rock, thus changing its composition causing some rock to melt and rise. It is this that fuels a chain of volcanoes, such as the Aleutian Islands, near Alaska.</div>
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Hotspots the Hottest places on Earth

The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot ...

Oil Shale

Oil, Rocks, sedimentary 3:31 PM

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<b>Oil shale</b>, also known as kerogen shale, is an organic-rich fine-grained sedimentary rock containing kerogen (a solid mixture of organic chemical compounds) from which liquid hydrocarbons called shale oil (not to be confused with tight oil—crude oil occurring naturally in shales) can be produced. </div>
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Shale oil is a substitute for conventional crude oil; however, extracting shale oil from oil shale is more costly than the production of conventional crude oil both financially and in terms of its environmental impact. Deposits of oil shale occur around the world, including major deposits in the United States. Estimates of global deposits range from 2.8 to 3.3 trillion barrels (450×109 to 520×109 m3) of recoverable oil.</div>
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Heating oil shale to a sufficiently high temperature causes the chemical process of pyrolysis to yield a vapor. Upon cooling the vapor, the liquid shale oil—an unconventional oil—is separated from combustible oil-shale gas (the term shale gas can also refer to gas occurring naturally in shales). Oil shale can also be burned directly in furnaces as a low-grade fuel for power generation and district heating or used as a raw material in chemical and construction-materials processing.</div>
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General composition of oil shales constitutes inorganic matrix, bitumens, and kerogen. While the bitumen portion of oil shales is soluble in carbon disulfide, kerogen portion is insoluble in carbon disulfide and can contain iron, vanadium, nickel, molybdenum, and uranium.Oil shale contains a lower percentage of organic matter than coal. In commercial grades of oil shale the ratio of organic matter to mineral matter lies approximately between 0.75:5 and 1.5:5. At the same time, the organic matter in oil shale has an atomic ratio of hydrogen to carbon (H/C) approximately 1.2 to 1.8 times lower than for crude oil and about 1.5 to 3 times higher than for coals.</div>
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Oil Shale

Oil shale , also known as kerogen shale, is an organic-rich fine-grained sedimentary rock containing kerogen (a solid mixture of organ...

