Volcanism. SOME BACKGROUND. The word Volcano comes from the little island of Vulcano in the Mediterranean Sea off the coast of Sicily. Centuries ago, the people living in this area believed that Vulcano was the chimney of the forge of Vulcan - the blacksmith of the Roman gods.
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These comprise the largest percent (60) of the Earth’s volcanoes.
They are typically steep sided and are built from alternating layers of lava and cinders.
These volcanoes can also be very explosive. Some of the world’s most majestic and beautiful mountains are this type of volcano. Mount Fuji in Japan and Mount St. Helens are composite volcanoes.
Cinder cones (Scoria Cones) are the simplest type of volcano.
They are built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as tephra or cinders around the vent to form a circular or oval cone.
These are the most explosive.
Shield volcanoes are the largest on Earth. The Hawaiian volcanoes are shield volcanoes. They are made up of the build up of lava flows over time.
They are not steep, their slopes are very gradual.
Their eruptions are not explosive
The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth’s surface.
When such a large volume of magma is removed for beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a Caldera.
Usually found at the top of a volcano – not always visible.
“A lava domeis a steep mass of very thick and pasty lava that is pushed up from the main vent.
The lava is so viscous (thick and pasty) that it does not flow but slowly rises higher with each movement of magma in the conduit.
Think of toothpaste that is slowly squeezed and then stopped and then squeezed again from the tube. This is how the lava dome in Mt. St. Helen's was formed.”
In some instances the magma never reaches the surface or breaks the ground above it. Huge reservoirs of magma are created to form ‘Batholiths’ or smaller reservoirs called ‘Laccoliths’. Both these are horizontal in structure. Vertical creations are called ‘Sills’. When these cool, they form intrusive igneous rock and are only discovered through erosion or mining.
The controlling factors on explosivity are viscosity, temperature, and the amount of dissolved gases in the magma.
Eruption of the Pu'u O'o volcano on the east zone of the larger Kilauea volcano, Hawaii.
Eruption of a voluminous plume of tephra is typical of explosive eruptions, as demonstrated in the 1980 eruption of Mt. St. Helens. These eruptive tephra plumes can travel tens of kilometers into the stratosphere.
How does the amount of dissolved gases affect the explosivity of volcanoes?
The dissolved gases in the magma provides the force for explosive eruptions.
As magma rises toward the surface, dissolved gases in the liquid rock begin to come out of solution (called exsolution), and bubbles begin to form in the magma. The magma gets “frothy”.
Not all magma has a lot of dissolved gases. The more the gas the more explosive the volcano.
Explosive eruptions, which get started by exsolution of gases, can be made more dramatic by sudden decompression, which lowers the confining pressure on the magma. Decompression is when the volcano blows its lid so to speak.
Decompression is similar to what can happen to a bottle of pop when the cap is removed: the sudden release of pressure can cause the CO2 to come out of solution explosively and rapidly, resulting in the pop to spray out of the bottle.
This happens when a volcanic mountain suddenly breaks open or the magma plug from a previous eruption gets pushed out of the way. Mt St. Helen’s is an example. An earthquake or tremour can open up the ground releasing this pressure and a violent eruption occurs.
How does viscosity affect explosivity?
Viscosity is defined as the ability of a substance to resist flow. In a sense, viscosity is the inverse of fluidity.
The higher the temperature, the more fluid a substance becomes, thus lowering its viscosity. The lava in this case is thin and runny and gases escape easily. Think of engine oil!
Magma may also contain a material that is crystal in structure called silca. Generally speaking, magma with a higher silica concentration has a higher viscosity. It resists flow up through the vent. The lava here is thick and sticky and gases cannot escape. Granitic rock is rich in silica.
Click here to read an excellent article on the study of why some volcanoes erupt violently.
The rapid eruption of expanding gases results in the obliteration and fragmentation of magma and rock. The greater the explosivity, the greater the amount of fragmentation.
Individual eruptive fragments are called pyroclasts ("fire fragments").
Tephra (Greek, for “ash”) is a term for any airborne pyroclastic accumulation.
If the explosivity of the magma is low, the liquid rock tends to flow out onto the surface as lava.
Every year about 60 volcanoes erupt, but most of the activity is pretty weak. How do volcanologists measure how big an eruption is? There is not any single feature that determines the "bigness", but the following eruption magnitude scale - called the Volcanic Explosivity Index or VEI - is based on a number of things that can be observed during an eruption. According to this scale, really huge eruptions don't happen very often, luckily!
This sedimentary rock has a different mineral composition (it is mainly granitic); it is less dense than the surrounding liquid rock (mainly basaltic). Therefore, after it melts in the mantle, it rises slowly toward the surface, breaking through the crust and erupting onto the surface.
Island arc formed by oceanic-oceanic subduction.
Volcanic arc formed by oceanic-continental subduction
This type of granitic magma has a high silica concentration and is therefore more viscous. It also tends to have high level of dissolved gases.
This accounts, therefore, for the more violent eruptions found near subduction zones, like the so-called Pacific Ring of Fire.
Mt. St. Helen’s, Krakatoa, Pompeii are examples of this type of volcanic activity.
Mantle plumes appear to be largely unaffected by plate motions. As lithospheric plates move across stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the mantle plume in the direction of plate movement.
Thus, a linear belt of inactive volcanic islands and seamounts will be produced. A classic example of this mechanism is demonstrated by the Hawaiian and Emperor seamount chains.
The "Big Island" of Hawaii lies above the mantle plume. It is the only island that is currently volcanically active. The seven Hawaiian Islands become progressively older to the northwest. The main phase of volcanism on Oahu ceased about 3 million years ago, and on Kauai about 5 million years ago.
This trend continues beyond the Hawaiian Islands, as demonstrated by a string of seamounts (the Hawaiian chain) that becomes progressively older toward Midway Island. Midway is composed of lavas that are approximately 27 million years old.
Northwest of Midway, the volcanic belt bends to the north-northwest to form the Emperor seamount chain. Here, the seamounts become progressively older until they terminate against the Aleutian trench.
The oldest of these seamounts near the trench is >70 million years old. This implies that the mantle plume currently generating basaltic lavas on the Big Island has been in existence for at least 70 million years!
The Foundation seamount chain is located near Easter Island in the south Pacific.
Lava flows tend to be mainly associated with basaltic lava because it has a lower viscosity due to a lower content of silica. Very low silica volcanoes spread their lava over very large areas. They create shield volcanoes.
Mauna Loa from the southeast
Mauna Loa satellite view
The amount of lava that is generated over a period of time is called the effusion rate.
A pyroclastic flow is a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice.
These awesome features are heavier-than-air emulsions that move much like a snow avalanche, except that they are fiercely hot, contain toxic gases, and move at phenomenal, hurricane-force speeds, often over 100 km/hour.
They are the most deadly of all volcanic phenomena. Sometimes the hot magma and pyroclastic flow melts the ice on high mountains. This creates a Lahar, which is categorized as a mudflow. This is also extremely dangerous because a Lahar not only carries with it the lava from the eruptions but with the added water, part of the mountain goes with it.
Active, extinct or dormant. FIGURE IT OUT!
Needless to say Volcanoes can be dangerous to human population. They also bring excellent soil for farming. The advantageous and disadvantages of living near a volcano are beyond this slide show. Sounds like a good essay.