Question

Describe the three processes that are responsible for the formation of magmas.

Short answer

The three processes responsible for the formation of magmas are: 1. Decompression Melting: This occurs when pressure on hot mantle rock decreases without significant change in temperature, often due to tectonic plate movement. This causes the rock to melt and form magma, commonly seen at divergent plate boundaries like mid-ocean ridges. 2. Flux Melting: This takes place when water is added to hot rock, lowering its melting temperature and causing it to melt into magma. This is typically seen at convergent plate boundaries, where oceanic crust subducts beneath another plate, releasing water and causing the overlying mantle to partially melt. 3. Heat Transfer Melting: This happens when mantle rocks are heated by intense heat from the Earth's core or an existing magma chamber, causing them to melt and form magma. This process is observed in hot spot volcanoes, where a plume of hot material rises through the lithosphere, forming magma as seen in the Hawaiian Islands.

Step-by-step solution

1. Melting of the mantle by lowering the pressure (Decompression Melting)

Decompression melting occurs when the pressure on hot mantle rock is decreased without a significant change in temperature. This most commonly happens as the tectonic plates move, causing the underlying mantle to rise and release pressure that was keeping the rock solid. As the pressure decreases, the melting temperature of the rock also decreases, allowing it to transform from solid to liquid and form magma. This is the primary mechanism for magma formation at divergent plate boundaries, such as mid-ocean ridges.

2. Melting by the addition of water (Flux Melting)

Flux melting occurs when water is added to hot rock, lowering its melting temperature and causing it to transform into magma. This is most common at convergent plate boundaries, where a tectonic plate carrying oceanic crust subducts beneath another plate. The water within the oceanic crust is released upon subduction, causing the overlying mantle to partially melt and form magma. This process of flux melting is responsible for the formation of magma in subduction zones and volcanic island arcs, such as the Pacific Ring of Fire.

3. Melting by increasing temperature (Heat Transfer Melting)

Heat transfer melting occurs when the mantle rocks are heated by the intense heat from the Earth's core or from an already existing magma chamber. The intense heat raises the temperature of the surrounding mantle rocks beyond their melting temperature, causing them to liquefy and form magma. This process can be seen in hot spot volcanoes, where a plume of hot material from the lower mantle rises up through the Earth's lithosphere, causing the formation of magma. An example of a hot spot volcano is the Hawaiian Islands, which are formed as the tectonic plate moves over the stationary hot spot.

Key concepts

Decompression Melting

When it comes to understanding how magma forms through decompression melting, think of it as relieving pressure on hot rocks beneath the Earth's surface. Imagine a camper inflating a balloon and letting it go to watch it zoom away. As the pressure inside the balloon drops, the air escapes quickly.
Likewise, in decompression melting, when the pressure on hot mantle rock decreases, the rocks begin to melt even if their temperature hasn't changed much. This is because the melting point of rocks drops with decreasing pressure.

• It's a primary process at divergent plate boundaries like mid-ocean ridges.
• The mantle rock, under high temperatures, rises towards the Earth's surface.
• The reduced pressure allows the rock to melt, forming magma without needing to raise its temperature.

This fascinating process illustrates how tectonic movements help churn the Earth's crust, leading to magma formation.

Flux Melting

Flux melting adds a different twist to how magma is created - through the addition of water. Visualize a baker adding water to flour to create dough: the water enables the dry ingredients to hold together and change form.
Similarly, in flux melting, water plays a big role in helping rock melt. Here’s how it works:

• This process occurs mainly at convergent plate boundaries.
• When an oceanic plate subducts beneath another plate, water trapped in the minerals is released due to increasing temperature and pressure.
• The released water lowers the melting temperature of the overlying mantle rock, causing it to melt into magma.

Commonly associated with volcanic activity in subduction zones, flux melting can lead to explosive volcanic eruptions in regions like the Pacific Ring of Fire.

Heat Transfer Melting

Heat transfer melting happens when heat from the Earth's core or existing magma raises the temperature of surrounding rocks to their melting point. Picture a glowing ember in a campfire igniting nearby pieces of wood. The wood catches fire from the intense heat without coming in direct contact with the ember.
In geologic terms, this process comprises

• Transferring heat from hot, partially molten material to surrounding cooler rocks.
• The temperature increase causes the surrounding rocks to reach their melting temperature and form magma.
• This process can be seen prominently in hot spot volcanic settings, like the Hawaiian Islands.

Heat transfer melting explains how geothermal dynamics within the Earth leads to magma formation, even without tectonic plate interactions.