When magma is heated in the mantle, it becomes less dense due to thermal expansion. As the temperature of the magma increases, the molecules within it begin to move more vigorously, causing the magma to occupy a larger volume. This decrease in density compared to the surrounding cooler, denser mantle rocks causes the heated magma to rise.
Direction of Movement:
- Movement Towards the Surface: The less dense magma rises through the denser surrounding material. This upward movement is driven by buoyancy, as the molten rock seeks to move towards areas of lower pressure, typically found closer to the Earth's surface.
- Formation of Magma Chambers: As it ascends, the magma may accumulate in pockets called magma chambers, which can eventually lead to volcanic activity if the pressure becomes sufficient for the magma to break through the surface.
Changes in Density:
- Heating: As magma is heated, its density decreases. For example, basaltic magma is less dense than the surrounding peridotite mantle, allowing it to rise.
- Composition Variation: The composition of the magma (such as the amount of dissolved gases or crystals present) can also change its density. For instance, magma that is more silica-rich tends to be more viscous and can also have different density characteristics compared to more mafic (basaltic) magmas.
In summary, heated magma in the mantle rises due to its decreased density compared to the surrounding mantle, driven by buoyancy and the pressure dynamics within the Earth.