Geologist's Hammer: Rock Hammer

Geologist's hammer, also known as a rock hammer, rock pick, geological pick, or informally a geo pick, is a vital tool for geologists during field work.

Geologist's hammers are specifically designed for splitting and breaking rocks. There are two main parts of a geologist's hammer:

Head: The head is usually made of forged steel and has two faces:

  • A flat face: This is used for breaking rock where precision is not required or for breaking down larger rocks into smaller pieces.
  • A pointed pick: This is used for more delicate work, such as extracting fossils or minerals from rock.

Handle: The handle is typically made of wood or fiberglass and may have a rubber grip for comfort and shock absorption.

Geologist's Hammer: Rock Hammer
A geologist's hammer used to break up rocks,
as well as a scale in the photograph

Usually, some combination of graffiti, lichen, moss, weathering or water stains have intermingled to cover or confuse the surface. In which case, breaking off a piece is the quickest and easiest way to see what is really there (at the risk of flying rock shards).

The other reason, however, is more fundamental. For many rocks and minerals, how they break is nearly as diagnostic as any other observation we can make.

Take, for instance, the difference between shale and slate. Shales are just layered mudstones; other than bedding they have no internal structure. Thus, they tend to break irregularly and often are somewhat crumbly.

In contrasts, slates — which are equally fine-grained — have re-crystallized so that their clay minerals are all aligned. As clays are thin, flat minerals, that means slates split easily into thin, brittle sheets, often ready-made for shingles.

For other sedimentary rocks, breaking them allows an inspection of their sands and sediments, and sometimes of the cements holding them together. Also, many fossils (especially shells and fish) can be found where a rock breaks.

But the real fun starts when we break igneous rocks like granite. Here, the minerals are all intermeshed and grown together. So if you break the rock, you have to break the minerals inside. This is guaranteed to reveal the true colors of those minerals, but it also shows something of how those minerals are put together as well.

Specifically, if you look closely at any broken piece of granite, you will see some dull, glassy minerals and also a number of flat, sparkly minerals. The knee-jerk assumption is that those flat reflectors are crystal faces that happened to be aligned with the break. But, look more closely and you’ll see at least some of those reflectors aren’t flat. They step up or down while still reflecting light at the same angle.
In reality, most of those facets are cleavage planes, places where the minerals broke along a systematic zone of weakness inside the crystal.

As such weaknesses mark variations in the chemical bonding of different atoms, they are repeated in some minerals at small intervals, and with specific orientations. Thus, micas (which have a single cleavage) peel off into thin sheets, and feldspars (which have multiple, intersecting cleavages) break either into tiny, block-like shapes or along stepped facets. On the other hand, minerals without cleavage, like quartz and garnet, do not break flatly and usually show more chipped or irregular surfaces.
So, as spring brings a sparkle to the rocks around us, take a closer look. Some of those glints may be more than just rain or snowmelt.

Rock Hammer Uses

Geologists use their hammers for a variety of purposes in the field, including:

Obtaining fresh rock surfaces: A geologist will use the hammer to break off a piece of rock to expose a fresh surface. This fresh surface is then used to determine the rock's composition, bedding orientation, and other features.

Collecting rock and mineral samples: The hammer can be used to break rocks apart in order to collect samples for further analysis in a laboratory.

Testing the hardness of rocks: By striking a rock with the hammer, a geologist can get a sense of how hard the rock is. This information can be helpful in identifying the rock type.

Scaling objects in photographs: Geologists often use their hammers for scale in photographs of rock outcrops. The hammer's size is well-known, so it can provide a reference for the size of the features in the photograph.

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