Evidence in Rocks

Background Knowledge:

One of the first things geologists in training are taught is how to identify rocks. This is because rocks are like Earth’s history books, they hold clues about events that occurred at different points over millions of years. Every rock tells a story, and geologists learn to read these.

Photo by Felix Kunze

Clues found in rocks that geologists look for include specific minerals which help them identify the rock and how it formed, traces of past life (fossils), and unique chemical markers.

Geologists study these 'rock clues' to learn about how mountains and rivers formed and changed over time, when extinct creatures lived, and what caused massive events like the Devonian extinction.

Fossil clues are often found in sedimentary rocks like sandstone, limestone, and shale, which formed from layers of mud, sand, or shells in ancient environments. Fossils are rarely found in other types of rock, like igneous or metamorphic, because the intense heat and pressure that form these rocks can destroy any fossils present—making sedimentary rocks like sandstone, limestone, and shale the best places to search for clues about Earth's past.

Learning to identify these rocks is the first step in uncovering the secrets they hold about life and landscapes from millions of years ago!

SANDSTONE- Usually tan, reddish, or grey. Sandstone has a gritty texture like sandpaper and may show visible grains or layers of sand packed together. Sometimes the layers are visible as stripes.

Sandstone (front)

Sandstone (back)

Sandstone (close-up)

SHALE- Dark-colored and thinly layered, sometimes it can break easily into flat, smooth sheets. Often feels smooth or flaky to the touch, and will make a dull, "thud" sound (compared to harder rocks like limestone or sandstone, which produce a sharper "clink" or "tap" sound).

Shale (front)

Shale (back)

Shale (close-up)

LIMESTONE- Usually light-colored and smooth, limestone often feels powdery and may have tiny fossils or shell fragments visible. It can fizz with a drop of hydrochloric acid or strong vinegar.

Limestone (front)

Limestone (back)

Limestone (close-up)

Work the Case:

The investigators want to put you to the test before bringing you out to the field with them. They left five unknown rock samples for you to identify. Click on the arrows floating on the images to see different views for the rock specimen. Use the cards with their notes to figure out whether the rock is a limestone, sandstone or shale.

Enter your data and check your answers by completing the Evidence in Rocks Google Form (note that there can be several of each type of rock!).

Rock layers provide many clues about the natural history of a place. The study of rock layers in the context of geology is called stratigraphy. Stratigraphy usually refers to sedimentary rock and soil layers or igneous rocks formed at the Earth’s surface. These layers are called strata.

Biostratigraphy is the method geologists use to date rock layers and fossils by focusing on the presence and arrangement of fossils within the strata of sedimentary rock formations or volcanic deposits.

Geologists use diagrams called “stratigraphic columns” (like the one on the right) to describe cross sections of rock formations. They include types of rocks found and clues they hold (like fossils, and chemical markers). By studying these fossils and chemical markers and their positions in the rock layers, scientists can piece together a timeline of Earth's history and better understand how life has evolved over time.

YOUR TURN!

You will examine three stratigraphic columns our investigators prepared for three locations that could have new evidence of the Devonian mass extinction. You want to look for sections in the strat column which are lacking a lot of fossils. See if you can determine which of them shows stronger evidence than the other two of a possible extinction event!

Use the arrows that appear on the images when you hover over them to swipe to the next strat column. The three location data was obtained from are:

Bactrian Mountain (Southern Nevada, USA)
Baldy Peak (Utah, USA)
Roberts Mountain (Central Nevada, USA)

Think about...

Which type of rock is most fossil data coming from?

How many meters deep is the sediment being analyzed?

Which of the three locations is showing signs of a possible extinction event based on fossil and oxygen record?

Got the TWO passkeys available for this section? Click on the arrow to advance to the next challenge which looks at fossil evidence.

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