Types of Drainage Patterns

earth, Geology, sedimentary 1:23 PM

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<b>Drainage pattern </b>a pattern created by stream erosion over time that reveals characteristics of the kind of rocks and geologic structures in a landscape region drained by streams.</div>
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Drainage pattern<b> </b>is the pattern formed by the streams, rivers, and lakes in a particular drainage basin. They are governed by the<b> topography of the land</b>, <b>whether </b>a particular region is dominated by <b>hard or soft rocks</b>, and the <b>gradient of the land</b>.<br />
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<b>Geomorphologists</b> and hydrologists often view streams as being part of drainage basins. A drainage basin is the topographic region from which a stream receives runoff, throughflow, and groundwater flow. Drainage basins are divided from each other by topographic barriers called a watershed.<br />
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A watershed represents all of the stream tributaries that flow to some location along the stream channel. The number, size, and shape of the drainage basins found in an area varies and the larger the topographic map, the more information on the drainage basin is available.<br />
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The pattern of tributaries within a drainage basin depends largely on the type of rock beneath, and on structures within that rock (folds, fractures, faults, etc.).<br />
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<span style="font-family: Georgia, "Times New Roman", serif;"> The main types of drainage patterns:</span></h4>
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<b>Dendritic patterns</b>, which are by far the most common, develop in areas where the rock (or unconsolidated material) beneath the stream has no particular fabric or structure and can be eroded equally easily in all directions. Examples would be granite, gneiss, volcanic rock, and sedimentary rock that has not been folded. Most areas of British Columbia have dendritic patterns, as do most areas of the prairies and the Canadian Shield. <br />
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<b>Trellis drainage</b> patterns typically develop where sedimentary rocks have been folded or tilted and then eroded to varying degrees depending on their strength. The Rocky Mountains of B.C. and Alberta are a good example of this, and many of the drainage systems within the Rockies have trellis patterns. <br />
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<b>Rectangular patterns</b> develop in areas that have very little topography and a system of bedding planes, fractures, or faults that form a rectangular network. Rectangular drainage patterns are rare in Canada. </div>
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<b>Parallel drainage</b> system is a pattern of rivers caused by steep slopes with some relief. Because of the steep slopes, the streams are swift and straight, with very few tributaries, and all flow in the same direction. Parallel drainage patterns form where there is a pronounced slope to the surface. A parallel pattern also develops in regions of parallel, elongate landforms like outcropping resistant rock bands. </div>
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<b>Radial drainag</b>e system, the streams radiate outwards from a central high point. Volcanoes usually display excellent radial drainage. Other geological features on which radial drainage commonly develops are domes and laccoliths. On these features the drainage may exhibit a combination of radial patterns.</div>
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The <b>centripetal drainage</b> system is similar to the radial drainage system, with the only exception that radial drainage flows out versus centripetal drainage flows in<br />
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A <b>deranged drainage</b> system is a drainage system in drainage basins where there is no coherent pattern to the rivers and lakes. It happens in areas where there has been much geological disruption. The classic example is the Canadian Shield. During the last ice age, the topsoil was scraped off, leaving mostly bare rock.<br />
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<b>Angular drainage</b> patterns form where bedrock joints and faults intersect at more acute angles than rectangular drainage patterns. Angles are both more and less than 90 degrees<br />
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<span style="color: white;"><span style="font-size: xx-small;"><span style="font-family: "georgia" , "times new roman" , serif;">The main types of drainage patterns:</span></span></span></h3>
<b><span style="font-family: Georgia, "Times New Roman", serif;">See also:</span></b><br />
<span style="font-family: Georgia, "Times New Roman", serif;"><a href="https://www.geologyin.com/2019/05/flysch-sequence-formation.html" target="_blank">Flysch Sequence Formation</a></span><br />
<span style="font-family: Georgia, "Times New Roman", serif;"><a href="https://www.geologyin.com/2015/01/how-to-draw-graphic-log.html" target="_blank">How to Draw a Graphic Log?</a></span><br />
<span style="font-family: Georgia, "Times New Roman", serif;"><a href="https://www.geologyin.com/2014/03/stenos-principles-of-stratigraphy.html" target="_blank">Steno's Principles of Stratigraphy</a></span></div>
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Types of Drainage Patterns

Drainage pattern a pattern created by stream erosion over time that reveals characteristics of the kind of rocks and geologic struct...

Cleavage of Minerals

Mineralogy 2:30 PM

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A <b>cleavage </b>plane is a plane of structural weakness along which a mineral is likely to split smoothly. Cleavage thus refers to the splitting of a crystal between two parallel atomic planes.<br />
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Cleavage is the result of weaker bond strengths or greater lattice spacing across the plane in question than in other directions within the crystal. Greater lattice spacing tends to accompany weaker bond strength across a plane, because such bonds are unable to maintain a close interatomic spacing.<br />
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Both the positioning of crystal faces in a mineral and the property of cleavage are derived from the crystalline structure of the species. However, despite the fact that every mineral belongs to a specified crystal system, not every mineral exhibits cleavage. A mineral such as quartz may demonstrate beautiful, well-developed crystals and yet possess no distinct planes of cleavage.<br />
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<b>Cleavage planes</b>, if they exist, are always parallel to a potential crystal face. However, such planes are not necessarily parallel to the faces which the crystal actually displays. Fluorite, for example, has octahedral cleavage yet forms cubic crystals. Nonetheless, the property of cleavage, if it is present, can offer important information about the symmetry and inner structure of a crystal. <br />
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The quality of a mineral's cleavage refers to both the ease with which the mineral cleaves and to the character of the exposed cleavage surface. The quality of a sample's cleavage is typically described by terms such as 'eminent,' '<b>perfect</b>,' '<b>distinct</b>,' '<b>difficult</b>,' '<b>imperfect</b>,' or '<b>indistinct</b>.' <br />
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'Eminent' cleavage describes the case in which cleavage always occurs readily and is in fact difficult to prevent from occurring. The mineral mica, for example, cleaves readily into thin, flat sheets.<br />
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A mineral which demonstrates 'perfect' cleavage breaks easily, exposing continuous, flat surfaces which reflect light. Fluorite, calcite, and barite are minerals whose cleavage is perfect. 'Distinct' cleavage implies that cleavage surfaces are present although they may be marred by fractures or imperfections. 'Difficult' or 'indistinct' cleavage produces surfaces which are neither smooth nor regular; samples possessing such cleavage tend to fracture rather than split. <br />
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Cleavage may be determined by the examination of surfaces which have actually broken. It may also be determined by inspection of the interlacing systems of cracks which permeate the structure of certain specimens. These systems of cracks are beautifully apparent within transparent crystals such as fluorite or calcite.<br />
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Read also:<br />
<a href="http://www.geologyin.com/2014/03/luster-of-minerals.html" target="_blank">Luster of Minerals</a><br />
<a href="http://www.geologyin.com/2014/05/the-mohs-scale-of-mineral-hardness.html" target="_blank">The Mohs scale of mineral Hardness</a><br />
<a href="http://www.geologyin.com/2014/12/the-complete-classification-of-minerals.html" target="_blank">The Complete Classification of Minerals</a><br />
<a href="http://www.geologyin.com/2014/11/crystal-structure-and-crystal-system.html" target="_blank">Crystal Structure and Crystal System</a></div>
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Cleavage of Minerals

