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This Page: Home Geohistory Enjoying the Geopark: What to Look For (Geohistory Section) Geology Basics – Three Different Types of Rocks

Geology Basics – Three Different Types of Rocks

When you hear the world “rock” you probably think about something of little value or purpose. You can walk down the street and see rocks lying about pretty much anywhere, and unlike living organisms, they don’t seem to have any defining features. You can observe their shape and color or measure their size, but it’s unclear what the significance of any of those properties are.

However, if you just learn how to look at rocks, they can become keys in unveiling the history and transformation of the earth and specific geographical areas.

Rocks can be divided into several different groups based on number of properties and traits, but one of the simpler ways of sorting rocks is grouping them into the three major categories of how they were formed.

The Three Types of Rocks

  • Metamorphic Rock
    • Oki Gneiss
  • Igneous Rock
    • Basalt
    • Alkali Rhyolite
  • Sedimentary Rock
    • Green Tuff
    • Diatomaceous Soil

Rocks formed through the high temperatures or pressures unleashed during continental collisions and volcanic eruptions are called Metamorphic Rocks. The Oki Gneiss found in Oki belongs to this group. When lava from volcanic eruptions or magma within the earth’s crust cools and hardens, igneous rock is formed. When an explosive eruption occurs, fragments from the volcano’s edifice will break off and pile up at the foot of the volcano to form pyroclastic rock. All of these rock forms can be found around the coastal areas of the Oki Islands.

The kind of rocks which sometimes contain fossils, formed when soil and sand pile up and harden overtime at the bottoms of lakes and oceans, are called sedimentary rocks. These rocks tell us about ancient lifeforms and how they lived.

In conclusion, these three types of rocks serve as records of the various stages and events in earth’s long history.

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