What Is A Rock Exactly?

What is a rock? We all know what rocks are right? The are lying on the ground everywhere! They are the mountains, and canyons that you can’t help but notice. We have all thrown them, sat on them, and dug them out of our gardens. However, to pin down an exact definition… that is something not all of us could do.


A rock is:

  • A solid
  • Naturally occurring
  • Is made up of minerals or mineral like matter.

Rock Composition
Some rocks are composed of just one mineral. Pyrite and quartz are two common rocks that fit this category. Most rocks are a solid mixture of several minerals like granite.

Rock Classification
Rocks are classified by how they are formed. There are three basic groups, igneous, sedimentary, and metamorphic.

In each group, distinctions are made for texture or grain size and chemical or mineral content.

That is pretty straightforward isn’t it? That is the textbook definition. However, there are some gray areas.

Exceptions to the What Is A Rock definition
What about mercury? It is not a solid at normal temperatures but if it gets cold enough it becomes solid.

pyrite
quartz

Then there is coquina. Coquina is a sedimentary rock made of seashells. The shells are made of minerals but they are not minerals.

Coal is considered a rock but it is not made of minerals it comes from organic matter plants.

These last two are called biogenic rocks.

Petrology is the study of rocks.

What Is A Rock? Perhaps the more important question is “why study rocks?

Rocks are the pages in the history book of the earth. They tell stories of times long past. From rocks, we have learned of dinosaurs, trilobites and thousands of other life forms that existed on earth once but are now no more.

We have learned that the earth is constantly moving, shifting, and changing. Rock is continually being created. It is heated, squeezed, and weathered into new rocks. Finally, it is remelted to start the process anew. By studying rocks, we learn to read the pages of earth’s history.

Learn more of the finer points to the question What Is A Rock?

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What Is A Rock?

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Metamorphic Rocks

Sedimentary Rocks

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