Types of Rocks — Reading Comprehension

Rocks are the solid materials that make up the Earth's crust and tell the story of our planet's past. Each rock type forms through a unique set of processes, revealing information about the conditions under which it was created. Geologists use clues from rocks to answer key scientific questions, such as how Earth's surface changes over time and where important natural resources are found. Understanding the different types of rocks helps us build safer structures, locate valuable materials, and learn about Earth's history. This passage introduces the three main rock families and explains why identifying rock types matters.

Formation of the Three Rock Families

There are three major groups of rocks: igneous, sedimentary, and metamorphic rocks. Igneous rocks form when molten material known as magma or lava cools and solidifies. This process can happen deep underground or after a volcanic eruption at the surface. Granite, which often makes up the continents, is an example of a coarse-grained igneous rock. Basalt, which forms the ocean floor, is a fine-grained igneous rock. Sedimentary rocks are created when small pieces of other rocks, called sediments, are compressed and cemented together over time, usually in rivers, lakes, or oceans. Sandstone and limestone are common sedimentary rocks, each recording details about the environment where they formed. Metamorphic rocks begin as igneous or sedimentary rocks but change due to intense heat and pressure inside the Earth. This transformation, called metamorphism, can produce rocks like marble and schist, which often have distinctive bands or layers.

Rock Identification: Texture and Composition

Scientists identify rocks by examining their texture and composition. Texture describes the size, shape, and arrangement of a rock's grains or crystals. For example, granite has large, visible crystals, while basalt's crystals are too small to see without a microscope. Sedimentary rocks might show layers or contain visible fragments. Composition refers to the types of minerals present in a rock. Different minerals form under different conditions, so knowing what minerals are in a rock provides clues about its origin. This scientific approach—using observations and evidence—helps geologists classify rocks and understand Earth's processes in detail.

Why Knowing Rock Types Matters

Recognizing and understanding rock types has important real-world applications. For example, many building materials, such as granite and limestone, come from specific rock types because of their strength and durability. Certain rocks contain valuable resources like metals, oil, or groundwater. Studying rocks also reveals evidence of past environments, such as ancient oceans or mountain-building events. This information helps scientists reconstruct Earth's history and predict how the planet might change in the future. Identifying rock types is an essential skill in geology, engineering, environmental science, and even in everyday life.

In summary, the three main types of rocks—igneous, sedimentary, and metamorphic—are each formed by different processes and tell unique stories about Earth's past. By identifying rocks using texture and composition, scientists can unlock information about natural resources, environmental changes, and the dynamic systems that shape our planet.

Interesting Fact: Some metamorphic rocks, like gneiss, can contain minerals that formed more than 1 billion years ago, making them among the oldest materials on Earth.

Which process forms igneous rocks?

What is the main way sedimentary rocks form?

Metamorphic rocks begin as which type(s) of rock?

Which detail best explains why knowing rock types is useful?

What does the word 'texture' mean in the passage?

What is 'metamorphism' as used in the passage?

If you find a rock with large, visible crystals, what type of rock is it most likely to be?

How do scientists use rocks to learn about Earth's past?

True or False: Sedimentary rocks can contain visible fragments of other rocks.

True or False: All metamorphic rocks start as igneous rocks.