Question

Describe two situations in 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 via decompression or flux melting, relating to divergent and convergent plate boundaries, respectively.

Step-by-step solution

Understand Decompression Melting

Decompression melting occurs when mantle rock rises and the pressure decreases while the temperature remains constant. This happens because the Earth's mantle is under less pressure closer to the surface, allowing rock to melt at lower temperatures. This process is common at mid-ocean ridges, where tectonic plates are diverging.

Explore Flux Melting

Flux melting happens when water or other volatiles are added to rock, lowering the rock's melting point without an increase in its temperature. This occurs at subduction zones, where an oceanic plate sinks beneath a continental or another oceanic plate. The water from the subducting plate lowers the melting point of the mantle above it, causing the mantle rock to melt and form magma.

Connect to Plate Tectonics

In decompression melting, the role of plate tectonics is evident at divergent boundaries where plates are moving apart. The reduced pressure on upwelling mantle allows it to melt, forming new oceanic crust. In flux melting, the tectonic activity at convergent boundaries involves a subducting oceanic plate that releases volatiles into the mantle wedge above it, facilitating melting and contributing to volcanic activity.

Key concepts

Decompression Melting

Decompression melting is a fascinating process where mantle rock melts due to a decrease in pressure rather than an increase in temperature. This phenomenon occurs because the mantle's pressure decreases as it rises towards the Earth's surface. The reduced pressure lowers the melting point of the rock, allowing it to transform into molten magma.

This type of melting frequently takes place at mid-ocean ridges, which are underwater mountain ranges formed by tectonic plates that are pulling apart. As the tectonic plates diverge, mantle material ascends to fill the gap. The rising mantle then experiences decompression, leading to melting.

• The mantle's asthenosphere rises to the surface at mid-ocean ridges.
• The pressure decrease allows mantle rock to melt without needing increased heat.
• This creates new oceanic crust, contributing to the ocean floor's dynamic nature.

Through decompression melting, we see the direct link between patterns of plate tectonics and the cycling of geological material on our planet.

Flux Melting

Flux melting is another exciting magma generation process that occurs when volatiles like water are introduced to hot mantle rock. These volatiles significantly lower the melting point of the rock, resulting in its melting without an increase in temperature. This mechanism is closely associated with subduction zones, which are areas where one tectonic plate is sliding below another.

In these zones, an oceanic plate subducts under a continental or another oceanic plate. The subducting plate carries water into the mantle from the ocean floor, where the added volatiles lower the melting point of the overlying mantle.

• Water from the subducting plate seeps into the mantle.
• Adding water lowers the effective melting temperature of the mantle.
• This process is crucial for the formation of volcanic arcs and island chains.

As a result, magmas generated by flux melting contribute to some of the most explosive volcanic activities on Earth.

Plate Tectonics

Plate tectonics is the grand, interconnected system that governs the movement of Earth's lithospheric plates. It plays a key role in both decompression and flux melting processes, highlighting how dynamic Earth truly is.

In decompression melting, the divergent boundaries where plates move apart are sites of new crust generation. Here, mantle rock is exposed to oceanic depressurization, leading to magma formation. This new crust pushes oceanic plates apart over time, reshaping the ocean floor.

• Divergent boundaries create new oceanic crust by decompression melting.
• Convergent boundaries involve flux melting due to subducting plates.
• Tectonic processes recycle crustal materials, influencing Earth's topography.

Understanding plate tectonics allows us to comprehend the sites and mechanisms of magma generation, offering insights into the planet’s geological history and ongoing geological activity.