A cleavage plane is a plane of structural weakness along which a mineral is likely to split smoothly. Cleavage thus refers to the spli...

Volcanology

volcano 8:13 PM

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<i><b>Volcanology  </b></i>is the study of volcanoes, lava, magma, and related geological, geophysical and geochemical phenomena. The term volcanology is derived from the Latin word vulcan. Vulcan was the ancient Roman god of fire.</div>
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A volcanologist is a person who studies the formation of volcanoes, and their current and historic eruptions. Volcanologists frequently visit volcanoes, especially active ones, to observe volcanic eruptions, collect eruptive products including tephra (such as ash or pumice), rock and lava samples. One major focus of enquiry is the prediction of eruptions; there is currently no accurate way to do this, but predicting eruptions, like predicting earthquakes, could save many lives.<br />
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A volcanologist sampling lava using a rock hammer and a bucket of water</div>
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Volcanology

Volcanology is the study of volcanoes, lava, magma, and related geological, geophysical and geochemical phenomena. The term volcanology...

Luster of Minerals

Mineralogy 2:00 AM

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Minerals may be categorized according to whether they are <b>opaque</b> or <b>transparent</b>. A thin section of an opaque mineral such as a metal will not transmit light, whereas a thin section of a transparent mineral will. <br />
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The absorption index of an opaque mineral is high. Light which is incident upon an opaque mineral such as a metal is unable to propagate through the mineral due to this high rate of absorption, and will thus be reflected. Opaque minerals typically reflect between 20% to 50% or more of the light incident upon them. In contrast, most of the light which is incident upon a transparent mineral passes into and through the mineral; transparent minerals may reflect as little as 5% of the incident light and as much as 20%. Typically those minerals which possess metallic bonding are opaque whereas those where ionic bonding is prevalent are transparent.<br />
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Relative differences in opacity and transparency are described as <b>luster</b>. The term luster refers to the quantity and quality of the light which is reflected from a mineral's exterior surfaces. Luster provides an assessment of how much the mineral surface 'sparkles'. This quality is determined by the type of atomic bonds present within the substance. It is related to the indices of absorption and refraction of the material and the amount of dispersion from the crystal lattice, as well as the texture of the exposed mineral surface.<br />
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Minerals are primarily divided into the two categories of <b>metallic</b> and <b>nonmetallic</b> luster. Minerals possessing metallic luster are opaque and very reflective, possessing a high absorptive index. This type of luster indicates the presence of metallic bonding within the crystal lattice of the material. Examples of minerals which exhibit metallic luster are native copper, gold, and silver, galena, pyrite, and chalcopyrite. The luster of a mineral which does not quite possess a metallic luster is termed <b>submetallic</b>; hematite provides an example of submetallic luster.<br />
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The property of streak can aid in distinguishing whether a specimen has a metallic or a nonmetallic luster. Metals tend to be soft, implying that more powdered material may be obtained from the streak sample of a metal than a nonmetal. Metals are also opaque, transmitting no light. Minerals which possess a metallic luster therefore tend to exhibit a thick, dense, dark streak whereas those which possess a nonmetallic luster tend to produce a thinner, less dense streak which is also lighter in color.<br />
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<b>Adjectives such as</b> "vitreous', 'dull', 'pearly', 'greasy', 'silky' or 'adamantine' are frequently used to describe various types of nonmetallic luster.</div>
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<dt style="text-align: left;"><span style="font-family: inherit;"><b>Dull</b> or <b>Earthy</b></span></dt>
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Minerals of dull or earthy luster reflect light very poorly and do not shine. This type of luster is often seen in minerals which are composed of an aggregate of tiny grains.</div>
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<dt style="text-align: left;"><b><span style="font-family: inherit;">Resinous</span></b></dt>
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A surface of resinous luster possesses a sheen resembling that of resin. Such materials have a refractive index greater than 2.0. Sphalerite (ZnS) demonstrates a resinous luster.</div>
