Describe how magmas are produced at continental rifts. (E)

Imagine you're holding a balloon tightly and then suddenly release some of the pressure. What happens? The balloon expands because the air inside decompresses. A similar process occurs deep within the Earth at continental rifts. As the lithosphere stretches, parts of the mantle experience a decrease in pressure. This drop in pressure, without the need for added heat, can cause the rock to melt, leading to what geologists call decompression melting.

The climb from high pressure to lower pressure settings means that solid mantle material starts to melt, creating magma. Since magma is more buoyant than surrounding solid rocks, it begins to ascend through the crust. This is crucial for forming new igneous rocks and can contribute to volcanic activity at the Earth's surface. Decompression melting is heavily reliant on the tectonic environment; at continental rifts, where the crust is pulled apart, it provides the necessary conditions for this phenomenon to flourish.

Have you ever noticed how warm air rises to the top in a heated room? Well, a somewhat parallel movement takes place beneath the Earth's surface, known as mantle upwelling. When hot mantle material from deep within the Earth ascends towards cooler regions nearer to the Earth's crust, we observe mantle upwelling.

This rise is driven by the difference in temperature and density between the hot mantle rocks and the cooler rocks above them. As the mantle rock moves upwards and pressure decreases, it can partially melt, forming magma. Mantle upwelling is a vital part of the process that creates magmas beneath continental rifts and tends to increase in activity when the overlying lithosphere is stretched or thinned through tectonic processes.

When you stretch a piece of dough, it gets thinner and weaker in the middle, right? A similar process happens during lithospheric thinning when the Earth's tectonic forces pull and extend the lithosphere at continental rifts. The lithosphere is composed of the crust and the very top portion of the mantle, and when it thins, the crust reduces in volume and breaks apart.

Lithospheric thinning is fundamental because it reduces the distance between the surface and the hotter mantle below. This facilitates decompression melting as the pressure is lowered on the mantle rocks. Furthermore, this thinning of the lithosphere alters the thermal gradients, meaning that heat from the mantle can induce additional melting of the lower crustal rocks, further contributing to magma production.

Let's think about making a chocolate sauce. You don't need to melt all the chocolate to get a sauce; just partially melting a portion of it will do the job. This concept can be applied to partial melting in geology. Partial melting occurs when only a fraction of a rock melts.

In this process, distinct minerals with lower melting points melt first. This happens primarily because different minerals have different melting points. Partial melting is essential in forming magmas at continental rifts because it creates melt with a different composition than the original solid rock material. This newly formed magma, which is lighter than the surrounding solid mantle, starts its journey upwards, potentially leading to volcanic eruptions at the Earth's surface.