Question

Describe two mechanisms by which mantle rock can melt without an increase in temperature. How do these magma-generating mechanisms relate to plate tectonics?

Short answer

Decompression melting and flux melting are key mechanisms that melt mantle rock without increased temperature, related to divergent and convergent plate boundaries, respectively.

Step-by-step solution

Understanding Decompression Melting

One common mechanism for mantle rock melting without an increase in temperature is decompression melting. This phenomenon occurs when the pressure on the mantle is reduced, allowing it to melt at the same temperature. In regions where tectonic plates are diverging, such as at mid-ocean ridges, the mantle is subjected to lower pressure as it rises towards the Earth's surface. This reduction in pressure allows the mantle rock to melt, forming magma beneath these ridges.

Exploring Flux Melting

Another mechanism is flux melting, which involves the addition of volatiles (such as water and carbon dioxide) into the mantle. These substances lower the melting point of the mantle rocks, causing them to melt without a temperature increase. This process is common in subduction zones where an oceanic plate slides beneath a continental plate, carrying water into the mantle. The water lowers the melting point of the mantle wedge above the subducting slab, leading to partial melting and magma formation.

Relating Mechanisms to Plate Tectonics

Both decompression melting and flux melting are closely related to plate tectonics. Decompression melting is typically associated with divergent plate boundaries where plates move apart and allow mantle material to rise and melt. Flux melting is linked with convergent plate boundaries, particularly subduction zones, where one plate is forced under another. Here, water released from the subducting plate induces melting in the overlying mantle. Thus, these melting processes are intrinsically tied to the movement and interaction of tectonic plates.

Key concepts

Decompression Melting

Decompression melting is an intriguing process where mantle rock melts without any increase in temperature. Imagine the Earth's mantle as a hot, solid layer with immense pressure bearing down on it. When this pressure is suddenly relieved, the rock can begin to melt.
This usually happens at mid-ocean ridges where tectonic plates are moving apart. As these plates diverge, they allow the underlying mantle rock to ascend.

• As the rock rises, the pressure decreases.
• Lower pressure means the melting point of the rock decreases.
• The mantle material then partially melts, producing magma.

This magma can eventually find its way to the surface, resulting in volcanic activity at these spreading centers. Decompression melting is a direct consequence of the dynamic nature of plate tectonics, which continually reshapes the Earth's lithosphere.

Flux Melting

Flux melting is another fascinating process where rocks can melt without an increase in temperature. This mechanism involves the addition of volatiles such as water and carbon dioxide to mantle rocks. These volatiles significantly lower the melting point of the rocks.
In subduction zones, where an oceanic plate subducts or dives beneath a continental plate, water trapped in the sediments and minerals is carried deep into the mantle.

• The subducting slab carries water into the mantle.
• Water acts as a flux, reducing the melting point of adjacent rocks.
• This leads to partial melting of the mantle wedge above the subducting slab.

This process generates magma that can rise to form volcanoes, typically seen along continental margins. Flux melting is intricately linked to the convergent boundaries of tectonic plates, highlighting a unique intersection of physics and geology.

Plate Tectonics

Plate tectonics is a grand theory that explains the movement of Earth's lithosphere, which is divided into several massive plates. These plates constantly interact with one another: they can move apart, collide, or slide past one another. This dynamic activity is the driving force behind many geological processes, including mantle rock melting.
Understanding how decompression and flux melting relate to tectonics helps us appreciate the vast scale and complexity of Earth's geological systems.

• At divergent boundaries, plates move apart, leading to decompression melting.
• At convergent boundaries, subducting plates initiate flux melting.
• The movements of these plates shape the Earth’s surface and influence volcanic activity.

Plate tectonics serves as the unifying framework that ties together these fascinating processes of rock melting and helps explain the formation of new crust and volcanic arcs on our planet.