Earth’s Layers — Reading Comprehension Lesson

Reading Passage

Earth’s Layers

Earth is not a solid, uniform ball of rock. It is composed of distinct layers, each with different composition, temperature, and physical properties. Scientists have identified four main layers: the crust, the mantle, the outer core, and the inner core. Although no one has ever drilled or traveled to these deep layers, scientists have learned about Earth’s interior by studying how seismic waves — vibrations produced by earthquakes — travel through the planet.

The crust is Earth’s outermost and thinnest layer. Oceanic crust, which lies beneath the ocean floors, is about 3 to 5 miles thick and made primarily of a dense, dark rock called basalt. Continental crust, which forms the continents, is thicker — typically 20 to 30 miles — and made of lighter rock called granite. The crust sits on top of the mantle; the boundary between them is called the Mohorovicic discontinuity, or Moho.

The mantle is Earth’s thickest layer, extending from the base of the crust to about 1,800 miles deep. It makes up about 84 percent of Earth’s total volume and is composed of silicate rock rich in iron and magnesium. Although the mantle is solid, it behaves like a very slow-moving fluid over millions of years, flowing in giant convection currents driven by heat from the core. These currents drive the movement of tectonic plates, causing earthquakes and volcanoes at plate boundaries.

At the center of Earth lies the core, which is composed primarily of iron and nickel. The outer core, from about 1,800 to 3,200 miles deep, is liquid — hot enough that iron remains molten. The movement of liquid iron in the outer core generates Earth’s magnetic field, which shields the planet from harmful solar radiation and allows compasses to work. The inner core, the very center of Earth, is solid despite being hotter than the surface of the Sun, because the enormous pressure at that depth keeps it from melting.

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