Subduction zones usually have a deep trench along the top.tectonic plates The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.upwelling The process by which material rises from Earth’s middle layer into its outer layer, where it will become part of the tectonic plates.volcano A place on Earth’s crust that opens, allowing magma and gases to spew out from underground reservoirs of molten material.
The magma rises through a system of pipes or channels, sometimes spending time in chambers where it bubbles with gas and undergoes chemical transformations.
You might think of them as the cracked eggshell jacketing a hard-boiled egg.
Like an eggshell, plates are relatively thin — on average only about 80 kilometers (50 miles) thick.
Over millions of years, though, those centimeters add up. For instance, roughly 250 million years ago, Earth had one giant landmass: Pangaea.
Plate movement split Pangaea into two huge continents, called Laurasia and Gondwanaland.
So, hot stuff in Earth’s middle “rises up — kind of like a lava lamp,” he explains.
“Once it gets back to the surface and cools off again, then it will sink back down.”The rising of hot rock from the mantle to Earth’s surface is called upwelling. Over time, the cooling outer crust becomes thicker and heavier.
The display case holds rocks collected by the Apollo 15 and 16 missions.
The 1.5-kilogram (3.3-pound) chunk of volcanic basalt in the center is 3.4 billion years old.