This week I moved to a topic somewhat related to heat, geology. The kids were all going on a trip to some caves this week, so I wanted to talk about some of the things they would be seeing there. For this class, I talked to the kids about the rocks first, then had them color a coloring sheet about rocks. I had small groups of kids come up while the rest were coloring to look at the rocks and minerals that I brought in. The kids really liked looking at these!
There are three types of rocks. Their formation process determines which group they are in, but all types of rocks can become another type. This is the rock cycle. Before I discuss the three types or rocks, I will talk about what minerals are.
Minerals are made up predominantly of one atom or molecule. They have a characteristic crystal structure. You can see some examples of minerals below.
Rocks are made up of two or more minerals. Sometimes small crystals are easily visible in the rock. Rocks do not have a specific shape.
Igneous rocks form from melted rock. Beneath the Earth’s crust is the mantel which consists of molten rock. This molten rock is heated by the Earth’s core and moves in convention currents. The crust is in several pieces and moves very slowly on top of the mantle. In some areas one plate moves below another and melts into the mantle. In other places, two plates push together to form mountains. In areas that have two plates moving away from each other, liquid rock comes to the surface. This is where volcanoes occur.
Intrusive and Extrusive Igneous Rocks
Magma can rise through the crust and stay there to cool very slowly, or it can come out of a volcano as lava and cool very quickly. When magma cools slowly, visible crystals form in the rock. You can see this in granite and the rock pictured below.
Rocks that form from lava cool very quickly. These are extrusive rocks. Obsidian and pumice are examples of extrusive rocks.
Sedimentary rocks form from little bits of rocks, the remains of plants or animals, or deposits of chemicals. These little pieces can enter a lake or the ocean and fall to the bottom. The weight of the water pushes down on these pieces and forms rocks. Examples of rocks that form from plants and animals are chalk and coal. Chalk is a type of limestone and it is formed from coccolith shells. Coal is formed from dead plant matter that was compressed and was not exposed to air. Sandstone is an example of a sedimentary rock formed from other rocks. Rocks exposed to wind and water break down and the pieces can be compressed into sedimentary rocks. Sandstone is an example of this. When molecules that are dissolved in water come out of solution, they can also form rocks. An example is this piece of specular hematite. Limestone can also be formed in this way.
Fossils can form in sedimentary rocks. If an animal dies and is buried in sediment, it can become part of the rock, and over time minerals replace the tissue of the animal. This leaves an image of the animal in the rock.
Metamorphic rocks form when igneous or sedimentary rocks are exposed to heat and pressure. Metamorphism starts at temperatures above 200 degrees C and 3,000 times atmospheric pressure. Rocks must be exposed to these conditions over very long periods of time. Some metamorphic rocks are thought to take tens of millions of years to form. The heat and pressure can change the molecules in a rock into new ones, or can change the crystal structure of the minerals in the rock. Metamorphic rocks can be foliated like the tigers eye pictured below.
Metamorphic rocks can also be non-foliated like the lapis lazuli pictured below.
All three types of rocks can enter the mantle, melt, and later move to the crust and cool to form igneous rocks. All three types of rocks can be weathered to form sedimentary rocks. Sedimentary and igneous rocks can be changed by heat and pressure into metamorphic rocks.
The last thing we talked about in class was how caves are formed. Water can be found underground, and sometimes it flows in rivers. This water dissolves the rock it flows past. This makes a larger and larger hole in the rock over time. When the water level goes down, people can walk through the tunnel (cave) that has formed.
Stalactites form from the ceiling in caves and stalagmites form on the cave floor below them. When water filters down through the soil from the surface of the ground, it picks up carbon dioxide. This water dissolves limestone and drips through the cave ceiling. As the water drips through, it leaves the minerals dissolved in it behind when it evaporates. This process forms stalactites and stalagmites very, very slowly. You can see a water stalactite (icicle) form in this video.
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