In contrast, the mantle is made of peridotite, which has a density of about 3. Therefore, when the mantle partially melts and re-crystalizes as basalt, the oceanic crust will sit on top of the mantle. Continental crust has an even lower density than oceanic crust because it is formed through multiple cycles of partial melting, leading to a granitic composition its density is about 2. The asthenosphere is the portion of the mantle that flows , allowing the brittle lithosphere above it to move in a process called plate tectonics.
Over time, oceanic crust cools, and therefore its density increases. When old oceanic crust collides with continental crust or with younger and therefore less dense oceanic crust, this pushes the old oceanic crust underneath the other crust due to its higher density.
At this point the old oceanic crust is cool enough that its density is higher than the mantle it sits on top of, so once it starts to bend it continues to sink into the mantle in a process called subduction. Scientists hypothesize that after being subducted, some oceanic crust sinks all the way to the bottom of the mantle. Eventually, nearly all oceanic crust is recycled in this process a very small amount of oceanic crust is preserved on the continents - these deposits are called ophiolites.
Interesting question! In reality, the mantle is solid, and the coloring is just meant to indicate that the rock is hot and flowing slowly under heat convention.
Trapasso, and Dorothy Sack states, "Extending down from the base of the lithosphere about kilometers mi farther into the mantle is the asthenosphere from Greek: asthenias , without strength , a thick layer of plastic mantle material. The material in the asthenosphere can flow both vertically and horizontally, dragging segments of the overlying, rigid lithosphere along with it. Topics: astheosphere , geology , heat , heat convection , lava , magma , mantle , molten , subduction , tectonic plate movement , temperature.
Under the continents is a layer of solid rock known as the upper mantle or asthenosphere. Though solid, this layer is weak and ductile enough to slowly flow under heat convection, causing the tectonic plates to move. The Antarctic Plate is a tectonic plate containing the continent of Antarctica, the Kerguelen Plateau and extending outward under the surrounding oceans. After this time, Gondwana slowly split apart to create Antarctica as a separate continent, and Antarctica has gradually moved away from the other southern continents towards its present polar position.
Begin typing your search term above and press enter to search. Press ESC to cancel. Skip to content Home Social studies Why do continents float on oceans? Social studies. Ben Davis January 27, Why do continents float on oceans? It is as solid as any rock you would pick up at the surface of the Earth. And to a 7th grader, saying that it is molten rock is incorrect and perpetuates the misconception that there is a magma ocean underneath the crust. Anyway, if you want we can continue this discussion in the chat, or you can open another question: "is there a magma ocean in the Earth?
We know this because we can measure the propagation of seismic shear waves through the mantle and they propagate through the whole mantle, meaning that it must be solid, because shear waves cannot travel through a liquid. S waves do not propagate through the outer core and this is how we know that it is liquid.
There is a huge difference between the two: one doesn't erupt in volcanoes whereas the second does. You can compare the two to chocolate slightly warned by hands that becomes soft and bends, but is still solid. Magma, on the other hands, is like chocolate on a fountain. You can even make a nice demonstration in class. Show 11 more comments.
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