Magma and Plate Tectonics

The first question this raises is: what exactly is this "material from the inside"? On our planet, it's magma, fluid molten rock. This material is partially liquid, partially solid and partially gaseous. To understand where it comes from, we need to consider the structure of planet Earth.


Graphic courtesy USGS

The earth is composed of many layers, roughly divided into three mega-layers: the core, the mantle and the outer crust:

  • We all live on the rigid outer crust, which is 3 to 6 miles (5 to 10 km) thick under the oceans and 20 to 44 miles (32 to 70)thick under the land. This may seem fairly thick to us, but compared to the rest of the planet, it's very thin -- like the outer skin on an apple.
  • Directly under the outer crust is the mantle, the largest layer of the earth. The mantle is extremely hot, but for the most part, it stays in solid form because the pressure deep inside the planet is so great that the material can't melt. In certain circumstances, however, the mantle material does melt, forming magma that makes its way through the outer crust.

In the 1960s, scientists developed a revolutionary theory called plate tectonics. Plate tectonics holds that the lithosphere, a layer of rigid material composed of the outer crust and the very top of the mantle, is divided into seven large plates and several more smaller plates. These plates drift very slowly over the mantle below, which is lubricated by a soft layer called the asthenosphere. The activity at the boundary between some of these plates is the primary catalyst for magma production.


Graphic courtesy NASA
The blue lines mark plate boundaries, the red triangles mark active volcanoes and the yellow dots show recent earthquakes.

Where the different plates meet, they typically interact in one of four ways:

  • If the two plates are moving away from each other, an ocean ridge or continental ridge forms, depending on whether the plates meet under the ocean or on land. As the two plates separate, the mantle rock from the asthenosphere layer below flows up into the void between the plates. Because the pressure is not as great at this level, the mantle rock will melt, forming magma. As the magma flows out, it cools, hardening to form new crust. This fills in the gap created by the plates diverging. This sort of magma production is called spreading center volcanism.

  • At the point where two plates collide, one plate may be pushed under the other plate, so that it sinks into the mantle. This process, called subduction, typically forms a trench, a very deep ditch, usually in the ocean floor. As the rigid lithosphere pushes down into the hot, high-pressure mantle, it heats up. Many scientists believe that the sinking lithosphere layer can't melt at this depth, but that the heat and pressure forces the water (the surface water and water from hydrated minerals) out of the plate and into the mantle layer above. The increased water content lowers the melting point of the mantle rock in this wedge, causing it to melt into magma. This sort of magma production is called subduction zone volcanism.

  • If the plates collide and neither plate can subduct under the other, the crust material will just "crumple," pushing up mountains. This process does not produce volcanoes. This kind of boundary can develop later into a subduction zone.

  • Some plates move against each other rather than push or pull apart. These transform plate boundaries rarely produce volcanic activity.
Click here for a great diagram of plate boundaries.