Question

Why do the gases in magma not form gas bubbles when the magma is deep within the crust?

Short answer

The gases in magma do not form gas bubbles when the magma is deep within the crust because the high pressure exerted on the magma at those depths keeps the gases dissolved in the magma and prevents them from forming gas bubbles. As the magma ascends and the pressure decreases, the gases become less soluble, allowing gas bubbles to form.

Step-by-step solution

Understanding Magma and Gases

Magma is a molten or partially molten mixture of minerals, gases, and sometimes crystals, which is found deep within the Earth's crust. The gases within magma are primarily composed of water vapor, carbon dioxide, and sulfur dioxide. These gases remain dissolved in the magma under high pressure conditions deep inside the Earth.

Formation of Gas Bubbles

The formation of gas bubbles within magma largely depends on two factors: the solubility of the gases and the pressure exerted on the magma. The solubility of gases in magma decreases with decreasing pressure, while the pressure exerted on the magma generally decreases as it ascends towards the Earth's surface.

Pressure and Solubility Relationship

When the magma is deep within the Earth's crust, the pressure exerted on it is extremely high due to the weight of overlying rock. This high pressure keeps the gases dissolved in the magma and prevents them from forming gas bubbles. The relationship between pressure and solubility can be described using Henry's Law, which states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

Effect of Depth on Gas Bubble Formation

As the magma rises through the Earth's crust towards the surface, the pressure on the magma decreases. The decrease in pressure causes the solubility of the gases within the magma to decrease as well. As a result, gas bubbles have the potential to form within the magma as it ascends. To summarize, the reason why gases in magma do not form gas bubbles when the magma is deep within the crust is due to the high pressure exerted on the magma at those depths. The high pressure keeps the gases dissolved in the magma and prevents them from forming gas bubbles. However, as the magma ascends and the pressure decreases, the gases become less soluble, allowing gas bubbles to form.

Key concepts

Magma Pressure

Deep within the Earth's crust, magma experiences extremely high pressure. This pressure is due to the immense weight of the rocks and materials above it.
This high pressure environment plays a significant role in how gases behave when trapped within magma.
When magma is situated at great depths, the pressure helps keep gases dissolved within it, preventing them from forming separate gas bubbles.

• High pressure is capable of keeping gases in a dissolved state within the magma.
• The deeper the magma, the greater the pressure due to more overlying rock layers.
• This pressure acts as a force holding the gases in place.

Understanding the pressure in magma helps explain why gas bubbles generally do not form until the magma moves closer to the Earth's surface.

Gas Solubility

Gas solubility refers to the ability of a gas to dissolve in a liquid, such as the gases dissolved in magma. Solubility is a crucial factor when discussing volcanic gases and how they transition from a dissolved state to forming bubbles.

• Gas solubility decreases when pressure decreases; this is why lifting pressure allows gases to separate and form bubbles.
• Solubility is greatly influenced by the type of gas as well as environmental conditions—water vapor, carbon dioxide, and sulfur dioxide are common gases in magma.

When magma is deep under the surface, the high pressure ensures that the gas solubility is high, meaning gases remain dissolved. As magma moves upwards during volcanic activity, the pressure decreases, reducing solubility and leading to bubble formation.

Henry's Law

Henry's Law provides a scientific explanation for the solubility behavior of gases. The law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.
In the context of magma, as long as the pressure remains high deep under the Earth's crust, the gases stay dissolved.

• Higher pressure equates to higher solubility, keeping gases dissolved in magma.
• As pressure decreases, as seen when magma rises, the solubility lessens and bubbles are likely to form.

Henry's Law is crucial for understanding how changes in pressure affect gas behavior in volcanic processes. It helps predict when and where gas bubbles might start forming as magma makes its journey towards the Earth's surface.