There is no stronger evidence for melting of portions of the crust and mantle than the fact that volcanoes exist on the Earth. Volcanoes involve ascent of molten rock (magma) from deep in the crust and upper mantle, and the process of melting is key for ongoing chemical differentiation of the Earth. The materials that ultimately reach the surface, and erupt from a volcano are strongly modified from the original material that melted, with only the lighter components, enriched in volatiles, reaching the surface. The rocks that form from volcanoes are thus different than the original rocks that melted to give the magma, and some of these rocks are so buoyant that they have progressively added to the Earth's continents, while oceanic floor is recycled back into the mantle.
So, melting occurs inside the crust and mantle. Yet, seismic S waves propagate through the crust and mantle, so it is not everywhere molten. In fact, melting occurs only in localized places, and requires somewhat special circumstances. Any rock material will melt, meaning to transform from solid phase to liquid phase, when the temperature exceeds the melting temperature of the rock. The melting temperature varies with pressure (generally it increases), so the temperature tends to only exceed the melting temperature over a localized depth range.
So, the temperature structure of the Earth is key to understanding the occurrence of melting. First we must ask why is the Earth hot inside to begin with? This takes us back to the discussions in the first third of the class, where we talked about the huge amount of heat resulting from accretion of the Earth, as planetisimals collided and built up the mass of the planet. That heat is called primordial heat, as it stems from the Earth formation itself. Associated with the earliest phase of the Earth, there was additional heat released when the iron sank to make the core. This release of gravitational energy gave additional heat to the early Earth, which is slowly dissipating, but has not yet escaped from the planet. The other source of heat for the Earth has been a continuously generating heat source, involving the decay of radioactive elements. Recall that the Earth formed with a distribution of very heavy, unstable elements, which have subsequently been undergoing spontaneous fission processes to decay to more stable forms. While the radioactive fuel is slowly being burned up, and there was more heat production early in the history of the Earth, there are still large amounts of radioactive materials continuing to heat the interior.
OK, given these three main sources of heat, how hot is it inside? This is actually not all that well known, as there are not many observations that we can make at the surface that tightly constrain the temperature at depth. Generally, we do know that the temperature increases with depth almost linearly through the lithosphere, down to depths of 75-100 km under oceans and 200-300 km under continents. In the lithosphere, which is stiff and does not flow, heat is transported by conduction, the transfer of heat by atomic vibrations without mass transport. At the base of the lithosphere the temperature has increased to on the order of 1000-1200 degrees, and if it continued to increase linearly, it would intersect the melting curve of mantle rocks which is about this temperature. But, as the temperature increases the rheology of the rocks changes, and they become lower viscosity and more deformable. At temperatures of around 1000 degrees, the rock becomes soft enough that it is easier for it to move plastically to transport heat out than it is for conduction to transport heat out. The efficient transfer of heat by flow, or convection, causes the rate of temperature increase with depth to greatly diminish, and through the rest of the mantle the temperature gradient is rather small, increasing the temperatures to about 2500 degrees near the base of the mantle (2600 km deeper than the lithosphere). It appears that the mantle rock melting curve is at higher temperatures throughout this region, so there is little or no melting.
At the base of the mantle the temperature again increases rapidly with depth, in the thermal boundary layer between the core and the mantle. This boundary layer again involves heat transport by conduction, as there is no mass flow across the core-mantle boundary. The temperature right at the base of the mantle is somewhere from 3000-4000 degrees. Within the outer core, which is vigorously convecting, the thermal gradient (geotherm) is small, with maximum temperatures of 5500-7000 degrees at the center of the Earth. One of the key tie points that we try to determine experimentally is the melting temperature of iron at pressures corresponding to the inner core-outer core boundary, as this transition from liquid to solid corresponds to the geotherm intercepting the melting temperature of iron (the geotherm is above the melting temperature in the outer core, and below it in the inner core. Unfortunately, the problem is complicated by both the difficulty of replicating the enormous pressures at depth and by uncertainty in the light alloying components of the core, which can strongly affect the melting temperature of the alloy.
So, there are perhaps 3 regions where melting occurs in the interior. One is in the outer core, which is fully molten, or at most a slurry with few particulates. Another region is possibly in the rapid temperature increase at the base of the mantle. There is now seismological evidence suggesting partial melting of the mantle in the core-mantle boundary vicinity in localized regions. The most important melting occurs up near the base of the lithosphere, where the geotherm increases temperature rapidly to the vicinity of the melting curve.
To understand the shallow melting processes better, we will consider the three basic environments in which melting takes place. The tectonic regions with volcanoes include:
The best known volcanoes are in Subduction Zones, in part because these tend to be explosive, deadly volcanoes. But some rift volcanoes are well known (Mt. Kilamanjaro), as are hotspots (Iceland, Hawaii, Tahiti). If we consider the major volcanic eruptions of the world, 15% are in continental rifts, 80% in subduction zones, and 5% in hotspots, but these numbers apply only to the subaerial eruptions. Far more volcanic activity occurs under the oceans.. There are from 15-20 large subaerial eruptions each year.
