Why does the Earth have tectonic plates?
Quick Answer
Earth’s outer shell cracked into plates because the hot mantle below slowly convects, and the plates are rigid enough to move as distinct units on top of it.
The Full Story
Earth’s interior is hot enough that the mantle, while solid, flows very slowly over millions of years in convection cells. The rigid outer layer, or lithosphere, is cool enough to be brittle and has broken into about 15 major plates and many smaller ones. As the mantle convects, it drags plates along and causes new crust to form at mid-ocean ridges where plates spread apart, while old crust is pulled down at subduction zones. This recycling system drives earthquakes, volcanoes and mountain building. Venus and Mars, by comparison, lack active plate tectonics, possibly because their lithospheres are too thick or their interiors too cool.
Key Facts
YouTube Angle
Suggested video title for this topic:
"Why Earth’s crust is cracked — the story of tectonic plates"