Question

In which of the following tectonic settings does magma form primarily as a consequence of decompression melting?

a. Mid-ocean ridges
b. Subduction zones
c. Transform faults
d. Continental rift zones

Answer 1

Magma forms primarily by decompression melting at mid-ocean ridges, where tectonic plates diverge and reduce pressure on the underlying mantle, leading to the formation of basaltic lava. This process is distinct from the formation of magma at subduction zones, which occurs via a different mechanism.

Step-by-step explanation:

Magma forms primarily as a consequence of decompression melting in tectonic settings at mid-ocean ridges. Tectonic plates diverge, or pull apart, causing the mantle underneath to experience reduced pressure. The decrease in pressure can result in the melting of the mantle if it is already sufficiently warm, leading to the creation of basaltic lava. This process characteristically occurs at mid-ocean ridges like the Mid-Atlantic Ridge, where two oceanic plates are moving away from each other.

In contrast, magma forms at subduction zones where one tectonic plate slides beneath another and is subjected to increased temperatures and pressure, causing the hydrated minerals in the rock to release water and lower the melting point of the overlying mantle, leading to magma formation. Transform faults are not generally associated with magma formation since they involve two tectonic plates sliding past each other horizontally. Similarly, continental rift zones may experience some magma formation due to the thinning of the crust and associated decompression melting, yet the classic scenario for decompression melting is best exemplified at mid-ocean ridges.

Answer 2

Magma forms primarily as a consequence of decompression melting at Mid-ocean ridges. Option A is the correct answer.

Step-by-step explanation:

Decompression melting occurs when hot rock rises toward the surface, experiencing a decrease in pressure. This decrease in pressure causes the rock's melting point to lower, leading to partial melting and the formation of magma.

Mid-ocean ridges are divergent plate boundaries where tectonic plates move away from each other. This allows hot mantle material to rise towards the surface, experiencing significant decompression melting and generating large amounts of magma. This magma eventually erupts on the seafloor, solidifying as new oceanic crust.

Subduction zones, where one plate dives beneath another, experience a different process called flux melting. Here, fluids released from the subducted plate lower the melting point of the surrounding rock, leading to magma formation.

Transform faults, boundaries where plates slide past each other, do not experience significant pressure changes and don't play a major role in magma generation.

Continental rift zones, where continental plates are pulling apart, can experience some decompression melting as the plates move away from each other. However, the amount of magma generated is typically much smaller compared to mid-ocean ridges.

Therefore, due to the significant role of decompression melting in magmatic activity, mid-ocean ridges are the primary tectonic setting where magma forms primarily as a consequence of this process.

Option A is the correct answer.