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.
Mixing - If two magmas with different compositions happen to come in contact with one another, they could mix together.
The mixed magma will have a composition somewhere between that of the original two magma compositions. Evidence for mixing is often preserved in the resulting rocks. Crystal Fractionation - When magma solidifies to form a rock it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change. Depending on how many minerals are lost in this fashion, a wide range of compositions can be made.
The processes is called magmatic differentiation by crystal fractionation. Crystals can be removed by a variety of processes. If the crystals are more dense than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind. Removal of crystals can thus change the composition of the liquid portion of the magma.
Let me illustrate this using a very simple case. Imagine a liquid containing 5 molecules of MgO and 5 molecules of SiO 2. Volcanic Eruptions In general, magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rocks.
Effusive Non-explosive Eruptions Non explosive eruptions are favored by low gas content and low viscosity magmas basaltic to andesitic magmas. Lava Flows Pahoehoe Flows - Basaltic lava flows with low viscosity start to cool when exposed to the low temperature of the atmosphere. Explosive Eruptions Explosive eruptions are favored by high gas content and high viscosity andesitic to rhyolitic magmas.
Blocks are angular fragments that were solid when ejected. Bombs have an aerodynamic shape indicating they were liquid when ejected. Bombs and lapilli that consist mostly of gas bubbles vesicles result in a low density highly vesicular rock fragment called pumice. Clouds of gas and tephra that rise above a volcano produce an eruption column that can rise up to 45 km into the atmosphere. Eventually the tephra in the eruption column will be picked up by the wind, carried for some distance, and then fall back to the surface as a tephra fall or ash fall.
If the eruption column collapses a pyroclastic flow will occur, wherein gas and tephra rush down the flanks of the volcano at high speed. This is the most dangerous type of volcanic eruption. The deposits that are produced are called ignimbrites if they contain pumice or pyroclastic flow deposits if they contain non-vesicular blocks.
Pyroclastic Deposits Pyroclastic material ejected explosively from volcanoes becomes deposited on the land surface. Fall Deposits. Material ejected into an eruption column eventually falls back to the earth's surface and blankets the surface similar to the way snow blankets the earth. The thickest deposits occur close to vent and get thinner with distance from the vent. By measuring the thickness at numerous locations one can construct an isopach map. Such isopach maps help to locate the source volcanic vent if it is not otherwise known and provides information about wind direction in the upper levels of the atmosphere during the eruption.
Fall deposits are usually fairly well-sorted, meaning that the clast size does not vary too much within the individual deposit. The clast size can be ash as in a cinder cone. They may also contain clasts of rock fragments called lithic fragments that are pieces of the volcanic structure ripped from the sides of the conduit during the explosive eruption. If the pyroclastic flows consist of solid clasts with high density along with ash fragments, they are called block and ash flows.
If the pyroclastic flows have low density clasts pumice along with ash, they are called ignimbrites. There are no definitive boundary between pyroclastic flows and surges as they grade into one another continuously. Similarly, ignimbrites grade into block and ash flows as the clast density increases. Pyroclastic Flow Deposits Pyroclastic flows tend to follow valleys or low lying areas of topography. The material deposited, thus tends to fill valleys, rather than uniformly blanket the topography like fall deposits.
Block and Ash Flow Deposits. Ignimbrites Ignimbrites contain blocks of pumice in an unsorted mixture of ash, lapilli, pumice blocks, and lithic fragments. Sometimes one finds concentrated zones of pumice or lithic fragments in the deposits. Surge Deposits. Because they move close to ground, friction with ground tends to produce cross stratification in the deposits. Individual layers can be well-sorted, but overall the deposits tend to be poorly sorted. Types of Volcanic Eruptions Volcanic eruptions, especially explosive ones, are very dynamic phenomena.
Some solidify within the earth plutonic or subvolcanic rocks. The properties of magmas include temperature, density, viscosity, gas content and abundance. The temperature of a magma is hard to measure. It can be done at a distance using an optical pyrometer glowing filament but this needs many corrections e.
The temperature of some lavas can be measured in the field using a thermocouple providing the eruption is not violent as in lava flows or lakes. The density of chilled magma volcanic glass is usually measured. It ranges from 2. Viscosity is the resistance of a fluid to motion. For example, syrup is more viscous than water.
Weathering How do turbidity currents work? The nature of volcanic eruptions is highly dependent on magma viscosity and also on dissolved gas content. Such magmas erupt as andesites and rhyolites or are intruded as granite masses. The more extensive silicate chain molecules render these magmas highly viscous, so when eruption occurs it is usually explosive e. Magma is less dense than the surrounding rock and rises up to the surface through cracks and crevices under the Earth's surface causing an eruption of lava.
The silica content of the magma determines how thick the magma is, how easily it flows and how easily dissolved gases within the magma can escape; therefore the silica content determines the characteristic of the volcanic eruption and the shape and size of the resulting volcanic cone.
If, however, the lava is very low in silica, the lava is not thick enough to build a cone at all and it will flow out over the Earth's surface much like a flood. The byproducts of this flow are commonly referred to as flood basalts.
Between 17 million and 6 million years ago the Columbia River flood basalts covered part of the surface of Washington, Idaho and Oregon to a depth of 9, ft.
Volcanic Eruptions: High Silica. Lava with high silica content is thick and viscous and does not readily flow.
Lava rises up toward the surface but is too thick to squeeze through the cracks and fissures in the Earth. As lava continues to rise upward, pressure continues to build.
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