This is an image of the ocean floor of the Earth, showing mountain ranges, subduction trenches, tectonic plates, and mid-ocean ridges.
Click on image for full size
Image from: U.S. Geological Survey
A Planet or a moon cools off slowly in time, just like the sidewalk cools off in the evening after a sunny day. The warmth contained inside a planet or moon controls what sort of activity the planet or moon has in the atmosphere or surface. As planetary bodies cool slowly, heat diminishes, and the activities diminish to nothing. Examination of a planet or moon for various kinds of activities tells scientists what stage a body is in it's history of cooling off.
Planets or moons get energy from
Energy from item (2) in this list causes the continents to drift
slowly around the surface of the Earth.
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