Basically, the Earth is a very big ball flying around the Sun, which is about 150,000,000 kilometers away. The furthest a human being has travelled from Earth is about 400,000 kilometers. However, we go around the Sun once a year anyway without a space rocket, so we are flying pretty fast! And you are doing it now!!
The center of the Earth is about 6,300 kilometers beneath you. However, the deepest we have ever drilled is only about 12 kilometers; we stopped drilling because it was starting to get too hot! We do know, however, that the outer shell of this big ball (the crust and surface parts), under your feet, is broken up into different plates. These plates are referred to as tectonic plates. Thin ones, usually over oceans, can be as thin as 15 kilometers in depth; thick ones, usually over land, can be as thick as 200 kilometers.
You can imagine these plates as a bit like the panels that compose an old-style leather football. Here’s a picture of the ball that was used in the 1936 Summer Olympics Football Tournament. Before they played the match, air was pumped into the ball so the players could get on with the game. Normally, the ball doesn’t burst or anything strange like that.
Unlike a football, the panels of which are stitched together, the Earth’s tectonic plates move around quite a bit. These movements have created our mountain ranges, earthquakes, volcanoes, and many of our islands. This movement can be a problem because, as you go down towards the center of the Earth, pressure and temperature builds up in absolutely unbelievable ways. Scholars think that, at really extreme depths, the pressure is so great that ordinary carbon is transformed into diamonds! If you could figure out a way to go down there, you could get really rich. Temperatures are so high that everything melts below the tectonic plates — until you reach the inner core; it is believed that the inner core is solid because of extreme pressures.
So, under the tectonic plates, there’s a lot of hot stuff and the pressure is very high. As the plates move around, hot molten rock called magma can be forced up through the surface. When the pressure is high enough, this can result in an explosion that creates a mountain. These explosions may throw up rock, ash, and so on. When molten rock emerges from a volcano, and when it later cools and solidifies, it is referred to as lava.
Mount Fuji is a stratovolcano. Stratovolcanoes generally have steeper sides formed as the result of occasional explosive eruptions.
Check out the slides here.