Rocks

Kinds of Rocks: Igneous, Sedimentary, Metamorphic
Earth's crust is made of minerals in rocks. We categorize rocks because they form differently: Igneous, Sedimentary, and Metamorphic.

Igneous rocks form from hot flexible chemical mixtures, where the elements gather into crystals.


 * We call it lava when we see this hot molten material flexing on the surface of the Earth. Lava produces extrusive igneous rocks.


 * We call it magma when it is underground and has not come out to meet air or water. Magma can produce intrusive igneous rocks.

Basalt is a common extrusive rock, which forms from lava.


 * Cooling lava produces small crystals fast, so it is hard to see individual crystals in a chunk of basalt.
 * Where lava flowed under air or water, it retains its flow texture - rough, smooth, bubbly, etc.
 * Lava can flow through cracks in existing rock, to can produce upright or flat-out expanses.
 * Idaho's Craters of the Moon is made of basalt.
 * Basalt that is mostly bubbles and floats is called pumice, and is an important economic resource.
 * Most of Earth's sea floor is made of basalt that is continually erupting under water.

Granite is a common intrusive rock, which forms from magma.


 * We can easily see the individual crystals in granite. Large crystals form slowly while the magma is trapped underground.
 * We find granite on continents that are very eroded. Mountains of granite exposed by glacial erosion are easy to see in Yosemite National Park in California.
 * The durability and beauty of granite make it an important economic resource and common building material.

Sedimentary rocks form when minerals and broken rocks spread into layers.


 * Bones and shells caught in the layers can form fossils.
 * Flow patterns (ripples, scree slopes, river channels, etc.) can leave clear marks in layered sedimentary rock.

Sedimentary rocks are the easiest to name! We just describe what we see!




 * Sandstone is rock made of sand. It will feel like sandpaper. Each grain of sand has a history, and each sandstone collects that history.
 * Mudstone is rock made of mud. Mud can be made of clay worn down from rocks in the mountains, or from tiny shrimp poops in a lake.
 * Conglomerate is rock made from a mix of big chunks of other rocks.



Metamorphic rocks form where heat and/or pressure transforms minerals into new material.


 * Beautiful patterns, swirls, shines, and gems are common in metamorphic rocks.
 * Study of metamorphic minerals helps us read the history of heat and pressure that altered entire continents.
 * Metamorphic rocks usually mess up fossils, and we will generally avoid them during this class!

How to observe sedimentary rocks
If you pick up a sedimentary rock from the ground, as you drive by a highway road cut, or as you stroll on the Bonneville Shoreline Trail, try these observations.

Large-scale features
Does a cliff of rock show layers, like an ice cream cake or a lasagna? Or is it just a massive wall of stuff?


 * Are the layers each about the same size? Is there a clear pattern (thick, thin, thick, thin)?
 * Are the layers each the same color and consistency? Or are there changes - gradual or patterned?

Does the cliff seem to resist weathering, and really stand out? Or does the sedimentary rock crumble and form a slippy slope?


 * If it stands out, look around: can you see other chunks or cliffs of the same stuff standing out on other hillsides nearby?
 * If it is crumbly, look around: can you see other places where the same rocks might be, but maybe they are hidden under grass?

Medium-scale features
Get face-to-face with a rock chunk. How big are the pieces?


 * Can you see individual cobbles or boulders, the size of your fist or bigger?
 * Can you see pebbles?
 * Can you identify any rock pieces? Do you see little pebbles of granite, or of sandstone?
 * Can you see any fossils? Bones? Shells? Patterns that might be traces from feet, or from wiggly invertebrates?

Feel the rock with your fingertips:


 * Can you feel rough grit, like sandpaper? Can you see the sand?
 * Can you feel smooth or scratchy patches where you don't see sand?

Can you see layers?


 * Sometimes there are big layers easy to see from a distance, and you'll see even more sub-layers up close.
 * Are the sediments in each layer the same, considering the qualities above?
 * What colors do you see? Do they appear different in each layer, or all the same?

Can you see bedding planes or sedimentary structures?


 * Some layers of rock are hard to distinguish, but some big cliffs of sedimentary rock show clear breaking points between layers.
 * "Bedding planes" are clear break-lines between layers of sedimentary rock. They can be smooth, or patterned.
 * Look for little ripples on an exposed slab of bedding.
 * Look for cross sections of ripples and other patterns.

Small-scale features
Get reeeeeealllly close to your rock. Use a little pocket magnifying lens if you have one!

If your rock has clasts you can see (sand, pebbles, etc.), what shape are they?


 * very round and spherical?
 * kind of round, or smooth and flat like a skipping stone?
 * pretty rough around the edges?
 * super rough around the edges?

How consistent are the clasts?


 * Are they perfectly sorted - all the grains in a single rock layer look the same size and shape?
 * Are they a wild mix - one rock layer has both large and tiny clasts, or both rough and smooth grains?
 * Are the clasts graded - with larger chunks at the bottom of a rock layer and gradually smaller grains at the top of the same rock layer?

Record your observations as written notes, hand-made drawings, and annotated photographs.

Learn how geologists interpret sedimentary rocks over at our page on Depositional Environments.