Volcanic Eruptions: Low Silica
Magma is less dense than the surrounding rock and rises up to the surface through cracks and crevices under the Earth's surface causing an eruption of lava.
The silica content of the magma determines how thick the magma is, how easily it flows and how easily dissolved gases within the magma can escape; therefore the silica content determines the characteristic of the volcanic eruption and the shape and size of the resulting volcanic cone.
Magma with low silica content is very fluid and easily erupts onto the surface through cracks and crevices at the surface. Over time the layers of lava build up a volcanic cone. Low silica volcanoes are characterized by "quiet" eruptions. Lava is able to easily flow out of the ground. These eruptions produce lots of lava, but very little, if any, ash. Gas readily escapes
from the lava. Over time low silica eruptions build a large broad cone
A typical low silica eruption produces large amounts of lava.
Tectonic Setting for Low Silica Eruptions include Spreading Centers such as the Mid-Atlantic Ridge or African Rift Zone and Aseismic Ridges or Hot Spots such as the Hawaiian Islands.
If, however, the lava is very low in silica, the lava is not thick enough to build a cone at all and it will flow out over the Earth's surface much like a flood. The byproducts of this flow are commonly referred to as flood basalts.
Between 17 million and 6 million years ago the Columbia River flood basalts covered part of the surface of Washington, Idaho and Oregon to a depth of 9,000 ft.
Volcanic Eruptions: High Silica
Lava with high silica content is thick and viscous and does not readily flow. Lava rises up toward the surface but is too thick to squeeze through the cracks and fissures in the Earth.
As lava continues to rise upward, pressure continues to build. Gas contained in the lava is not released because it is "trapped" in the viscous fluid. Pressure will continue to build under the volcano until it exceeds the strength of the overlying surface material. At this point the lava will explode outward much as champagne from a bottle when the cork is released.
These types of eruptions are very destructive and cause many deaths. The tectonic setting that creates explosive eruptions is a subduction zone. Recent explosive eruptions include: Mt. St. Helens, Mt. Pinatubo and Monserrat. The 4 drawings below illustrate how a high silica eruption occurs.