Krajala
KrajalaHi Krajala,
This is a question that we've answered a number of times before so if my explanation doesn't help, please check the "previously-asked-questions" list.
There are a number of classifications that you may find in books. I think the classes of volcanoes should be: 1) strato (or composite) volcanoes; 2) shield volcanoes; 3) rhyolite caldera complexes; 4) monogenetic fields; 5) flood basalt provinces; and 6) mid-ocean ridges. This order has no real significance.
Strato volcanoes are the most common type on Earth. They are composed of about equal quantities of ash and thick stubby lava flows or lava domes. This inter-layering results in the name. They are characterized by viscous magma which causes most of their eruptions to be explosive. Viscous magma allows gas pressures to build to high levels and when it is finally released the erupions are powerful. Viscous magma can form steep slopes and ash can pile up in steep piles, so the slopes of strato volcanoes often approach 35 degrees. The volcanoes themselves tend not to be very big. Strato volcanoes occur all along subduction zones, most particularly around the Pacific Ocean.
Shield volcanoes are composed of basalt lava, which is hot and fluid. This means that gas pressures cannot be built very high so that eruptions are not explosive. The fluid lava flows easily down the slopes of the volcano so the slopes are not very steep--usually 5-10 degrees. Shield volcanoes are mostly associated with hotspots--places where magma from the mantle burns a hole through the overlying lithospheric plate. Shield volcanoes tend to be very big.There are a few shield volcanoes along some subduction zones but they are not the big kind.
Rhyolite caldera complexes are also pretty large. Fortunately they don't erupt very often because when they do they are extremely explosive and the volumes of erupted ash are on a scale that is hard to comprehend. We have not had to live through one of these. They usually consist of a large caldera, sometimes 30-40 km across, that formed during a huge eruption. So much magma is erupted that the ground surface above collapses inward to form these huge depressions. The ash and pyroclastic flows from these eruptions often are carried hundreds of km from the vent. Rhyolite complexes often have repose periods between big eruptions of hundreds of thousands of years. Some of them also experience smaler eruptions on a more frequent scale. Many of these smaller eruptions occur along the caldera boundary.
Monogenetic fields occur where the supply of magma to the surface is very low. In these locations there is not enough magma, nor does it erupt often enough, to maintain any one preferred pathway. This means that each small batch of magma works its own way to the surface to erupt any old place (instead of always coming up the same conduit to eventualy build a single volcano). Many monogenetic field eruptions form cinder cones, but they also form tuff cones, maars, lava fields, or lava domes, depending on the environment they come up into and the viscosity of the erupting magma. Cinder cones also form on the flanks of strato volcanoes, shield volcanoes, and in rhyolite caldera complexes so it is misleading to say that "cinder cone" is a type of volcano (although many books do just that).
Flood basalts are another type of volcanic eruption that fortunately we've never had to experience. They are huge volumes of basaltic lava that cover hundreds of square kilometers to depths of sometimes 200 m. They were erupted with very little explosive activity and in fact, the actual vents are quite difficult to locate--it's almost as if these huge volumes of basalt just welled up out of the ground. The old school of thought was that these were fast-moving very thick lava flows that were emplaced in a matter of weeks to months. It is now starting to look like that instead, they were emplaced very slowly over periods of perhaps a few hundred to a thousand years. Instead of flowing over the ground as thick flows, they reached their thicknesses by inflating themselves with the later lava.
Mid-ocean-ridges are places where oceanic crust is formed. Some folks consider the system of spreading centers to be one single ~70,000 km-long volcano. We have never seen an actual mid-ocean ridge eruption but we know that they occur. The basalt at the ridges is young and covered with very little sediment. As you move away from the ridge the basalt gets older and older, and the sediment cover gets thicker and thicker.
Hopefully this information will help a little bit.
Sincerely,
Scott Rowland
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