What is a plate?Often on this page you’ll hear me talking about plate motion, plate tectonics, or pl

What is a plate?Often on this page you’ll hear me talking about plate motion, plate tectonics, or plate collisions as processes that form mountains and reshape the Earth’s surface. But, what exactly is a plate, to a geologist?A plate is a portion of the Earth’s outer layer that is rigid and can break into pieces. This is a cross sectional view of how these rigid plates might move. First thing to realize about almost all images like this – the Earth is mostly solid, it’s not molten. The Earth’s mantle is hot, over 1200°C. At that temperature, the rocks are soft enough to slowly flow and change shape, even though they’re mostly solid.That’s the biggest difference between a plate and the rocks below; the plate is cold. It’s so cold that it can’t flow over geologic times; it behaves rigidly, bending and breaking like any other solid; this image even shows small faults where it is being bent. The portion of the planet that is cold enough to be part of a rigid plate depends on how old the plate is; the oldest ones are stable and probably more than 100 kilometers thick, the youngest ones in the ocean are only about 10 kilometers thick. Over time, as the plate gets older, heat diffuses out through the surface, cooling it to greater depths.Those are the key objects in plate tectonics. Rigid, cold objects that move on top of warmer material – still solid, but able to flow. When we talk about one plate subducting beneath another, it’s the cold layer at the surface being pulled down into the planet. When we talk about plates colliding, it’s these cold surface layers running into each other, folding and faulting.Plate tectonics is the main way that the Earth removes heat. The crust at the surface loses heat out to space over time, and when that material sinks back into the planet, it slightly cools off the inside. When plates move apart, they trigger volcanism and upwelling that brings hot material to the surface where it melts and erupts, again removing heat.Interestingly, Earth seems to be the only planet in the solar system with anything like our current arrangement of plates. Mars has large fractures, large volcanoes, and the remnants of large craters, but the surface of Mars is mostly stagnant, pieces do not sink into the planet. Venus has a young surface, including active volcanism, but no evidence of large-scale crustal shifting. Mercury’s surface is old, covered by craters and volcanic rocks, but again no evidence that the crust broke into pieces.Scientists are still working to understand the situation that created plate tectonics and plate motion on Earth but not on other rocky worlds. Some have suggested the first plates were broken by asteroid impacts, others have suggested that plate tectonics can occur naturally under the right conditions. Geologists aren’t even certain about when plate tectonics started on the Earth; many of us think it has gone on for more than 3 billion years, but beyond that time we have so few intact rocks that it’s hard to know whether what was happening was exactly plate-like behavior or not.-JBBImage credit:https://en.wikipedia.org/wiki/Oceanic_trench#/media/File:Subduction_Trench_Schematic.jpg -- source link
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