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Rock Types

Rock Types

Introduction: Step outside and chances are you are stepping on rock or something made from rocks. If you step onto soil, it is made partly from broken down rock. Step onto concrete, it is made with crushed rocks. Maybe you will step on a walkway made from patio stones or perhaps onto a sandy beach. The oldest rocks found on earth are over three billion years old. It's no wonder the pyramids were constructed with rock!

Geologists classify rocks into three types based on how the rocks formed: igneous, sedimentary, and metamorphic.

Igneous rocks form when molten earth material cools and hardens. Magma deep within the earth can slowly cool over millions of years. During this time, mineral crystals grow large and form igneous rocks. Lava on the earth's surface can also cool and form igneous rocks. Lava cools quickly, so these rocks usually have small or no mineral crystals. Most of the earth's crust is made of igneous rocks.

Sedimentary rocks form from silt, sand, rock pieces, and even plant and animal remains. These particles, or sediment, get compacted and cemented together into a rock. Sedimentary rocks can also form when minerals in water crystallize. Sedimentary rocks are the most common types of rocks found on the earth's surface where water and wind create pieces of sediment. Often, they form as layers of rock when younger sediment forms on top of older sediment.

Metamorphic rocks form from heat, pressure, and chemical reactions. Bury a preexisting rock deep enough in the earth and it will change into a metamorphic rock. Faulting from movement of tectonic plates can change rocks into metamorphic rocks. Sedimentary, igneous, and even metamorphic rocks can change into metamorphic rocks. But, if they melt, they will not form metamorphic rocks. They will form igneous rocks instead!

Directions for This Lesson: In this lesson, you will learn about the three types of rocks. First, try the practice questions to determine what you already know. Then, watch the video lesson to learn more. Finally, apply what you learned in the activity and practice sections.


Required Video:

Practice what you have learned by completing the post-lesson worksheets.