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The formation of magma

The formation of magma is dependent on three physical parameters which are temperature, pressure, and changes in composition of water and carbon dioxide. Each of these factors may cause rocks to melt, forming magma. An increase of temperature is usually the mechanism for magma formation within continental crust. In addition, it is also the most commonly thought-of reason as to why rocks melt into magma (Tarbuck, et al. 113).

Temperature increases happen due to upward intrusions of rocks from the mantle which can occur due to movement of tectonic plates more commonly known as earthquakes, although these earthquakes don’t necessarily have to be felt on the surface. A decrease in pressure may cause rocks to rise during the convection of the solid mantle. If this happens, the rocks will slightly cool while expanding in an adiabatic process. However, the cooling process is only around 0.3 degrees Celsius per kilometer while the increase of temperature caused by the sudden upward surge can reach up to 4 degrees per kilometer (Tarbuck, et al. 118).

This makes the rocks melt as they rise due to decompression. Adding water in rock composition lowers down the solidus temperature at a given pressure, thereby making it easier for rocks to melt. A similar effect may be observed in the addition of carbon dioxide, although water is the most responsible substance for the creation of magma through composition alteration (Grove et al. 75).

Norman L. Bowen was a petrologist in the early 1900s who worked with powdered rock material that was heated and melted and then allowed to cool to a target temperature at which he recorded the kinds of minerals that were formed in the rocks produced. Repetition of this process at different temperatures led him to formulate a reaction series that is still acceptable today (Hamblin & Christiansen 51). Through Bowen’s reaction series, we can find out the relative conditions in which a particular rock is formed based on the minerals present in it. Bowen’s series is divided into the continuous and the discontinuous branch.

Based on the illustration provided, the minerals at the top are those that crystallize first, followed by the rest along the temperature gradient of high to low. The continuous branch (right) shows the same mineral from high to middle crystallization temperature while the discontinuous branch shows different minerals found per temperature gradient. Simply put, the higher temperature minerals are the easiest to transform into magma and are thus the most unstable on the surface while the low temperature minerals are found in conditions that are similar to the surface and are thus much more stable.

Bowen’s Reaction Series Works Cited: Grove, T. L. Chatterjee, N. Parman, S. W. and Medard, E. “The influence of H2O on mantle wedge melting”. Earth and Planetary Science Letters, v. 249, p. 74-89: 2006. Hamblin, W. K. , and E. H. Christiansen. Earth’s Dynamic Systems. 9th ed. Upper Saddle River: Prentice Hall, 2001. Tarbuck, Edward. D. , Frederick K. Lutgens, and Tasa Dennis. Earth: An Introduction to Physical Geology. 7th ed. Upper Saddle River, NJ: Prentice Hall, 2002.

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