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

Mantle rock can melt through decompression melting and flux melting, both related to tectonic plate movements. Decompression occurs at divergent boundaries, while flux melting happens at convergent boundaries.

Step-by-step solution

Pressure Reduction and Decompression Melting

Mantle rock can melt through a process called decompression melting. As the pressure decreases, which happens when tectonic plates move apart at mid-ocean ridges or rift valleys, the solid rock is able to reach its melting point even though the temperature remains constant. This is because the decreased pressure lowers the melting point of the rock.

Addition of Volatiles and Flux Melting

Another mechanism is flux melting, which involves the addition of volatiles such as water and carbon dioxide. These volatiles lower the melting point of mantle rock and cause it to melt, even without an increase in temperature. This typically occurs at subduction zones where oceanic plates dive beneath continental plates, carrying water-rich sediments down into the mantle.

Relationship with Plate Tectonics

These processes are intrinsically linked to plate tectonics. Decompression melting is common at divergent boundaries where plates are moving apart, allowing the mantle to ascend and melt due to lower pressure. Flux melting occurs at convergent boundaries where subducting plates introduce volatiles into the mantle, leading to melting and magma formation.

Key concepts

Decompression Melting

Decompression melting is a fascinating process that occurs when mantle rock melts due to a reduction in pressure. Imagine the mantle as a solid rock under immense pressure deep within the Earth. When tectonic plates move apart, such as at divergent boundaries like mid-ocean ridges or rift valleys, this pressure is reduced. Although the temperature remains constant, the decrease in pressure allows the rock to melt because its melting point is lowered. This process is essential for the generation of new oceanic crust as mantle rock melts, creating magma that rises to form new crust at these boundaries.

Flux Melting

Flux melting is another intriguing mechanism that allows mantle rock to melt without raising the temperature. This process involves the addition of substances known as volatiles—usually water and carbon dioxide. These volatiles play a crucial role in lowering the melting point of mantle rock. Typically, this occurs at subduction zones, where an oceanic plate slides beneath a continental plate. As the oceanic plate descends, it brings water-rich sediments into deeper, hotter parts of the mantle. The added volatiles cause the mantle rock to melt, creating magma that can lead to volcanic activity.

Plate Tectonics

The theory of plate tectonics is a cornerstone of understanding Earth's geology. It explains how large plates of the Earth's lithosphere move and interact with each other. These interactions are dynamic and responsible for various geological phenomena. For instance, tectonic plates can move apart at divergent boundaries, come together at convergent boundaries, or slide past each other at transform boundaries. These movements not only shape the Earth's surface but also drive processes like decompression and flux melting, leading to magma formation and volcanism.

Subduction Zones

Subduction zones are fascinating areas where two tectonic plates meet, and one plate is forced or "subducts" beneath the other. Typically, an oceanic plate subducts beneath a continental plate due to density differences. This process is responsible for intense geological activity, including earthquakes and volcanism. As the oceanic plate descends, it carries water and sediments into the mantle, facilitating flux melting. The introduction of volatiles enables the mantle rock to melt at lower temperatures, producing magma that can form powerful volcanic eruptions.

Divergent Boundaries

Divergent boundaries are regions where tectonic plates are moving apart. These boundaries are commonly found along mid-ocean ridges and continental rift zones. As the plates separate, mantle rock from below ascends to fill the gap. The reduced pressure at these boundaries causes decompression melting of the mantle rock, resulting in magma formation. This newly formed magma solidifies to create new oceanic crust, contributing to the continuous renewal and spreading of the ocean floor. Divergent boundaries play a crucial role in the Earth's plate tectonic cycle and help drive the continuous process of crustal formation.