Question

Describe the three processes responsible for the formation of magmas. (C)

Short answer

The three main processes responsible for the formation of magmas are partial melting of the mantle and crust, decompression melting, and flux melting. Partial melting occurs when variations in temperature, pressure, and composition cause a fraction of solid mantle or crust to turn into magma. Decompression melting happens when solid mantle material rises upward, reducing pressure and causing the material to melt and form magma, commonly at divergent plate boundaries and hotspots. Flux melting occurs at subduction zones, where volatile components are introduced and lower the melting point of the surrounding rocks, leading to magma formation.

Step-by-step solution

1. Partial Melting of the Mantle and Crust

Partial melting is the process where only a fraction of a solid material, such as the Earth's mantle or crust, turns into a liquid (magma). This happens due to variations in temperature, pressure, and composition in the Earth's mantle and crust. As solid rock in the mantle or crust rises towards the surface, it can experience a decrease in pressure. This decrease in pressure, combined with the presence of water or other volatiles that lower the melting point of the rock, can cause partial melting and the formation of magma.

2. Decompression Melting

Decompression melting occurs when solid mantle material rises upward due to convection currents. As the pressure decreases with depth, the mantle material's melting point also decreases, leading to melting or the creation of magma. This process occurs mainly at divergent plate boundaries, where tectonic plates are pulling apart, and at hotspots, like volcanic islands, where mantle plumes bring hot material up from the deep mantle.

3. Flux Melting

Flux melting is a process where magma is generated due to the introduction of volatile components, such as water or carbon dioxide, which lowers the melting point of the surrounding rocks. This process typically occurs at subduction zones, where one tectonic plate is pushed under another and the subducting plate releases water and other volatile materials into the overlying mantle. The presence of these volatiles lowers the melting point of the mantle rock, resulting in the formation of magma. In summary, the three main processes responsible for the formation of magmas are partial melting of the mantle and crust, decompression melting, and flux melting. Each of these processes occurs in different tectonic settings and contributes to the creation of magma, which can eventually lead to volcanic eruptions and the formation of new crust.

Key concepts

Partial Melting of Mantle and Crust

Imagine the Earth’s interior as a colossal but intricate oven that heats up rocks. But not all rocks will melt uniformly; some will start to transform into a viscose, molten material while adjacent parts remain solid. This phenomenon is known as partial melting. In this process, variations in temperature, pressure, and rock composition play crucial roles. Generally, where temperatures are high but not enough to melt the rock entirely, or where pressure drops as rocks ascend towards the surface, partial melting can occur. Add water or other volatile substances into the equation, which aid in lowering the melting points of these rocks, and a recipe for creating magma is at hand. Such magmatic material then poses a risk of erupting through volcanoes or forming igneous rocks upon solidification.

Decompression Melting

When you open a bottle of soda and watch the fizz arise, you're observing a release of pressure – leading to the escape of gas. A similar principle applies to decompression melting beneath Earth’s surface. Here, the solid mantle material experiences a decrease in pressure as it moves upward, mostly because of the natural convective motions within the mantle. Like unscrewing a soda cap, the drop in pressure diminishes the melting point of the mantle material. It’s most common at divergent boundaries – where tectonic plates drift away from each other – or at hotspots, which are like Earth's natural chimneys that transport heat from deep below. When the conditions are right, melting ensues, adding to Earth’s cache of magma.

Flux Melting

Flux melting is akin to adding salt to ice; it generally lowers the freezing point, allowing ice to melt at temperatures it would otherwise remain solid. In geologic terms, volatile substances like water or carbon dioxide have the 'salt effect' by lowering the melting point of rocks. This process is notably active in subduction zones, where an oceanic plate submits under a continental plate, dragging water and other volatiles with it into the mantle. This 'subducted water' then acts as the flux, prompting the overlying rocks to melt at lower temperatures than they typically would, thereby generating magma. This magmatic activity is a fundamental part of the earth’s continuous cycle of crust creation and destruction.

Tectonic Settings

The formation of magma is intricately connected to the restless dance of tectonic plates. It’s the layout and movement of these gigantic Earth slabs that set the stage for where and how magma is formed. Divergent boundaries give us decompression melting, as seen in the mid-ocean ridges; convergent boundaries set the scene for flux melting, shown by the volcanic arcs that form alongside subduction zones; and transform boundaries, where plates slide past one another, might not directly create magma but can influence the stress and heat flow that contribute to magma formation elsewhere. Each of these tectonic settings provides a different mechanism and environment for the creation of the magmatic material underlying much of Earth’s dynamic geological activity.

Volcanic Activity

The grand finale of magmatic processes often takes center stage in the form of volcanic activity. Magma that makes its way toward the Earth’s surface eventually has to go somewhere, and that’s where volcanoes come into play. These fiery natural features are the Earth's way of releasing internal heat and pressure. When the stored energy in the magma chamber exceeds the strength of the overlying rock, eruptive activity occurs. Volcanic eruptions can be explosive, sending ash and lava high into the atmosphere, or effusive, with lava gently spilling out of the vent. The aftermath contributes to the shaping of landscapes, forming new land, and even affecting climate patterns. Volcanic activity is not merely a spectacle but also a poignant reminder of the planet’s constantly shifting nature.