The physical features of magma include its composition (liquid rock with suspended solid crystals and dissolved gases), phase (solid, liquid, and gas), and behavior related to temperature and pressure.
Composition and Phase
Magma isn't just liquid rock. It's a complex mixture typically composed of:
- Liquid Rock: This is the main component, consisting of various molten silicate minerals.
- Suspended Solid Crystals: Minerals that have already begun to solidify within the magma. The type and amount of crystals depend on the magma's composition and temperature.
- Dissolved Gases: Gases like water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂) are dissolved within the liquid rock. These gases play a crucial role in volcanic eruptions.
As magma rises toward the surface, the surrounding pressure decreases. This pressure drop causes the dissolved gases to come out of solution, forming bubbles. Therefore, magma near the surface is a mixture of solids, liquids, and gases.
Temperature and Pressure Effects
The physical characteristics of magma are highly dependent on temperature and pressure:
- Temperature: Higher temperatures result in a less viscous magma (more fluid-like), while lower temperatures lead to a more viscous magma (more resistant to flow). Different magmas have different temperature ranges at which they remain liquid.
- Pressure: High pressure keeps gases dissolved within the magma. As pressure decreases, gases are released, increasing the volume of the magma and contributing to explosive eruptions.
Viscosity
Viscosity is a critical physical property of magma, defining its resistance to flow.
- High Viscosity: Magmas with high viscosity (like rhyolitic magma) are thick and pasty. They tend to trap gases, leading to explosive eruptions.
- Low Viscosity: Magmas with low viscosity (like basaltic magma) are runny and allow gases to escape more easily, leading to effusive eruptions (lava flows).
Viscosity is affected by:
- Temperature: Higher temperatures decrease viscosity.
- Composition: Higher silica content increases viscosity.
- Crystal Content: A higher proportion of crystals increases viscosity.
Density
Magma density is another important physical property.
- Lower Density: Magma is less dense than the surrounding solid rock. This density difference is the primary reason magma rises towards the surface.
- Composition: The chemical composition of the magma affects its density. Magmas rich in iron and magnesium are generally denser.
- Temperature: Higher temperature leads to lower density.
- Presence of Gas Bubbles: The presence of gas bubbles decreases the density of magma.
In summary, the physical features of magma are best characterized by its multi-phase composition, its temperature and pressure-dependent viscosity, and its relatively lower density compared to surrounding rock, which drives its movement toward the Earth's surface.
