Types of Eruption

Apart from hotspots over mantle plumes, volcanoes frequent crustal plate boundaries, making a “ring of fire” around the Pacific. The ocean ridge system is a chain of submarine volcanoes.  Where it breaks the surface, in Iceland for example, it coincides with a mantle plume. The volcanoes of the Rift Valley of east Africa represent a new ocean trying to open. Where ocean crust dives beneath a continent, it takes with it water locked in minerals. As the rocks heat and melt, the wet magma rises like uncorked champagne to produce some violent eruptions.

Mt. St. Helens in Washington State is one of many volcanoes above the sub ducting Pacific plate. The wet magma ascends periodically like a pressure cooker letting off steam. Up to May 1980 geologists had monitored 10,000 small earthquakes in the region and had used lasers to measure the growing bulge on the mountains north flank. By May 12 parts of the bulge were 138 m higher than before and very unstable suddenly on May 18, the entire north flank collapsed in three great landslides only seconds apart. The second exposed pressurized molten magma which erupted in a tremendous lateral blast, flattening trees up to 30 Km away, the third block to slide exposed the top of the magma column itself, which erupted upwards sending ash more than 19Km high and coating 50,00Km² with 540 million tons of ash.

Volcanoes

Although the earth’s mantle is solid, it can still flow slowly in the same way as a glacier does. Plumes of hot mantle rock rise and as they do so the pressure drops and some of the minerals begin to melt. Not everything melts, so the composition of the melt or magma is different from the bulk composition of the mantle. If the angles between the remaining grains (the dihedral angles) are big enough, the magma can flow out and upwards, accumulating in large volumes called magma chambers. The nature of the subsequent volcanic eruptions depends on the source and chemistry of the magma.

The ratio different helium isotopes in bubbles of gas contained in the mantle plume suggests that some of them come from great depth, possibly the base if the lower mantle. These produce the vast basalt flows on which the Hawaiian Islands are built. In the past they have produced even bigger eruptions. Sixty five million years ago millions of cubic kilometers of basalt erupted over what is now western India. The effects on climate of large amounts of volcanic gases are considerable, and may have caused the demise of the dinosaur. Such shield volcanoes produce copious quantities of runny, alkaline lava which spreads over a wide area. Acidic lavas are more viscous and produce more explosive eruptions and ash clouds, particularly if they contain a lot of water or dissolved gas.

Earth Quakes & Earthquake Zones

The continents slowly slip past one another, rising, falling, straining and distorting as they go.  Their passage is seldom smooth.  With little lubrication, strain builds up until the rock can withstand it no longer and the cracks. The resulting earth quake can release tremendous amounts of energy in the form of seismic waves that run through the ground, shaking buildings with destructive force. Movement along the fracture or fault line can extend for many meters, and can occur horizontally and vertically. Seismic seabed movement can generate seemingly harmless waves in deep water, but the waves travel at u to 800 Km/h. they are slowed down in shallow water but their amplitude increases so that they reach height of 50 meters. These tidal waves, or tsunamis, wreak great havoc on coastal areas.

Most earthquakes, like volcanoes, take place in specific zones that correspond to the boundaries of tectonic plates. Shallow earthquakes happen where one plate moves against another on the surface, as in California, and where displacement is horizontal. Deeper earth quakes occur where one plate is sliding beneath another, described as subduction zone. This is happening along the west coast of South America, where the ocean crust dives beneath the continent. Volcanoes occur in the same regions, as magma is produced at both constructive (where crust is being generated) and destructive (where it is disappearing) margins. The most violent eruptions occur at destructive margins, and  are known as Andesite volcanoes (after the Andes mountains).