Moving Plates — Reading Comprehension Lesson

Reading Passage

Moving Plates

Earth’s lithosphere — the rigid outer shell consisting of the crust and uppermost mantle — is broken into about 12 major tectonic plates and several smaller ones. These plates are constantly moving, carried along by convection currents in the hot, semi-solid mantle beneath them. The movement is slow — about one to four inches per year — but over millions of years it adds up to enormous distances. The continents have moved thousands of miles from their original positions over Earth’s 4.6 billion year history, assembling and breaking apart multiple supercontinents in the process.

Where tectonic plates move apart, rift zones form. Magma rises from the mantle to fill the gap, solidifying into new oceanic crust in a process called seafloor spreading. The Mid-Atlantic Ridge is an active rift zone running down the center of the Atlantic Ocean; Iceland sits on top of this ridge and is literally being pulled apart. As plates rift apart, they may eventually separate a continent into two pieces. The East African Rift Valley is an example of a continent in the early stages of rifting — millions of years from now, eastern Africa may separate from the rest of the continent, creating a new ocean.

Where an oceanic plate collides with a continental plate, the denser oceanic plate is forced downward beneath the lighter continental plate in a process called subduction. Subduction zones are sites of intense geological activity: the descending plate heats up and releases water into the overlying mantle, triggering melting and volcanism. Chains of volcanoes called volcanic arcs form above subduction zones — the Cascade Range in the Pacific Northwest, including Mount St. Helens and Mount Rainier, is a volcanic arc formed above the subducting Juan de Fuca Plate. The Ring of Fire — a horseshoe of volcanoes and earthquake zones encircling the Pacific Ocean — marks where the Pacific Plate is being subducted.

When two continental plates collide, neither is dense enough to subduct, so both are crumpled and pushed upward into mountain ranges. The Himalayas — home to Mount Everest, the world’s tallest peak — formed when the Indian subcontinent crashed into Asia about 50 million years ago, and they are still rising today. Scientists have multiple lines of evidence for plate movement: GPS satellites can directly measure the slow drift of continents; the ages of seafloor rocks increase symmetrically away from mid-ocean ridges; magnetic stripes in the seafloor record reversals of Earth’s magnetic field and show how the seafloor has spread; and the same fossil species are found on continents that are now far apart.

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