Plate Tectonics — Reading Comprehension Lesson

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Plate Tectonics

Earth’s outer shell, called the lithosphere, is broken into a mosaic of large pieces called tectonic plates. There are about 12 major plates and several smaller ones, and they are constantly moving — very slowly, at rates of about one to four inches per year, roughly the speed at which human fingernails grow. This movement is driven by convection currents in the hot, semi-molten rock of the mantle beneath the plates. The theory of plate tectonics, developed in the 1960s, revolutionized earth science by providing a single framework that explains earthquakes, volcanoes, mountain building, and the distribution of fossils across continents.

The idea that continents move was first proposed by German scientist Alfred Wegener in 1912. He noticed that the coastlines of South America and Africa fit together like puzzle pieces, and that the same fossils and rock types were found on both continents. He proposed the theory of continental drift — that all continents were once joined in a single supercontinent he called Pangaea, which began breaking apart about 200 million years ago. Although his evidence was compelling, scientists rejected his idea for decades because he could not explain what force could move continents. The discovery of seafloor spreading in the 1960s provided the missing mechanism.

Tectonic plates interact at their boundaries in three ways. At divergent boundaries, plates move apart. Magma rises from the mantle to fill the gap, creating new oceanic crust and mid-ocean ridges. The Mid-Atlantic Ridge is a divergent boundary where the North American and Eurasian plates are moving apart. At convergent boundaries, plates move toward each other. If one plate is oceanic, it sinks beneath the other in a process called subduction, forming ocean trenches and volcanic arcs. If both plates carry continental crust, they collide and crumple upward to form mountain ranges like the Himalayas. At transform boundaries, plates slide horizontally past each other, causing earthquakes along faults like the San Andreas Fault in California.

Plate tectonics has shaped Earth throughout its history and continues to do so today. The arrangement of continents affects ocean circulation and climate. Mountain ranges influence weather patterns. Volcanic eruptions deposit nutrients in soils and release gases into the atmosphere. Plate tectonics also concentrates valuable mineral deposits — many ore deposits form at plate boundaries where magma intrudes into the crust. Scientists use GPS satellites to measure plate movement with millimeter precision, seismographs to map plate boundaries, and ocean drilling to study seafloor sediments. Understanding plate tectonics is essential for assessing earthquake and volcanic hazards and understanding how our planet works.

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