Volcanoes — Reading Comprehension Lesson

Reading Passage

Volcanoes

Volcanoes are openings in Earth’s crust through which molten rock, ash, and gases escape from deep within the planet. They form at two main types of locations: at the boundaries of tectonic plates, where plates collide or pull apart, and over “hot spots” — unusually hot areas in the mantle where magma plumes rise toward the surface. The Ring of Fire, a horseshoe-shaped zone around the Pacific Ocean, contains about 75 percent of the world’s active volcanoes and is the site of most of the world’s major earthquakes.

Magma is molten rock beneath Earth’s surface. When it erupts from a volcano, it is called lava. Not all volcanoes look or behave the same — their shape and eruptive style depend primarily on the composition of their magma. Shield volcanoes, like those in Hawaii, have gently sloping sides built up by many flows of low-viscosity lava that spreads far and wide. They erupt frequently but relatively quietly. Composite volcanoes, also called stratovolcanoes, like Mount St. Helens and Mount Fuji, are steep-sided, cone-shaped mountains built from alternating layers of lava and ash. They can remain quiet for centuries and then explode violently.

A volcanic eruption can take many forms. Effusive eruptions produce steady flows of lava that move slowly enough for people to observe and sometimes outrun. Explosive eruptions are far more dangerous — they blast fragments of rock and magma (called tephra or pyroclasts) into the air, produce pyroclastic flows (fast-moving avalanches of superheated gas, ash, and rock that travel at hundreds of miles per hour), and can send ash clouds into the stratosphere that affect climate worldwide. The 1815 eruption of Mount Tambora in Indonesia was so powerful it caused crop failures across the Northern Hemisphere the following year, known as the “Year Without a Summer.”

Despite their destructive power, volcanoes play vital roles in Earth’s systems. Volcanic activity over billions of years released the gases that formed Earth’s early atmosphere and oceans. Volcanic soil is among the most fertile on Earth, which is why densely populated regions often sit near active volcanoes. Volcanic islands like Hawaii and Iceland were built entirely by volcanic activity rising from the ocean floor. Today, scientists use seismographs, GPS, satellite imagery, and gas sensors to monitor volcanoes and provide early warning of eruptions, saving many lives that would otherwise be lost.

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