Rock Cycle — Reading Comprehension Lesson

Reading Passage

Rock Cycle

Rocks are not permanent. Over millions of years, rocks are continuously formed, broken down, and transformed into new types through a process called the rock cycle. There are three main types of rock: igneous, sedimentary, and metamorphic. Each type forms through different processes, and through the rock cycle any rock can eventually be transformed into any other type. The rock cycle is driven by Earth’s internal heat and by energy from the sun, which powers weathering, erosion, and the water cycle.

Igneous rocks form when magma cools and solidifies. If magma cools slowly deep underground, it forms coarse-grained rocks like granite, with large visible crystals that had time to grow. If lava cools quickly at the surface, it forms fine-grained rocks like basalt or glassy rocks like obsidian, with little or no crystal structure. Igneous rocks make up about 95 percent of Earth’s crust by volume. The word “igneous” comes from the Latin word for fire.

Sedimentary rocks form at or near Earth’s surface through the accumulation and cementation of sediment — fragments of other rocks, mineral grains, and the remains of organisms. Wind, water, and ice erode existing rocks, transport the particles, and deposit them in layers. Over time, the weight of overlying layers compresses the sediment and minerals dissolved in groundwater cement the particles together in a process called lithification. Sedimentary rocks often contain fossils because organisms can be buried and preserved in accumulating sediment. Sandstone, limestone, and shale are common sedimentary rocks.

Metamorphic rocks form when existing rocks are subjected to extreme heat and pressure deep within Earth — but not enough to melt them entirely. The heat and pressure cause minerals to recrystallize into new minerals and new textures without the rock becoming liquid. Marble is metamorphic limestone; slate is metamorphic shale; quartzite is metamorphic sandstone. All three rock types can be returned to magma if they are subducted deep enough into Earth’s mantle, completing the cycle. The same atoms have been cycling through Earth’s rock cycle for billions of years.

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