Field Guides

Guide to Studying Geologic Rock in the Field

Wash U’s Arts and Sciences Dean and geologist Jennifer Smith gave the Missouri’s Natural Heritage class general tips to studying geologic rock in the field during the 2016 trip to Elephant Rocks and Johnson’s Shut-Ins. Below is a basic guide condensing her tips.

1. First, check if the rock you are studying is “in place” or “in situ.”

  • “In situ” means that the current location of the rock is also the place of origin.
  • If the rock is small and looks as if it could have been moved from another location, it is not a reliable source to study the rock of the land.
  • If the ground is rocky and it seems like the layers below it are composed of the same rock, it is most likely a reliable representation of the rock of the area.

2. Look at the color and examine the components of the rock using a magnifying glass.

  • This part is important in identifying the rock and finding what elements make up the rock.
  • For example, granite is a red rock formed by magma that cooled below the surface of the earth. Thus, the crystals and different colored elements are distinguishable.
  • Red, pink, and orange color variations of granite come from differences in amounts of trace elements.

3. Note how porous the rock is.

  • Porosity can indicate how easily elements such as water can pass through the rock and how resistant the rock is.
  • Resistant rock is not easily eroded.
  • This is relevant in cases where there might be linear holes that go through layers of rock, showing how an insect or animal burrowed through the rock, leaving spaces that were filled in by sedimentary rock that then eroded because it was weaker than the main rock.

4. Note the vegetation on top of the rock.

  • The type of plants above the surface may be telling of the rock’s chemical nature.

5. You can use a rock hammer to break through the surface of the rock and examine a fresh area.

  • This is relevant if you want to study the rock itself because surface age of a rock is not equal to the rock age.
  • A piece of rock can be cut from an old rock and the newly exposed surface will be younger than the already exposed surface and have a different appearance.
  • You can compare the surface ages of different parts of the same rock in the same area by looking at the weathering of the rock. At Elephant Rocks, more lichen was indicative of an older surface age.

The most difficult task of a geologist is determining if a seemingly irregular aspect of the rock is different enough for it to be significant or indicative of something we cannot see. This guide is only a beginner’s guide to finding what aspects are important as you examine geologic rock you may come across as you hike through the natural environments of Missouri. Happy identifying!

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