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<dt style="text-align: left;"><b><span style="font-family: inherit;">Pearly</span></b></dt>
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Pearly luster appears iridescent, opalescent, or pearly. This is typically exhibited by mineral surfaces which are parallel to planes of perfect cleavage. Layer silicates such as talc often demonstrate a pearly luster on cleavage surfaces.</div>
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<dt style="text-align: left;"><b><span style="font-family: inherit;">Greasy</span></b></dt>
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A surface which possesses greasy luster appears to be covered with a thin layer of oil. A light-scattering surface which is slightly rough, such as that of nepheline, may exhibit greasy luster.</div>
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<dt style="text-align: left;"><b><span style="font-family: inherit;">Silky</span></b></dt>
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Silky luster occurs when light is reflected off of an aggregate of fine parallel fibers; malachite and serpentine may both exhibit silky luster.</div>
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<dt style="text-align: left;"><b><span style="font-family: inherit;">Vitreous</span></b></dt>
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Vitreous luster occurs in minerals with predominant ionic bonding and resembles the reflective quality of broken glass. The refractive index of such minerals is 1.5 to 2.0. Many silicates possess this type of luster; quartz and tourmeline both demonstrate vitreous luster.</div>
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<dt style="text-align: left;"><span style="font-family: inherit;"><b>Adamantine</b> or <b>brilliant</b></span></dt>
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A brilliant luster such as the sparkling reflection of diamond is known as adamantine. Minerals of adamantine luster have high refractive indices (1.9-2.6) and are highly dispersive and translucent. Covalent bonding or the presence of heavy metal atoms or transition elements may result in adamantine luster.</div>
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Luster of Minerals

Minerals may be categorized according to whether they are opaque or transparent . A thin section of an opaque mineral...

The Streak of Minerals

Mineralogy 9:58 PM

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Streak plates with pyrite (left) and rhodochrosite (right)</div>
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The <b>streak </b>(also called "powder color") of a mineral is the color of the powder produced when it is dragged across an unweathered surface. This trail's color generally is much more consistent than that of its parent mineral and therefore is an important mineral identification tool.<br />
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If no streak appears, then the mineral's streak is said to be white or colorless. Streak is especially important for identifying opaque and colored materials and is less useful for silicate minerals, most whereof have a white streak and are too hard to easily powder.<br />
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The apparent color of a mineral can greatly vary because of trace impurities or a disturbed macroscopic crystal structure. Small amounts of an impurity that strongly absorbs a particular wavelength can radically change the wavelengths of light that the specimen reflects, thereby changing the apparent color. Dragging the specimen to produce a streak breaks into randomly oriented microscopic crystals, reducing the small impurities' effect.<br />
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The surface whereover the mineral is dragged is called a "<b>streak plate</b>" and generally is made from unglazed porcelain tile. The unglazed underside of a porcelain bowl or vase or the back of a glazed tile also works. Sometimes a streak is more easily or accurately described by comparing it with the "streak" made by another streak plate.</div>
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<b>Read also:</b><br />
<a href="http://www.geologyin.com/2014/03/luster-of-minerals.html" target="_blank">Luster of Minerals</a><br />
<a href="http://www.geologyin.com/2014/05/the-mohs-scale-of-mineral-hardness.html" target="_blank">The Mohs scale of mineral Hardness</a><br />
<a href="http://www.geologyin.com/2014/12/the-complete-classification-of-minerals.html" target="_blank">The Complete Classification of Minerals</a><br />
<a href="http://www.geologyin.com/2014/11/crystal-structure-and-crystal-system.html" target="_blank">Crystal Structure and Crystal System</a></div>
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The Streak of Minerals

Streak plates with pyrite (left) and rhodochrosite (right) The streak (also called "powder color") of a mineral is the c...

