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Go outside, grab a shovel, and start digging a hole. After a few meters, you'll discover that it's hard work. But, keep going. After your hole is one hundred meters deep, take a break and feel the earth. It will probably still feel cool, but not cold. Pick up that shovel again, and dig deeper. When your hole is one thousand meters (one kilometer) deep, the earth should start to feel warm. If you can keep digging, you'll find that the earth continues to feel warmer as you dig deeper.
Scientists know that the outer layer of the earth is made of solid rock and that it warms as you dig deeper. Actual evidence exists down to a depth of about eight kilometers (five miles). Mine shafts and holes in the surface of the earth have opened a window for scientists to see inside the earth. So, don't worry if you can't dig any deeper! No one ever has.
Based on many years of investigations, scientists now estimate that the crust of the earth, the solid outer layer, is about seventy kilometers (forty-four miles) deep at its thickest point. Some parts of the crust, like the floor of the oceans, may be as little as ten kilometers (six miles) deep.
Imagine wearing a concrete bodysuit all of the time. After a while, you'd feel extremely hot because of the tremendous pressure you'd feel on your body. If you think about it, the mantle wears a huge, heavy bodysuit that is about seventy kilometers thick. It's not too hard to believe, then, that there is extreme pressure and heat in the mantle. Most of the mantle, over 99%, stays solid, but the high temperature and the push and pull forces cause some of the mantle to melt. This molten rock is known as magma.
If you were wearing that concrete bodysuit, you'd probably fight to break free from it. You'd probably wiggle your fingers and stretch your toes to try and poke some small holes. The outer layer of the earth has some weak spots and cracks in it. When the magma in the mantle pushes finds those weak spots, it pokes through, forming a volcano. Once the magma bursts free it becomes lava. Magma and lava are basically the same substance. Both are melted rock. The different names help scientists know where the melted rock is located.
Big pieces of the crust, called plates, form along the cracks. The huge plates, like giant rafts, drift on the liquid magma in the mantle. Eventually, two plates will bump into each other and one plate's edge will slip beneath the edge of the other plate. That collision causes a subduction volcano to form. The plate that slips underneath suddenly feels the tremendous pressure of wearing the concrete bodysuit. Some of its rock will literally start to melt, forming more magma and more pressure. The result is a volcano.
Meanwhile, in another part of the crust, two plates can pull away from each other, opening a rift in the crust. Rift volcanoes allow magma to ooze up from the mantle along the rift. Someday, the two plates may shift their drift and push back together to close the rift.
It's also possible for volcanoes to form right in the middle of a crustal plate. Hot-spot volcanoes result from magma melting a hole in the plate and oozing upward. The Hawaiian Islands, for example, are all the result of hot-spot volcano formations. Hawaii is not located along an edge of a crustal plate. Rather, it is near the center of the giant Pacific crustal plate.
Hot-spot volcanoes, like the Hawaiian Islands, erupt often, but gently. Since the crust is thin and weak enough for the magma to melt through, the pressure is also weak. So the eruption of the volcano is less forceful. Only rarely do hot-spot volcanoes erupt with a forceful spray of rocks and cloud of ash. Instead, the magma reaches the surface and oozes out, forming rivers of lava or gentle fiery fountains of lava. The lava flows quickly and, as it cools, hardens to form volcanic rocks. All of the Hawaiian Islands, for example, are actually just huge volcanic rocks, produced by lava flowing, cooling, and hardening over millions of years.
Unlike hot-spot volcanoes, subduction volcanoes usually erupt with tremendous force. Lava bursts forth, literally shattering the crust of the earth into pieces of rock that fly through the air and rain down around the volcano. A huge cloud of ash, the product of intense heat reactions with the rock, rises into the sky. The thick cloud, like any cloud in the sky, blocks the sun and darkens the sky for many kilometers around the volcano.
In the United States, the most destructive eruption ever was when Mount Saint Helens, a volcano in Washington, exploded in 1980 on a Sunday morning. Its ash cloud rose nineteen kilometers (twelve miles) into the sky. Winds carried the cloud as far as 402 kilometers (250 miles) away, darkening the daytime sky.
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