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The Theory of Plate Tectonics
The Theory of Plate Tectonics
The theory of plate tectonics is one of the most important ideas in Earth science. It explains how Earth’s outer shell, called the lithosphere, is broken into large pieces known as tectonic plates. These plates fit together like a giant jigsaw puzzle and float on top of the softer, slowly flowing layer beneath them called the asthenosphere.
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Continental vs Oceanic Lithosphere
Continental vs Oceanic Lithosphere
Earth’s lithosphere is the rigid outer layer of the planet. It includes the crust (the very top) and the uppermost mantle beneath it. But not all lithosphere is the same—there are two main types: continental lithosphere and oceanic lithosphere.
Continental Lithosphere
Continental Lithosphere
Continental lithosphere makes up the landmasses we live on. It is:
Thicker (about 30–70 km on average, and even thicker under mountains).
Less dense (made mostly of granite-like rocks).
Older (some parts are billions of years old).
Oceanic Lithosphere
Oceanic Lithosphere
Oceanic lithosphere forms the ocean floor. It is:
Thinner (about 5–10 km thick).
More dense (made mostly of basalt, a heavier rock).
Younger (rarely more than 200 million years old, because it is constantly being recycled at plate boundaries).
In short: continental lithosphere is thick, light, and old, while oceanic lithosphere is thin, heavy, and young—and their interactions shape Earth’s surface.
The lithospheric plates are always moving, though usually only a few centimeters per year—about the same speed your fingernails grow. As they move, they interact in different ways.
Convection Currents
Convection Currents
Have you ever seen a lava lamp? The light bulb at the bottom of the lamp warms the wax, making it less dense so it rises through the liquid. When the wax reaches the top, it cools, becomes denser, and sinks back down. This cycle repeats, creating a slow, up and down motion in the lamp. Something similar happens inside the Earth, but much more slowly—this process is called convection currents.
Deep inside Earth, heat from the hot core warms the lower mantle. As mantle rock heats up, it becomes less dense and rises toward the surface. When it cools, it becomes denser and sinks back down. This continuous cycle of rising and sinking rock forms convection currents in the mantle. These currents are powerful enough to move the lithosphere.
Convection currents are Earth’s “engine.” They carry heat from the interior of the Earth to the surface and act like giant conveyor belts that slowly but steadily move the tectonic plates.
Plate tectonics helps explain many of Earth’s surface features: mountain ranges, deep ocean trenches, earthquakes, and volcanoes. It also shows that Earth’s surface is dynamic and constantly changing, not fixed and unchanging.
In short, plate tectonics is the unifying theory that connects geology, geography, and Earth’s history—it shows us that our planet is active, alive, and always reshaping itself.
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