Andradite

Minerals 11:10 PM

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<b>Locality:</b> Dashkesan (Dashkezan) Co-Fe deposit, Dashkesan, Daşkəsən (Daskasan; Dashkyasan) District, Azerbaijan. </div>
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From the Bill Larson garnet suite, a deep wine-red andradite garnet that could not be more perfectly displayed, perched on its natural matrix base. At 1.5 cm, it is complete and undamaged, with pretty striations on its faces.</div>
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Andradite

Locality: Dashkesan (Dashkezan) Co-Fe deposit, Dashkesan, Daşkəsən (Daskasan; Dashkyasan) District, Azerbaijan. From the Bill Lar...

Acicular Natrolit from India

Minerals 8:39 PM

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<span style="font-family: Georgia,"Times New Roman",serif;"><b>Natrolite </b>is a tectosilicate mineral species belonging to the zeolite group.</span><br />
<span style="font-family: Georgia,"Times New Roman",serif;"><br />Formula: Na2Al2Si3O10 · 2H2O<br />System: Orthorhombic<br />Colour: White, Colorless, Red, ...<br />Hardness: 5 - 5½<br />Member of: Zeolite Group</span><br />
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<br /><span style="font-family: Georgia,"Times New Roman",serif;">Needle stone or needle-zeolite are other informal names, alluding to the common acicular habit of the crystals. </span><br />
<span style="font-family: Georgia,"Times New Roman",serif;"> </span><br /><b>From Nasik District, Maharashtra, India</b><br />
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Acicular Natrolit from India

Natrolite is a tectosilicate mineral species belonging to the zeolite group. Formula: Na2Al2Si3O10 · 2H2O System: Orthorhombic Colour...

Volcanic Ash the Tiny Disaster

sedimentary, volcano 9:00 PM

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<b>Volcanic ash</b> consists of fragments of pulverized rock, minerals and volcanic glass, created during volcanic eruptions, less than 2 mm (0.079 inches) in diameter.<br />
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<tr><td class="tr-caption" style="text-align: center;">Ash plume from Mt Cleveland, a stratovolcano.</td></tr>
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The term volcanic ash is also often loosely used to refer to all explosive eruption products (correctly referred to as tephra), including particles larger than 2mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere.<br />
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The force of the escaping gas shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometers away.<br />
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<br /><br />Volcanic ash is formed during explosive volcanic eruptions, phreatomagmatic eruptions and during transport in pyroclastic density currents.<br /></div>
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Explosive eruptions occur when magma decompresses as it rises, allowing dissolved volatiles<br />
(dominantly water and carbon dioxide) to exsolve into gas bubbles. As more bubbles nucleate a foam is produced, which decreases the density of the magma, accelerating it up the conduit. Fragmentation occurs when bubbles occupy ~70-80 vol% of the erupting mixture.<br />
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When fragmentation occurs, violently expanding bubbles tear the magma apart into fragments which are ejected into the atmosphere where they solidify into ash particles. Fragmentation is a very efficient process of ash formation and is capable of generating very fine ash even without the addition of water.</div>
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Volcanic Ash the Tiny Disaster

Volcanic ash consists of fragments of pulverized rock, minerals and volcanic glass, created during volcanic eruptions, less than 2 mm (...

Fantastic Azurite

Minerals 8:00 PM

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<b><span style="font-family: "georgia" , "times new roman" , serif;">Azurite |‪#‎Geology </span></b><br />
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Formula: Cu3(CO3)2(OH)2 <br />
System:Monoclinic <br />
Colour:Azure blue, blue, light ... <br />
Lustre:Vitreous <br />
Hardness:3½ - 4 <br />
A secondary copper mineral frequently found in the oxidized zones of Cu-bearing ore deposits. <br />
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Locality: Altenberg, Pöllan, Paternion, Gailtaler Alpen & Karnische Alpen, Carinthia, Austria<br />
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Chris Auer photo and collection <br />
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Fantastic Azurite

Azurite |‪#‎Geology Formula: Cu3(CO3)2(OH)2 System:Monoclinic Colour:Azure blue, blue, light ... Lustre:Vitreous Hardness:3...

