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What happens to the magma inside the earth once it is released to the surface? Does the earth generate more or will all the magma eventually come out?
Question Date: 2002-01-14
Answer 1:

What a great question you have asked. In a way you want to know if the earth will ever run out of magma. A short answer to this is would be - not in any near future but yes, eventually if the earth continues to cool down.

But let me explain this to you in more detail: You have probably heard of the Rock Cycle and you know that magma is nothing else but molten rock in the earth mantle. Because magma is always regenerated in the rock cycle for the next million of years we will have enough of it. A short description of the rock cycle is explained like this:

Magma cools and crystallizes to form igneous rock. Igneous rock undergoes weathering (or breakdown) to form sediment. The sediment is transported and deposited somewhere (such as at the beach or in a delta, or in the deep sea). The deposited sediment undergoes lithification (the processes that turn it into a rock). These include cementation and compaction. As the sedimentary rock is buried under more and more sediment, the heat and pressure of burial cause metamorphism to occur. This transforms the sedimentary rock into a metamorphic rock. As the metamorphic rock is buried more deeply (or as it is squeezed by plate tectonic pressures), temperatures and pressures continue to rise. If the temperature becomes hot enough, the metamorphic rock undergoes melting. The molten rock is called magma. This completes the cycle.

Now how come that the earth will eventually run out of magma? The earth was formed around 4.5 billion years ago by the accumulation of rocky bodies like meteoroids and asteroids. At that time the earth was very hot - mostly molten - and did not have a crust, mantle and core. Over time the core was formed as heavier metals sank out of the magma. The crust is the slowly cooled down mantle on the outside. Now, the earth will eventually cool down completely causing the mantle to solidify. Since the mantle is the source for magma (molten rocks) this mean also no more magma. This has happened to Mercury already but it will take a very long time for the earth to completely cool down.

Answer 2:

About 30 cubic kilometers of magma is erupted onto the surface or emplaced at shallow levels (within a few km of surface) each year on earth. There is roughly a 1: 5 ratio of erupted vs intrusive magma. The Earth has been 'doing this" for the last 4.5 Billion years!!! Overall the Earths OCEANIC CRUST has been made over about 60 times. Oceanic crust has an average age of about 80 million yrs; oceanic crust is shoved back into the Earth (subduction) at the same rate that it is generated. This is the plate tectonic cycle.

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