The Earth’s crust is broken into pieces, known as tectonic plates. The two boundaries are the edges of tectonic plates. When two plates move apart, or when one plate goes below another, lava is pushed to the surface, creating the volcano.

Volcanic eruptions, mountain ranges and earthquakes are signs that the tectonic plates are moving. For example, the Atlantic ridge has lava coming out all the time because the American plates are moving away from the African and European plates. If you look at a map , you will see that Africa and South America look like they fit together; this is because they were originally next to each other and drifted apart.

Volcanic eruptions in mountain ranges (like the Andes) occur because one plate is going under another plate (subduction zone), which pushes lava to the surface. The lava is under a lot of pressure, which is why the eruptions are so impressive. Oceanic plates usually go under continental plates, because oceanic plates are denser.

You will also find mountain ranges near subduction zones (e.g. the Andes, the Rockies…) because the oceanic plate under pushes the continental plate upwards.

Earthquakes occur when two tectonic plates slide against each other. The plates are usually stationary, but when enough pressure builds up, they slide violently at areas called fault lines, causing earthquakes. Our major fault line in California is the San Andreas fault line.