Chalcedony (Agate)

Minerals 4:25 PM

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Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original molten mass, which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls</div>
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Locality: Patagonia, Argentina</div>
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Collection: M. Schäfer, Piesbach</div>
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<b><i><span style="font-family: Georgia, "Times New Roman", serif;">Photo: Edgar Müller</span></i></b></div>
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Chalcedony (Agate)

Most agates occur as nodules in volcanic rocks or ancient lavas, in former cavities produced by volatiles in the original molten mass, ...

Order of Geologic Events

Stratigraphy 8:54 PM

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<span style="font-family: Georgia, "Times New Roman", serif;"><br />Applying basic geologic principles: Laws of original horizontality, superposition, and cross-cutting arelationships explain the order of this diagram with the order of formation:</span><div>
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<span style="font-family: Georgia, "Times New Roman", serif;"><span style="font-size: 16px;">What is the order of geologic events that produced the sequence of rocks below?</span></span></h4>
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Order of Geologic Events

Applying basic geologic principles: Laws of original horizontality, superposition, and cross-cutting arelationships explain the order of...

beautiful Amethyst

Minerals 3:00 PM

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<b>Locality: </b>Don Nicolas Mine, Villa Benegas, Mina Clavero, San Alberto Department, Córdoba, Argentina</div>
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Dimensions: 10.5 cm x 6.5 cm x 5 cm</div>
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Quartz (Amethyst) scepter from Mina Don Nicolás, Villa Benegas, Mina Clavero, San Alberto, Córdoba</div>
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A very special specimen due to the considerable size of the crystals, their parallel growth, the scepter </div>
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formation, and the quality and intensity of the color.</div>
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beautiful Amethyst

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Principle of Original Horizontality

Geology, Stratigraphy 11:42 PM

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<span dir="auto">Principle of original horizontality</span></h1>
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<span dir="auto">The Principle of <b>Original Horizontality</b> was proposed by the Danish geological pioneer Nicholas Steno (1638–1686). This principle states that layers of sediment are originally deposited horizontally under the action of gravity. </span></div><div><span dir="auto"> </span></div><div><span dir="auto">The principle is important to the analysis of folded and tilted strata.</span><span dir="auto"> From these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science of plate tectonics; that movement and collisions of large plates of the Earth's crust is the cause of folded strata.</span></div><div><span dir="auto"> </span></div>
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As one of Steno's Laws, the Principle of Original Horizontality served well in the nascent days of geological science. However, it is now known that not all sedimentary layers are deposited purely horizontally.</div><div> </div>
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For instance, coarser grained sediments such as sand may be deposited at angles of up to 15 degrees, held up by the internal friction between grains which prevents them slumping to a lower angle without additional reworking or effort. This is known as the angle of repose, and a prime example is the surface of sand dunes.</div>
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Principle of Original Horizontality

Principle of original horizontality The Principle of Original Horizontality was proposed by the Danish geological pioneer Nicholas ...

How Others See Geologist

Fun 10:54 PM

How Others See Geologist

What is Mining?

