Based on our analyses of the bathymetric maps and lava and sediment samples from our expedition, we now have clues that high-silica lavas may have erupted here during at least two different time periods.
Preliminary work suggests that the rhyolite dome discovered in is roughly 25, years old. Two dives ago we visited another mound of probable rhyolite that lies further off-axis than the rhyolite dome. We will test our hypothesis that this flow is older by using radiocarbon analyses of microfossils that we collected directly above the flow with long push cores from the ROV Doc Ricketts. We will soon have an age constraint for this high-silica flow and for dozens of flows with more common compositions.
These characteristics lead us to suspect that this is a high-silica lava. A column of magma intruded into the crust and cooled slowly after it was emplaced. It has been exposed and peeled away from the wall through subsequent tectonic rifting and erosion. As defined above, block and ash flows consist of an unsorted mixture of blocks and ash with the blocks being mostly rock fragments. Surges tend to hug the ground as they flow over the surface and thus tend to produce thicker deposits in valleys with thinner deposits over ridges.
This helps to distinguish surge deposits from flow deposits and fall deposits. Volcanic eruptions, especially explosive ones, are very dynamic phenomena. That is the behavior of the eruption is continually changing throughout the course of the eruption. This makes it very difficult to classify volcanic eruptions. Nevertheless they can be classified according to the principal types of behavior that they exhibit. An important point to remember, however, is that during a given eruption the type of eruption may change between several different types.
Hawaiian - These are eruptions of low viscosity basaltic magma. Gas discharge produces a fire fountain that shoots incandescent lava up to 1 km above the vent. The lava, still molten when it returns to the surface flows away down slope as a lava flow. Hawaiian Eruptions are considered non-explosive eruptions. Very little pyroclastic material is produced. Strombolian - These eruptions are characterized by distinct blasts of basaltic to andesitic magma from the vent. These blasts produce incandescent bombs that fall near the vent, eventually building a small cone of tephra cinder cone.
Sometimes lava flows erupt from vents low on the flanks of the small cones. Strombolian eruptions are considered mildly explosive, and produce low elevation eruption columns and pyroclastic fall deposits. Vulcanian - These eruptions are characterized by sustained explosions of solidified or highly viscous andesite or rhyolite magma from a the vent. Eruption columns can reach several km above the vent, and often collapse to produce pyroclastic flows.
Widespread pyroclastic falls are common that contain mostly angular blocks. Vulcanian eruptions are considered very explosive. They may also produce surges with resulting surge deposits.
Pelean eruptions are considered violently explosive. Plinian - These eruptions result from a sustained ejection of andesitic to rhyolitic magma into eruption columns that may extend up to 45 km above the vent.
Eruption columns produce wide-spread fall deposits with thickness decreasing away from the vent, and may exhibit eruption column collapse to produce pyroclastic flows and surges. Plinian ash clouds can circle the Earth in a matter of days.
Plinian eruptions are considered violently explosive. Phreatomagmatic - These eruptions are produced when magma comes in contact with shallow groundwater causing the groundwater to flash to steam and be ejected along with pre-existing fragments of the rock and tephra from the magma. Because the water expands so rapidly, these eruptions are violently explosive although the distribution of pyroclasts around the vent is much less than in a Plinian eruption.
Surge deposits are usually produced. Phreatic also called steam blast eruptions - result when magma encounters shallow groundwater, flashing the groundwater to steam, which is explosively ejected along with pre-exiting fragments of rock.
No new magma reaches the surface. Surge deposits may result from these eruptions. Questions on this material that could be asked on an exam. Natural Disasters. Volcanoes, Magma, and Volcanic Eruptions. Characteristics of Magma Types of Magma Types of magma are determined by chemical composition of the magma.
Temperature of Magmas Temperature of magmas is difficult to measure due to the danger involved , but laboratory measurement and limited field observation indicate that the eruption temperature of various magmas is as follows: Basaltic magma - to o C Andesitic magma - to o C Rhyolitic magma - to o C.
