Dear Aunt Lori,

Do you like rocks? What's your favorite kind?

Totally NOT Dwayne Johnson

Dear Totally NOT Dwayne Johnson,

Yes! There are a lot of really cool rocks out there with fossils, interesting formations, compounds, colors, and more! We can learn a lot from rocks and all the different ways they form.

For starters we need to understand the different types of rocks: sedimentary, igneous, and metamorphic.

Sedimentary

Sedimentary rocks - as the name suggests - are created when sediment (small pieces of existing rock or organic matter) pack tightly together. Within the category of sedimentary rocks are clastic, organic, and chemical rocks.

Clastic rocks are formed from small pieces of existing rocks (clasts) pack tightly together. The presence of clastic rocks tells us that there is something nearby causing the erosion and weathering of existing rocks that carry clasts to that spot. This could be done through different types of erosion: wind, water, ice, or animal activity.

Organic sedimentary rocks are formed in a similar way, but are formed from organic matter being tightly compressed until it hardens into rock. These types of rocks are often made from bones, shells, or even pieces of plants. The presence of these rocks can help us discover what species of plants and animals were once living in the area by looking into the makeup of the rock.

Chemical sedimentary rocks are formed by what's called chemical precipitation. A solution with a compound like silica, calcium carbonate, or salts pools in an area. As the temperature rises in the puddle, the water evaporates and only the compound is left. That compound can then pack together to form a rock. This type of rock is common in the Earth's crust as water travels through it.

Igneous

Igneous rocks are very common near volcanoes - that's because they are formed when molten hot material, such as lava, cools and hardens. They can be formed inside the Earth's crust (plutonic/intrusive) or on the surface of Earth's crust (volcanic/extrusive).

The Earth's crust (the layer we live on) is composed of tectonic plates above a layer of magma. Sometimes the tectonic plates shift just enough to create a little "pocket" of magma. This pocket doesn't reach the surface and eventually cools, hardens, and compresses into an intrusive igneous rock.

Sometimes when those tectonic plates shift, they move enough to allow magma to erupt to the surface, which is called lava. This lava erupts from a volcano and it eventually cools and hardens into volcanic (extrusive) igneous rock.

Both of these rocks are more likely to occur along the edges of a tectonic plate.

Metamorphic

Metamorphic rocks have undergone a metamorphosis (change). These are existing rocks that have been subjected to extreme pressures and/or heat which causes a change in the rock. They are further divided into foliated and nonfoliated.

Foliated metamorphic rocks are rocks that had flat or elongated minerals that were put under enormous pressure. When this happens, the minerals form layers perpendicular to the direction of the pressure. Nonfoliated metamorphic rocks are formed in the same way, but don't contain flat or elongated minerals, and they do not line up to create layering.

The presence of these rocks can give various pieces of information about the area, depending on the minerals found in the rock.

Favorites

As for my favorite rock? I really like river rocks! They're a testament to the strength of water and how it can sculpt something so smooth. The water sculpting it also indicates that there are probably sedimentary rocks downstream!

For gemstones, my favorites are blue sapphires and London blue topaz. The reasoning is very simple for these - I think they're pretty!

Aunt Lori

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