Rock Classification Lesson

Classifying Rocks Reading Lesson

Classifying Rocks Reading

1 Have you ever picked up an interesting rock? Maybe you wondered where it came from or how it was made. Look around. Rocks are everywhere. This is not surprising. We live on the crust of Earth, which is made up entirely of rock. Some of these rocks have been around for billions of years. Most arent quite that old. New rocks are slowly formed all the time. Old rocks are slowly worn away by erosion. It does not matter how old a rock is. They were all formed in some way.

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2 There are three categories of rocks. They are based on the way the rocks were formed. Volcanic action results in igneous rock. Layer upon layer of deposited materials result in sedimentary rock. Extreme heat and pressure will cause some rocks to change into metamorphic rock. These different processes give each rock type a different look. This lets us classify rocks into one of the three categories.

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3 Deep under Earth’s crust, the temperature is hot enough to melt rocks. This molten rock is called magma. Magma in the crust slowly cools and hardens, and forms a rock. Sometimes volcanic action brings magma through the crust. When this happens, the molten rock is no longer called magma. On the surface, it is called lava. As the lava flows, it also cools and hardens. Rocks that are formed by magma or lava are igneous rocks. This is the first category of rock. Granite is an example of an igneous rock.

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4 Sedimentary rock is the second category. These rocks cover about three quarters of Earth’s land surface. Sedimentary rocks are made from pieces of other rocks and shells. Rain, wind, freezing, and plant roots can all cause erosion of rocks. Erosion causes rocks to be slowly worn away. These small pieces of rock are now called sediment. They are eventually carried by rivers to the oceans and lakes. As the flowing water slows down, it drops the sediment. The sediment settles to the bottom, forming layers. Over millions of years, more and more layers of sand and mud are deposited. The weight from the upper layers turns the lower layers of sediment into rock. This makes some sedimentary rocks look layered. Also, there are often fossils in sedimentary rocks.

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5 Metamorphic rocks are the third and final category. Metamorphic rocks are the least common rocks on Earth. These rocks were once an igneous, sedimentary, or even a metamorphic rock. They were changed by extreme pressure and heat while deep inside Earth. Enough heat and pressure will change a rock’s look, structure, and composition, which changes it into a different rock. Granite is an igneous rock. Granite will become metamorphic gneiss when it is placed under heat and pressure. Limestone is a sedimentary rock. Limestone will become metamorphic marble when it is placed under heat and pressure. Think of metamorphic rocks as rocks that have changed.

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6 Take a good look the next time you pick up a rock. Think about how it could have been formed. Was it formed by molten magma cooling and hardening? Was it formed by layers of sediments pressing down on each other over time? Was it changed by heat and pressure into an entirely different rock? On Earth, there are only these three types of rocks. Each rock is either igneous, sedimentary, or metamorphic.

Multiple Choice

As a volcano erupts, it produces lava. As this lava flows, it cools down and hardens, forming a rock. How could you classify this rock?

• As molten magma

• As a sedimentary rock

• As a metamorphic rock

• As an igneous rock

Multiple Choice

Sometimes fossils can be found in sedimentary rocks. Which statement best explains why?

• Heat and pressure from deep within Earth cause fossils to form.

• The remains of dead plants and animals settle to the bottom of an ocean or lake and are covered by mud. Pressure eventually forms these layers into rock.

• Lava may flow over an animal or plant, and as it cools and hardens, a fossilized rock is formed.

Multiple Choice

A metamorphic rock can also be thought of as a rock that changes. What causes the rock to change?

• Layers of sediment building up over time and pressing down, forming rock.

• Rain, wind, freezing, freezing, thawing, and plant roots growing cause the rock to erode away, leaving behind sediments.

• Extreme heat and pressure from deep within Earth cause changes to the rock’s s appearance, structure, and composition.

Multiple Choice

Based on the context in paragraph 4, fossil refers to _____.

• how igneous rocks were formed over time

• the remains of dead plants and animals settle to the bottom of an ocean or lake and are covered by mud

• a trace or print, or the remains of a plant or animal of a past age preserved in earth or rock.