Less dense material, also containing bits of fresh magma and ash moves even faster and can rise thousands of feet in the air (Zeilinga de Boer, 6).
Plate Techtonics.
Volcanic activity is from the movement of tectonic plates that move at a rate of several centimeters per year (Zeilinga de Boer, 8). The rigid outer portion of the Earth called the lithosphere is broken into several different plates, which are in constant motion relative to each other. The hot mobile layer of partially molten rock, from which magma is derived, is called the asthenosphere (Camp). The movement of the plates is likely because of the currents of within the mantle. These currents are driven by the heat from the earth's core. .
Volcanism is typically widespread along plate boundaries, where plates are divergent (spreading), convergent (coming together) and transforming (moving horizontally). Divergent boundaries generally produce non-explosive eruptions, where magma is filling the separation of the plates or feeding lava flows (Encarta). Most of these boundaries are within the oceans basin and can account for 75 to 80 percent of the magma that rises to the earth's surface (Zeilinga de Boer, 14). In convergent boundaries, there is subduction of the plates, where one plate will slide under another. The subducted plate will ascend into the asthenosphere, where it begins to melt into magma. These boundaries are often the zones of earthquakes and explosive eruptions.
Between 100 and 150 kilometers is where magma is generated. Blobs of magma rise through the asthenosphere, until they reach the lithosphere. As magma accumulates in the lithosphere eventually it generates enough pressure to arch it, forming pockets called magma chambers (Zeilinga de Boer, 10). The chambers continue to expand and as the magma resides in the chamber it reacts with the surrounding rock changing the chemistry and it becomes more viscous or resistant to flow.