The subduction zone volcanoes include many of the famous names: Fuji, Mt. St. Helens, Vesuvius, Pelee, Katmai, Krakatao. These volcanoes are distinctive in being the classic, conical shaped mountains built up of layer upon layer of lava flows, in what are called strato-volcanoes. Mt. Fuji, which last erupted in 1707 is a classic volcanic cone, one which you can hike up in a few hours (usually at night so that you can catch the sunrise, if you are lucky and have a clear morning). Yet it is an active volcano which we fully expect to continue to erupt again and again in its long geological history. Mt. Fuji is located over the subduction zone where the Pacific plate underthrusts beneath Japan, and it owes its existence to the melting process that occurs in this convergent zone. The Aleutians, Tonga, Marianas, Kuriles, and other island arcs have similar volcanoes. Subduction zones along continental margins also result in dramatic strato-volcanoes, such as in the Cascades and the Andes. But, why are there volcanoes in these regions?
This question has puzzled geologists for a long time. The subducting oceanic lithosphere in a subduction zone is as much as 1000 degrees colder than the surrounding mantle near depths of 100-200 km. Thus, it is well below the melting temperature for basaltic rocks that make up the oceanic crust, and this is true for the depleted peridotites that make up the rest of the oceanic lithosphere as well. Clearly, volcanoes in island and continental arcs are not caused by melting of the oceanic plate as it sinks into the mantle. The surrounding mantle is actually cooled by the presence of the sinking plate as well, since material is dragged down along with the plate, thus displacing colder material from shallow to deeper depths. So, what is the cause of the melting that feeds volcanoes such as Mt. Fuji?
The answer lies in understanding the sensitivity of melting temperature of rock to the presence of volatiles such as water. If a small amount (say 2-5%) of water is added to a rock, the melting temperature at a given depth can be reduced by hundreds of degrees. Thus, rather than explain the melting as a result of locally higher than average temperatures, we account for the melting by locally lowering the melting temperature by adding water to the rocks. Water is bound up in hydrous minerals in the sediments that accumulate on oceanic plates, and there is actually water that circulates down into the oceanic crust. When the plate sinks into the mantle, the water is transported downward. As the plate heats up slowly the hydrous minerals can become unstable, releasing the water in dehydration reactions. This water is then expelled from the slab and leaks out into the overriding wedge of material, lowering its melting point. At depths of 100-150 km, where the water is expelled from the slab, melting occurs due to the lowered melting temperature (despite the cooling effect from the downwelling slab). The melting reduces the density of the molten rock and it then rapidly ascends, feeding into magma chambers and conduits that build up pressure and eventually erupt in arc volcanoes.
As the magma melts, ascends, ponds in magma chambers, and eventually erupts, it can chemically differentiate significantly, so that the rocks that form from the magma bear little resemblance to the mantle in the wedge that melted. Some of the changes include interactions with the overriding crust through which the magma must traverse so that it can erupt. The most common rock type in island arc volcanoes is andesite. This is a rock that is very sticky, the result of a high silica content. The stickiness, or high viscosity, allows the layers of flows to build up a large cones. It also causes the volcano to plug itself up, which allows pressure to build up between eruptions.
As a result, arc volcanism tends to be explosive. This is the combined result of high silica content rocks, which are sticky, and the presence of lots of volatiles and gases in the magma. Arc volcanoes tend to have long periods of repose, with eruption intervals of 100-1000 years. This time scale makes it challenging to anticipate future violent eruptions, as we typically have very limited history of eruptions to extrapolate from. The source magmas of arc volcanoes are rich in water and gases both due to volatiles that were carried down by the subducting slab, and due to interactions with the overriding plate as the magma ascends.
Arc eruptions are an important part of the process of continental growth, as they extrude large masses of chemically differentiated rocks. Mt. St. Helens erupted about 0.5 cubic km of new rocks, but that is a modest amount for a large volcano. The 1883 eruption of Krakatao ejected 6.0 cubic km, while the 1912 eruption of Katmai gave out 12.0 cubic km. But far greater eruptions have happened in the past. The explosion that produced the Long Valley Caldera gave out about 600 cubic km. Ash from this event spread thick over states as far away as Nebraska. A repeat event would be devastating for any country, and perhaps for the entire hemisphere in which it took place. To put the volcanic energy release into perspective, the violent 1980 eruption of Mt. St. Helens gave off energy equivalent to 27,000 nuclear bombs like that dropped on Hiroshima. This would involve 1 such explosion each second for 9 hours.
A caldera is the large ring-like collapse structure that results when the surface layers overlying a massive magma chamber collapse in, as the chamber empties during a big eruptions.
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