That water then rises up into the mantle above it, causing it to melt at a lower temperature and, bam! Basalt is produced in the process called flux melting. The largest volcanic system on Earth is the mid-ocean ridge system , where you don't have any subduction to bring water down into the mantle to help melting along.
Now, why do you get basalt there? This time we have to use another method to melt that peridotite - we need to decompress it at constant temperature. This is called adiabatic ascent. The mantle is convecting, bringing hot mantle from depth up towards the surface and as it does so, the mantle material stays hot, hotter than the surrounding rocks. So, keep that mantle material hot and decompress it and you get melting to form basalt! So, underneath mid-ocean ridges and at hotspots like Hawaii , the mantle is upwelling, causing decompression melting to occur.
Let's review : Under normal conditions, mantle rock like peridotite shouldn't melt in the Earth's upper mantle -- it is just too cool. However, by adding water you can lower the melting point of the rock. Alternatively, by decompressing the rock, you can bring it to a pressure where the melting point is lower.
Magma can intrude into a low-density area of another geologic formation, such as a sedimentary rock structure. When it cools to solid rock, this intrusion is often called a pluton. A pluton is an intrusion of magma that wells up from below the surface.
Plutons can include dikes and xenoliths. A magmatic dike is simply a large slab of magmatic material that has intruded into another rock body. A xenolith is a piece of rock trapped in another type of rock.
Many xenoliths are crystals torn from inside the Earth and embed ded in magma while the magma was cooling.
Lava cools to form volcanic rock as well as volcanic glass. This magma solidifies in the air to form volcanic rock called tephra. In the atmosphere, tephra is more often called volcanic ash. As it falls to Earth, tephra includes rocks such as pumice. In areas where temperature, pressure, and structural formation allow, magma can collect in magma chamber s.
Most magma chambers sit far beneath the surface of the Earth. The pool of magma in a magma chamber is layered. The least-dense magma rises to the top.
The densest magma sinks near the bottom of the chamber. Over millions of years, many magma chambers simply cool to form a pluton or large igneous intrusion. If a magma chamber encounter s an enormous amount of pressure, however, it may fracture the rock around it.
The cracks, called fissure s or vents, are tell-tale signs of a volcano. Many volcanoes sit over magma chambers. An eruption reduce s the pressure inside the magma chamber. Large eruptions can nearly empty the magma chamber. The layers of magma may be document ed by the type of eruption material the volcano emits. Gases, ash, and light-colored rock are emitted first, from the least-dense, top layer of the magma chamber.
Dark, dense volcanic rock from the lower part of the magma chamber may be released later. In violent eruptions, the volume of magma shrinks so much that the entire magma chamber collapses and forms a caldera. All magma contains gases and a mixture of simple element s. Being that oxygen and silicon are the most abundant elements in magma, geologists define magma types in terms of their silica content, expressed as SiO 2.
These differences in chemical composition are directly related to differences in gas content, temperature, and viscosity. This type of magma has a low gas content and low viscosity, or resistance to flow. Mafic magma also has high mean temperatures, between o and o Celsius o and o Fahrenheit , which contributes to its lower viscosity. Low viscosity means that mafic magma is the most fluid of magma types. This lava cools into basalt , a rock that is heavy and dark in color due to its higher iron and magnesium levels.
The Hawaiian Islands are a direct result of mafic magma eruptions. This results in a higher gas content and viscosity. Its mean temperature ranges from o to o Celsius o to o Fahrenheit. This more gaseous and sticky lava tends to explode violently and cools as andesite rock. Intermediate magma most commonly transforms into andesite due to the transfer of heat at convergent plate boundaries.
Andesitic rocks are often found at continent al volcanic arcs, such as the Andes Mountains in South America, after which they are named. As a result, felsic magma also has the highest gas content and viscosity, and lowest mean temperatures, between o and o Celsius o and o Fahrenheit.
These trapped bubbles can cause explosive and destructive eruptions. These eruptions eject lava violently into the air, which cools into dacite and rhyolite rock. Much like intermediate magma, felsic magma may be most commonly found at convergent plate boundaries where transfer of heat and flux melting create large stratovolcano es. Magma exists as pockets and plumes beneath the surface of the Earth.
Photograph by Carsten Peter, National Geographic. In Figure 7. Flux-induced partial melting of rock happens in subduction zones. Minerals are transformed by chemical reactions under high pressures and temperatures, and a by-product of those transformations is water. Relatively little water is required to trigger partial melting. In laboratory studies of the conditions of partial melting in the Japanese volcanic arc, rocks with only 0. Viscosity refers to the ease with which a substance flows.
A substance with low viscosity is runnier than a substance with high viscosity. As magma loses heat to the surrounding rocks and its temperature drops, things start to change. Silicon and oxygen combine to form silica tetrahedra. With further cooling, the tetrahedra start to link together into chains, or polymerize.
These silica chains make the magma more viscous. Magma viscosity has important implications for the characteristics of volcanic eruptions. This is a quick and easy experiment that you can do at home to help you understand the properties of magma. It will only take about 15 minutes, and all you need is half a cup of water and a few tablespoons of flour.
Add 2 teaspoons 10 mL of white flour and stir while continuing to heat the mixture until boiling. The white flour represents silica. The mixture should thicken like gravy because the gluten in the flour becomes polymerized into chains during this process.
Add that mixture to the rest of the water and flour in the saucepan. Stir while bringing it back up to nearly boiling temperature, and then allow it to cool. This mixture should slowly become much thicker Figure 7. Kushiro, I. Origins of magmas in subduction zones: a review of experimental studies.
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