What is volcanoes ?

Volcanoes are conduits in the Earth’s crust through which gas-enriched, molten silicate rock-magma-reaches the surface from beneath the crust.

The melting of the rock generates magma. Rock may melt by (1) raising its temperature, (2) lowering the pressure on it, or (3) increasing its water content.

There are three types of magma: basalt, andesite, and Rhyolite

The fliuidity of a liquid is measured by its viscosity, its internal resistance to flow. The lower the viscosity of a magma, the more fluid is its behavior. The viscosity of magma increases with the content of silica. Basalt has highest temperature which causes atoms to spread farther apart, thus decreasing density and increasing fluidity, so more of it reaches the surface (80%) while Rhyolite is more viscous and they tend to be traped deep below the surface. In addition, gases in basalt is relatively easy to escape and therefore the basaltic volcano eruption is relatively peaceful, whereas the gas is very difficult of escape from the Rhyolite volcano eruption, and also the magma is very sticky and resistant to flow because of high viscosity. So how the entrapped gas escapes from the sticky magma ? explosion.

As volcanoes erupt, the magma rises toward the surface, the decrease in pressure frees more dissolved gases. If gases escape easily, the magma will flow on the surface as lava. Otherwise, if gas is trapped and held in the magma, it may blasted into the air as pyroclastic debris (pyro = fire, clastic = fragments).

As we discussed before, Lava flows are typical of basaltic magma eruption. There are three types of basaltic lava: pahoehoe, aa, and block lava.

Pahoeheo is basaltic lava which solidifies at the top, but which is still fed from below by pipe vesicles. Such lava has a smooth, wrinkled surface, is less than 15 m thick and flows at rates of a few meters per minute.

Aa forms as a thin river of lava, less than 10 m thick, with a spiky, cinder-like topping.

Block lava consists solely of Andesite or rhyolite up to 300 m thick, and has a surface coating consisting of smooth-sided fragments. It has the slowest travelling speed of all lava flows, moving only a few meters per day.

If gases escape very difficultly, volcanoes explosion may happen. The materials blast into air can be varied with a wide range in size: from dust to huge blocks and bombs.

The total energy of a volcanic event can be broken into four categories:

1. energy released by volcanic tremors and earthquakes;
2. energy necessary to fracture the overburden;
3. energy expended in ejecting materials
4. energy expended in producing atmospheric shock waves and , occasionally, tsunami.

based on the behavior of volcano eruption, the eruptive styles of volcanoes can be classified into several types:

Submarine eruption: Pillow lava

Icelandic-type: it is the most peaceful eruption. Develop plateaus of nearly horizontal volcanic rock layers.

Hawaiian Type: it is quite similar to icelandic-type. Also quite peaceful eruption. Shield volcano

Stromobolian type: named after Mount Stromboli in Italy. the magma materials are basal to andesite with low-to-moderate viscosity, develop cones landform.

Vulcanian-type: vulcanian-type eruptions alterate between thick,highly viscous lavas and masses of pyroclastic materials blown out of the volcano. Vulcanian-type eruptions are also common first phasees in the eruptions of other volcanoes as they "clear their throats" before emitting larger eruptions.

Vesuvian-type: it is more violent blasts of moderate to high-viscosity magma loaded with trapped gases, the landforms developed are volcanic cone and stratovolcanoes. Stratovolcanoes are steep-sided, symmetrical volcanic peaks built of alternating layers of erupted debris capped by high-viscosty lava flows that solidify to form protective caps. Like Mt. Fuji in Japan is stratovolvanoes.

Pelean type: it is the most violet eruption. The magma erupted is basically Rhyolite with very high viscosity.

Volcanoes are distributed along (1) the oceanic spreading centers; (2) subduction zones and (3) hot spots.

There are many hazardous phenomenon produced directly, or as secondary effects by volcanic eruptions, which include:

1. Earthquake, like the earthquakes occurring in Hawaii, which are associated with volcano eruptions.
2. Tsunami

Tsunami are water wave phenomena generated by the shock wave associated with volcano eruptions or earthquakes or submarine landslides

3. Lava flow. Because the lava flow areas in volcanic areas are very fertile, and have attracted intense agricultural usage and dense settlement. So, re-eruption of volcanoes in these areas has, thus, led to the destruction of this agricultural land and loss of life. Much of this destruction can be avoided simply by mapping and avoiding those areas most frequently inundated by lava flows.
4. Ballistics and ash clouds
5. Pyroclastic flows and base surges.

Pyroclastic flow is an avalanche of an exceedingly dense mass of hot, highly gas-charged and constantly gas-emitting fragmental lava, much of it finely divided, extraordinarily mobile, and practically frictionless, because each particle is separated from its neighbors by a cushion of compressed gas.

6. Gases and acid rains
7. Lahars
8. Lahars are mud flow which may occur at the time of the eruption (primary lahars) or several year afterwards (secondary lahars). Primary lahars can be generated by pyroclastic flows.
9. Glacier bursts

1. Long-term predictions

2. Short-term predictions

The precursors for volcanoes group into five categories:

Land deformation, seisactivity, geomagnetic and geo-electric effects, temperature and gases.