The Earth's outer mantle is composed of magma.Haven't you asked yourself how? This article will explain the production or formation of magma in the Earth's mantle and its rising or movement towards the Earth’s crust.
What is Magma?
Magma is a hot semi-liquid material that mostly compose the Earth's mantle. It is composed of different minerals with a small percentage of dissolved gasses such as carbon dioxide, water vapor, and sulfur.
Three types of Magma according to Chemical Composition
a. Basaltic Magma/Mafic Magma
It is
composed of silicon dioxide, and a high percentage of iron, calcium, magnesium, and a low percentage of silicon dioxide. Its
temperature is ranging from 10000C to 12000C.
b. Andesitic Magma
It has a moderate amount of silicon dioxide, iron, calcium, magnesium, and silicon dioxide. Its temperature is ranging from about 800oC to 1000oC.
c. Rhyolitic magma
It contains a high percentage of sodium and potassium, but a low percentage of magnesium, iron, and calcium. It has a temperature ranging from 650oC to 800oC.
Formation of Magma during the Earth’s Formation
When was the formation of magma started? When there was
enough primordial heat during the Earth's accretion and differentiation, the
Earth's protoplanets near-surface started to melt, forming a magma ocean. According
to the most accepted terrestrial model, extensive heat from the combination of
primordial and radioactive heat formed a vast magma ocean covering the Earth's
surface. As the Earth cooled, the upper part of the magma ocean crystallized,
forming the Earth's crust while it's lower part remained as hot, liquid magma. The pressure and high
temperature inside the Earth's crust keep the magma in its liquid state. As the
mantle temperature increase due to radioactivity, it causes a more significant
upper mantle melting. This results in the release of more magma on the Earth's
surface, forming thicker crusts.
Special conditions required for the Formation of Magma
1. Decompression melting: reducing pressure at a constant temperature
A low-pressure area has a lower melting point than a
high-pressure area. Magma formation through decompression melting usually happens when a solid mantle melts as its rises to an area with lower pressure through a convection process. The melting occurs because
of decompression or the reduction of pressure.
2. Flux melting (melting of rocks due to the addition of volatiles)
Hot solid rocks usually melt, forming magma, at a lower temperature when added to a volatile substance such as carbon dioxide and water. Volatiles can decrease a rock's melting point and break the chemical bonds in a rock. This happens in subduction zones where water overlying a subducting seafloor lowers the melting point of the mantle. This melts parts of the mantle, generating magma that rises to the subduction zone surface.
3.
Heat transfer melting
This happens when rock melts as it encounters a
rising magma.
References:
https://www.nationalgeographic.org/encyclopedia/magma/#:~:text=Magma%20is%20extremely%20hot%20liquid,planet's%20mantle%20consists%20of%20magma
https://www.intechopen.com/books/earth-crust/introductory-chapter-earth-crust-origin-structure-composition-and-evolution
https://www.nationalgeographic.com/science/earth/surface-of-the-earth/earths-interior/