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What is Geology?

Geology is the study of the origin, history and the structure of the solid earth.

What is the Earth made of?

The earth formed approximately 4.5 billion years ago. It was so hot when it formed that it was made up of molten (melted) rock. While the earth was cooling, the chemicals in the earth became differentiated, or separated. The lighter materials remained close to the surface making up the crust. The heavy, dense materials created a core at the center of the earth, composed of mainly nickel and iron. In the middle is the mantle, which contains less dense material than the core, but more dense material than the crust.


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Aren't all rocks the same?

There are many different kinds of rocks. The three main groups of rocks include:

  • Igneous rocks: formed from the cooling of magma (under the surface) or lava (on the surface).
  • Sedimentary rocks: formed when mineral grains cement together. (Ex: sand cementing together to form a new rock is called sandstone)
  • Metamorphic rocks: formed when either sedimentary or igneous rocks have been changed because of high temperatures, high pressures, or both.


Lava flow of Pu' u 'O'o volcano, Aug 1990. Lava cools to
become igneous rock.
(Photo taken by T. Maddox)

Earthquakes and Volcanos


Geologic Time Scale | What is Geology? | Exploringthe Lehigh Valley | Rock on! Educational Material

Pennsylvania Geology | Which Way is North? | Geologic Explorations | Dino Inquiry | Wonderful World of Rocks and Minerals

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