why does high silica magma tend to
form volcanic domes with steep sides
Try to imagine viscous granitic magma , which can be as 'cool' as 700°, flowing out from an eruption. Being viscous, it does not travel very far before it solidifies. This results in the steep domes.
High silica magma, also known as felsic magma, tends to form volcanic domes with steep sides due to its unique physical properties and composition.
1. High viscosity: Felsic magma has a high silica content, which increases its viscosity or resistance to flow. The high viscosity of the magma makes it difficult for gas bubbles to escape, resulting in the buildup of pressure below the surface.
2. Gas-rich composition: Felsic magma tends to be rich in volatiles, such as water vapor and carbon dioxide. These gases are released as the magma rises, leading to explosive eruptions. However, due to the high viscosity of felsic magma, the gas bubbles cannot escape easily, creating a build-up of pressure within the magma.
3. Explosive eruptions: When the pressure within the magma reaches a critical level, it causes a violent and explosive eruption. The high viscosity of felsic magma traps the gas bubbles, causing the pressure to build up rapidly. Instead of flowing smoothly and spreading out, the magma is forcefully ejected in explosive eruptions.
4. Dome formation: During an explosive eruption, the felsic magma is ejected out of the volcanic vent, but it does not spread far due to its high viscosity. Instead, it piles up and accumulates around the vent, forming a steep-sided volcanic dome.
5. Lack of lateral spreading: Unlike low viscosity mafic magma, which can flow easily and spread out, felsic magma's high viscosity inhibits lateral spreading. This results in the magma being confined to a smaller area around the vent, causing it to build up vertically and form steep-sided domes.
In summary, high silica magma forms volcanic domes with steep sides due to its high viscosity, gas-rich composition, explosive eruptions, and limited lateral spreading capability.
High silica magma tends to form volcanic domes with steep sides primarily due to its physical properties and behavior during volcanic eruptions. The high silica content makes the magma more viscous (thick and sticky) compared to low silica magma.
When high silica magma rises to the Earth's surface, it often encounters resistance due to the surrounding cooler rocks and the pressure from the overlying rocks above. Since the magma is thick and less fluid, it has difficulty flowing easily and expelling gases. As a result, the gas bubbles in the magma become trapped, causing the pressure to build up.
The high viscosity combined with the buildup of gas pressure leads to explosive eruptions, where the magma is forcefully ejected from the volcano. However, due to the molten rock's high viscosity, it cannot flow very far before it solidifies and piles up around the volcanic vent.
This accumulation of solidified lava forms a steep-sided dome-shaped structure known as a volcanic dome. The steep sides result from the magma's limited ability to flow outwards and spread horizontally. Instead, it piles up vertically, creating a mountain-like formation with a characteristic dome shape.
The steep sides of volcanic domes also tend to resist erosion more effectively, preserving their shape over time. The solidified lava is more resistant to weathering and erosion compared to less viscous lava types. This factor contributes to the preservation of the steep sides of high silica magma domes even after the volcanic activity ceases.
In summary, high silica magma forms volcanic domes with steep sides because of its high viscosity, which restricts its flow and causes explosive eruptions. The limited horizontal flow results in the vertical accumulation of magma around the vent, forming a dome-shaped structure.