Seafloor Spreading: Mid-Ocean Ridge (MOR), Oceanic Crust and Ophiolites

Mid-Ocean Ridge Basalts (MORB)

MORB is the most abundant type of igneous rock on Earth. It is formed at mid-ocean ridges, which are long, narrow mountain ranges that run through the ocean basins. Mid-ocean ridges are formed where tectonic plates are moving apart. As the plates move apart, magma from the mantle rises to the surface and erupts, forming new oceanic crust. MORB is a dark, fine-grained basalt that is rich in iron and magnesium.

Oceanic Crust

The oceanic crust is the layer of rock that covers the ocean floor. It is made up of MORB and other volcanic rocks. The oceanic crust is much thinner than the continental crust, and it is also much younger.

The oceanic crust is constantly moving, as it is created at mid-ocean ridges and destroyed at subduction zones. Subduction zones are areas where tectonic plates collide and one plate sinks beneath the other. As the plate sinks, it melts and releases water vapor. This water vapor rises through the overlying plate, lowering its melting point and causing it to melt. The molten rock eventually rises to the surface and erupts as volcanoes.

Ophiolites

Ophiolites are slices of oceanic crust that have been thrust onto continents. Ophiolites are typically composed of a layered sequence of rocks, including MORB, gabbro, diabase, and pillow basalts. Ophiolites are also often associated with serpentinite, which is a rock that is formed by the alteration of mantle rocks.

Ophiolites are important because they provide a direct record of the formation and evolution of the oceanic crust. Ophiolites can also be used to study the processes that occur at mid-ocean ridges and subduction zones.
Seafloor Spreading: Mid-Ocean Ridge (MOR), Oceanic Crust and Ophiolites
Ophiolitic Sequence

A typical oceanic crust (or ophiolitic sequence) is made of four layers:
  • Sedimentary layer: This  is the topmost layer, and it is made up of sediments that have been  deposited from the ocean. The thickness of the sedimentary layer can  vary depending on the location, but it is typically only a few  kilometers thick.
  • Basaltic lavas: This  layer is made up of basaltic lavas that have erupted from the mantle.  The basaltic lavas are typically pillow-shaped, which is caused by the  rapid cooling of the lava as it comes into contact with the cold ocean  water.
  • Sheeted dikes: This  layer is made up of a series of vertical dikes that were injected into  the basaltic lavas. The dikes are made up of the same material as the  basaltic lavas, but they are typically more coarsely crystalline.
  • Gabbroic layer: This is  the bottommost layer of the oceanic crust, and it is made up of gabbro.  Gabbro is a type of intrusive igneous rock that is formed from the slow  cooling of magma.

 

Origin of MORB magmas

MORB magmas are formed by the partial melting of mantle rock at mid-ocean ridges. The mantle is a layer of hot, semi-solid rock that lies between the Earth's crust and core. As the mantle rises and melts, it forms magma. This magma is then able to erupt onto the surface, forming MORB.

The composition of MORB magmas depends on the composition of the mantle rock that they are formed from. The mantle is not uniform in composition, and there are two main types of mantle rock: depleted mantle and enriched mantle.

Depleted mantle is the most common type of mantle rock. It is formed when molten rock rises to the surface and erupts as MORB. The molten rock leaves behind a residue of depleted mantle, which is lower in incompatible elements (such as potassium, rubidium, and barium) and higher in compatible elements (such as magnesium and iron).

Enriched mantle is less common than depleted mantle. It is formed when subducted oceanic crust melts and releases fluids into the mantle. The fluids enrich the mantle with incompatible elements.

Types Spreading Ridges

Mid-ocean ridges can be classified into two main types: fast-spreading ridges and slow-spreading ridges. Fast-spreading ridges have a spreading rate of greater than 4 centimeters per year. Slow-spreading ridges have a spreading rate of less than 4 centimeters per year.
 


Fast Slow
Topography
No axial valley, high “domes”, overlapping spreading centers, smooth.
Deep axial valley, irregular topography, narrow ridge.
Tectonics
Limited extension, lateral grabens
Important extension, axial graben
Oceanic crust thickness
Thick
Thin
Sequence
Complete, basalts-gabbros-dunites-harzburgites
Incomplete, discontinous gabbro intrusions or basaltic pillows on lherzolite substratum
Peridotite type
Harzburgite
Lherzolite
Melt fraction
High
Low
 
Fast-spreading ridges are typically associated with HOT (high-temperature) mantle. HOT mantle is hotter and more fluid than LOT (low-temperature) mantle. As a result, MORB magmas from fast-spreading ridges are typically more fluid and have a higher eruption rate than MORB magmas from slow-spreading ridges.

Slow-spreading ridges are typically associated with LOT (low-temperature) mantle. LOT mantle is colder and more viscous than HOT mantle. As a result, MORB magmas from slow-spreading ridges are typically less fluid and have a lower eruption rate than MORB magmas from fast-spreading ridges.

Two extreme cases, Harburgite Ophiolite Type and Lherzolite Ophiolite Type.

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