Geology, Mining 8:43 PM

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<b>Mining </b>is the extraction of valuable minerals or other geological materials from the earth, usually from an orebody, lode, vein, seam, reef or placer deposit. These deposits form a mineralized package that is of economic interest to the miner.<br />
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Ores recovered by mining include metals, coal, oil shale, gemstones, limestone, chalk, dimension stone, rock salt, potash, gravel, and clay. Mining is required to obtain any material that cannot be grown through agricultural processes, or created artificially in a laboratory or factory. Mining in a wider sense includes extraction of any non-renewable resource such as petroleum, natural gas, or even water.</div>
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<b><span style="font-family: "georgia" , "times new roman" , serif;">Why mine?</span></b></h4>
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Mining is a money making business.  Not only do mining companies prosper, but governments also make money from revenues.  Workers also receive income and benefits. </div>
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<span style="font-family: "georgia" , "times new roman" , serif;">Mining Techniques</span></h4>
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Mining techniques can be divided into two common excavation types: surface mining and sub-surface (underground) mining. Today, surface mining is much more common, and produces, for example, 85% of minerals (excluding petroleum and natural gas) in the United States, including 98% of metallic ores.<br />
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Targets are divided into two general categories of materials: placer deposits, consisting of valuable minerals contained within river gravels, beach sands, and other unconsolidated materials; and lode deposits, where valuable minerals are found in veins, in layers, or in mineral grains generally distributed throughout a mass of actual rock. Both types of ore deposit, placer or lode, are mined by both surface and underground methods.<br />
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Some mining, including much of the rare earth elements and uranium mining, is done by less-common methods, such as in-situ leaching: this technique involves digging neither at the surface nor underground. The extraction of target minerals by this technique requires that they be soluble, e.g., potash, potassium chloride, sodium chloride, sodium sulfate, which dissolve in water. Some minerals, such as copper minerals and uranium oxide, require acid or carbonate solutions to dissolve.<br />
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<span style="font-family: Georgia, "Times New Roman", serif;"> Exploration</span></h4>
A mining operation begins with prospecting and exploration -- stages with long periods of investment and high risk of failure. However, prospecting and exploration are necessary forms of investment and insurance for the future of any mining company. Success in mineral exploration or the acquisition of high -potential mineral properties by negotiation determines the survival of mining companies and industrial nations. <br />
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What is Mining?

Mining is the extraction of valuable minerals or other geological materials from the earth, usually from an orebody, lode, vein, se...

The Terminology of Coated Grains in Limestone

Carbonate, Rocks, Terminology 3:00 PM

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<tr><td class="tr-caption" style="text-align: center;">Pisolite by Dirk Wiersma</td></tr>
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<b><span style="font-family: Georgia, "Times New Roman", serif;">Ooid (oolith)</span> -</b> a spherical to ellipsoidal grain, 0.25 to 2.00 mm in diameter, with a nucleus covered by one or more precipitated concentric coatings (cortical layers) with radial and/or concentric orientation of constituent crystals. Nuclei typically consist of detrital terrigenous grains, skeletal fragments, or pellets and peloids, and coatings can have a variety of compositions. A rock composed dominantly of ooids is termed an “<b>oolite</b>”. That term is commonly misused, however, to describe the constituent ooid grains. <br /><br /><span style="font-family: Georgia, "Times New Roman", serif;"><b>Spastolith or deformed ooid</b> </span>- An ooid or other coated grain that has been deformed, generally by shearing the concentric laminations away from each other or from the nucleus. In rarer cases, the deformation is tectonic. <br /><br /><span style="font-family: Georgia, "Times New Roman", serif;"><b>Superficial ooid</b> </span>- An ooid with an incomplete or very thin cortical coating; specifically one in which the thickness of the accretionary coating is less, commonly far less, than the radius of the nucleus.vlarger than 2 mm and (in some usages) less than 10 mm in diameter. </div>
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A <b>pisolite </b>is a rock containing abundant pisoids. <br /><br /><span style="font-family: Georgia, "Times New Roman", serif;"><b>Oncoid</b></span> - In North American usage, an oncoid is a coated grain of algal (but not red algal) or microbial origin that is coarser than 2 mm in diameter; a spheroidal form of microbial stromatolite showing a series of concentric (often irregular or scalloped) laminations. These unattached stromatolites are produced by mechanical turning or rolling, exposing new surfaces to microbial/algal growth. Common European usage is less genetic, and in that usage a microbial/algal origin is not a prerequisite. An oncolite is a rock composed of oncoids; the term, however, is often used improperly as a synonym for “oncoid”. <br /><br /> <div style="text-align: left;">
<b><span style="font-family: Georgia, "Times New Roman", serif;">Rhodoid (rhodolith) - </span></b>An irregularly laminated calcareous nodule composed of encrusting coralline algae arranged in more or less concentric layers about a core; spheroidal but knobby surfaced, and up to several centimeters in diameter; form in warm to cool, clear, shallow sea water down to depths of 150-200 m. </div>
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The Terminology of Coated Grains in Limestone

Pisolite by Dirk Wiersma Ooid (oolith) - a spherical to ellipsoidal grain, 0.25 to 2.00 mm in diameter, with a nucleus cover...

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