|MadSci Network: Earth Sciences|
Dear Nitin, In order to answer your question we need to establish a few terms and concepts: (1)Lava is molten rock that erupts at the Earth’s surface at volcanoes. Molten rock that has not erupted at the surface is called magma. (2)A cross-section through the Earth would show that it is divided into a thin outer lithosphere, a thick layer called the mantle, and the core. (3)The lithosphere actually consists of the crust and the top part of the mantle, which are both composed of solid, rigid rock and are fused together as a rigid unit. The thickness of the lithosphere varies, but averages about 50 km. (4)Below the lithosphere, between about 50 km and 250 km depth, is a layer of the mantle called the asthenosphere. The asthenosphere is not rigid but can flow, rather like toothpaste being squeezed from a tube. (5)Below the asthenosphere is the rest of the mantle, which extends down to about 2900 km to the boundary with the outer core. (6)The mantle is composed of a rock called peridotite, which is rich in iron and magnesium. (7)Heat is continually being produced within the Earth’s mantle and lithosphere. This heat causes the mantle to churn, rather like water boiling in a pan only, of course, much more slowly. The main point is that the rocks of the mantle actually flow. (8)Mid-ocean ridges are regions where new ocean crust is created by upwelling of magma from the mantle. Subduction zones are regions where old ocean crust dives back down into the mantle. Here is a website with a diagram of the lithosphere and the upper part of the mantle, including the asthenosphere, that you may find helpful: HYPERLINK "http://geology.er.usgs.gov/eastern/tectonic.html" http://geology.er.usgs.gov/eastern/tectonic.html Perhaps the asthenosphere is the zone you are thinking about when you describe the lithosphere as floating on lava. Although the lithosphere does not float on an ocean of liquid magma, it can be said to float on the asthenosphere. The asthenosphere is a weak layer of the mantle which is thought to consist of about 95% solid rock (peridotite) and 5% magma. That may not sound like a lot of melt, but it is enough to make the asthenosphere mechanically weak, so that it can flow like a fluid. Think of the asthenosphere as a sort of slush of mostly solid rock mixed with a small amount of liquid magma. The asthenosphere exists because at the depth where it occurs, roughly between 50 and 250 km, the conditions of temperature and pressure are just right to cause the rock to partially melt. Below the asthenosphere the mantle is hotter but also under greater pressure, and melting does not occur. Above the asthenosphere the mantle is under lower pressure but the temperature is also lower, and again melting does not occur. You are correct that hot, liquid magma is less dense than the rock that surrounds it, and that being less dense it will tend to rise to the surface. But because the asthenosphere lies beneath the lithosphere there are only certain regions where magma from the asthenosphere is able to find its way to the surface. Also, other parts of the mantle in addition to the asthenosphere can partially melt under certain conditions and this magma can reach the surface. Magma from the asthenosphere reaches the surface along mid-ocean ridges, where heat flow is very high and where the depth from the surface to the asthenosphere is only about 50-60 km. Magma from very deep in the mantle can reach the surface at hot spots, such as Hawaii and Réunion Island, where plumes of very hot material from the boundary between the mantle and the core reach the surface. Hot spot plumes will punch through the entire mantle and the crust on their way to the surface. At mid-ocean ridges and hot spots the lava erupted at the surface is mostly a type of lava called basalt. Basalt is formed by the partial melting of the peridotite of the mantle, and its composition – rich in iron and magnesium – reflects its origin in the mantle. Away from mid- ocean ridges and hot spots, magma from the asthenosphere is unable to reach the surface because the asthenosphere lies deeper, heat flow is less, and the magma is confined under higher pressure by a greater thickness of overlying lithosphere. Volcanoes associated with subduction zones, such as the volcanoes of Indonesia, produce lavas from melting of rocks of the lithosphere, including rocks of the crust or of lithospheric mantle lying above the asthenosphere. But whatever the type of volcano, the lava erupted comes from pockets of magma which form as a result of local melting of the rocks of the mantle or lithosphere. It does not come from anything resembling an ocean of magma beneath the surface. A final point is that because the mantle flows, any magma which is lost to the surface will be replaced by fresh material rising from deeper in the mantle. The whole process is a cycle, in which melting of fresh magma takes place continually at mid-ocean ridges and hot spots, rises to the surface to form new crust, and is replaced by old crust being carried back down into the mantle at subduction zones.
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