Kinds of Rocks: Igneous, Sedimentary, and Metamorphic edit

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

Igneous Rocks edit

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.

Examples of igneous rocks edit


  • This is a common extrusive rock, which forms from lava.
  • Lava cools so quickly that it only produces very small crystals, so it is difficult to see individual crystals in 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, and 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.
  • Here is a 3D model of basalt. The green crystals are peridotite, and the grey rock is the basalt.


  • Granite is a common intrusive rock, which forms in 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.
  • Here is a 3D model of granite.

Sedimentary Rocks edit

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!

Examples of sedimentary rocks edit



  • A 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.
  • Here is a 3D model of mudstone
  • A rock made from a mix of big chunks of other rocks.
  • Here is a 3D model of conglomerate

Metamorphic Rocks edit

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!
  • Here is a 3D model of a metamorphic rock with garnet inside it.

How to Observe Sedimentary Rocks edit

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 edit

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, is the rock patterned or are the changes gradual?

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 edit

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 edit

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.