Indeed, were that not so, then volcanoes would be popping up all over the shop! Rocks, in both the mantle layer and the crust, begin to melt only in exceptional circumstances. Since some rock-forming minerals have lower melting temperatures than others, it is normal for partial melting to take place, the resulting magma being squeezed out of the parent rock and upward toward the surface.
This provides the flux needed to lower the melting temperature. The magma produced, being less dense than the surrounding rock, moves up through the mantle, and eventually into the crust. As it moves toward the surface, and especially when it moves from the mantle into the lower crust, the hot magma interacts with the surrounding rock.
This typically leads to partial melting of the surrounding rock because most such magmas are hotter than the melting temperature of crustal rock. In this case, melting is caused by an increase in temperature. Again, the more silica-rich parts of the surrounding rock are preferentially melted, and this contributes to an increase in the silica content of the magma.
As the temperature drops, usually because the magma is slowly moving upward, things start to change. Silicon and oxygen combine to form silica tetrahedra, and then, as cooling continues, the tetrahedra start to link together to make chains polymerize. As the magma continues to cool, crystals start to form. This is an 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 this represents silica and stir while the mixture comes close to boiling. It should thicken like gravy because the gluten in the flour becomes polymerized into chains during this process.
Intraplate magmatism is thought to be caused by hot spots formed when thin plumes of mantle material rise along narrow zones from deep within the mantle. The hot spot remains stationary in the mantle while the plate moves over the hot spot. Decompression melting caused by the upwelling plume produces magmas that form a volcano on the sea floor above the hot spot.
The volcano remains active while it is over the vicinity of the hot spot, but eventually plate motion results in the volcano moving away from the plume and the volcano becomes extinct and begins to erode.
Examples of questions on this material that could be asked on an exam. What could cause melting to occur in each of the following tectonic settings for each setting give all possible mechanisms? Magmatism and Plate Tectonics From the discussion above it should be obvious that magmatism is closely related to plate tectonics.
Diverging Plate Boundaries Diverging plate boundaries are where plates move away from each other. Because the Pacific Plate is one of the faster moving plates, this type of volcanism produces linear chains of islands and seamounts, such as the Hawaiian - Emperor chain, the Line Islands, the Marshall-Ellice Islands, and the Austral seamount chain.
Structure of the Earth and the Origin of Magmas. Magmas do not form everywhere beneath the surface of the Earth. This is evident from looking at the world distribution of volcanoes. Thus, magmas must require special circumstances in order to form.
Before we talk about how and where magmas form, we first look at the interior structure of the Earth. The Earth's Internal Structure Evidence from seismology tells us that the Earth has a layered structure.
There are two types of body waves: P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel. Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth.
Thus they can give us information about the properties of rocks near the surface, but not about the properties of the Earth deep in the interior.
Because seismic waves reflect from and refract through boundaries where there is sudden change in the physical properties of the rock, by tracing the waves we can see different layers in the Earth. This allows us to look at the structure of the Earth based on layers of differing physical properties.
Also note from the velocity equations that if density increases, wave velocity decreases. Thus, the other properties, incompressibility and rigidity must increase with depth in the Earth at a greater rate than density increases.
Layers of Differing Chemical Composition Crust - variable thickness and composition Continental 10 - 70 km thick, underlies all continental areas, has an average composition that is andesitic. Evidence comes from Seismic wave velocities, experiments, and peridotite xenoliths foreign rocks brought to the surface by magmas. Experimental evidence suggests that the mineralogy of peridotite changes with depth ant thus pressure in the Earth.
This occurs because Al changes its coordination with increasing pressure, and thus new minerals must form to accommodate the Al. At greater depths, such as the km discontinuity and the km discontinuity, olivine and pyroxene likely change to high pressure polymorphs. Despite these changes in mineral assemblage, the chemical composition of the mantle does not appear to change much in terms of its major element composition. Core - km radius, made up of Iron Fe and small amount of Nickel Ni.
Evidence comes from seismic wave velocities, experiments, and the composition of iron meteorites, thought to be remnants of other differentiated planets that were broken apart due to collisions. Layers of Differing Physical Properties Lithosphere - about km thick up to km thick beneath continents, thinner beneath oceanic ridges and rift valleys , very brittle, easily fractures at low temperature. Inner core - km radius, solid Where do Magmas Come From?
Origin of Magmas Again, magmas do not form everywhere beneath the surface, so special circumstances are necessary. Lowering the Solidus Temperature As we saw in our discussion of phase diagrams, mixtures of components begin melting at a lower temperature than the pure components.
Continental Rift Valleys or Extensional Zones are areas, usually located in continental crust where extensional deformation is occurring. These areas may be incipient spreading centers and may eventually evolve into oceanic ridges, such as has occurred in the Red Sea region. Progress in this petrological direction can be integrated with geochemical studies to sharpen our ability to distinguish the magmatic products of mantle plumes from those of lithosphere flexure and fracture Hofmann and Hart, ; Hirano and Koppers, MgO and FeO contents of magmas and the compositions of olivines that they would crystallize.
Primary magma compositions are those expected for peridotite melting. Small melt droplets are mostly illustrative of fractional melting, but those with the lowest FeO contents are from Keiding et al.
Sign In or Create an Account. User Tools. Sign In. Advanced Search. Skip Nav Destination Article Navigation. Research Article December 01, Google Scholar. Geology 39 12 : — Article history first online:. Figure 1. View large Download slide.
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Temperatures in ambient mantle and plumes: Constraints from basalts, picrites and komatiites.
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