Viscosity of Magmas Viscosity is the resistance to flow opposite of fluidity. Higher SiO 2 silica content magmas have higher viscosity than lower SiO 2 content magmas viscosity increases with increasing SiO 2 concentration in the magma. Lower temperature magmas have higher viscosity than higher temperature magmas viscosity decreases with increasing temperature of the magma.
How Magmas Form in the Earth As we have seen the only part of the earth that is liquid is the outer core. If the mineral contains no water H 2 O or carbon dioxide CO 2 and there is no water or carbon dioxide present in the surroundings, then melting occurs at a single temperature at any given pressure and increases with increasing pressure or depth in the Earth. This is called dry melting. If water or carbon dioxide are present within or surrounding the mineral, then melting takes place at a single temperature at any given pressure, but first decreases with increasing pressure.
Since rocks are mixtures of minerals, they behave somewhat differently. Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures. Thus, it is possible to have partial melts, from which the liquid portion might be extracted to form magma. The two general cases are:. Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt.
Melting of wet rocks is similar to melting of wet minerals, except there is range of temperature range over which partial melting occurs. Again, the temperature of beginning of melting first decreases with increasing pressure or depth, then at high pressure or depth the melting temperatures again begin to rise. Three ways to Generate Magmas From the above we can conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way.
Chemical Composition of Magmas The chemical composition of magma can vary depending on the rock that initially melts the source rock , and process that occur during partial melting and transport. Initial Composition of Magma The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting.
Magmatic Differentiation But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma. Assimilation - As magma passes through cooler rock on its way to the surface it may partially melt the surrounding rock and incorporate this melt into the magma. Because small amounts of partial melting result in siliceous liquid compositions, addition of this melt to the magma will make it more siliceous.
Yokoo, A. Processes prior to outbursts of vulcanian eruption at Showa Crater of Sakurajima Volcano. Jpn 58 , — Denlinger, R. Cyclic eruptive behavior of silicic volcanoes. Geology 27 , — Lensky, N. Remarkable cyclic ground deformation monitored in real time on Montserrat and its use in eruption forecasting. Druitt, T. Volcanic Processes, Products and Hazards — Geol. Clarke, A. Connor, C. Edmonds, S. Watson, I. Nakada, S. Overview of the — eruption at Unzen Volcano. Anderson, K. Cyclic ground tilt associated with the — eruption of Mount St Helens.
Scott, D. Magma solitons. McKenzie, D. The generation and compaction of partially molten rock. Michaut, C. Ascent and compaction of gas-rich magma and the effects of hysteretic permeability. Earth Planet. Watts, R. Miller, A. Spieler, O. The fragmentation threshold of pyroclastic rocks. Umakoshi, K. High-frequency earthquake swarm associated with the May dome extrusion at Unzen Volcano, Japan.
These eruptions can last for days and create a sustained and tall eruption plume, which drops huge amount of tephra , fallen volcanic material, on surrounding areas. Additionally, a Plinian eruption can produce extremely fast moving lava flows that destroy everything in their path. In Hawaiian Eruptions the lava is more basic and basaltic, with low gas pressures and low silica content.
These eruptions are generally not explosive or destructive and DO NOT throw huge amounts of Tephra or pyroclastic material in the air. Instead they produce low-viscosity, low-gas-content lava that flows over large areas producing gently sloping shield volcanoes and lava plateaus.
Eruptions can form fire fountains , Lava thrust up to 50m in the air for many hours. The general eruption style is a steady lava flow from a central vent, which can produce wide lava lakes , ponds of lava forming in craters or other depressions. In Strombolian Eruptions, named after Stromboli in Italy, the effects are impressive but not that dangerous. They put small amounts of lava 15 to 90 meters in the air, in very short bursts. The lava has a fairly high viscosity, so gas pressure has to build to a high level before it will thrust the material upward.
These regular explosions can produce impressive booming sounds, but the eruptions are relatively small. Find out more here. Types of magma.
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