Overview
This lecture explores how volcanoes form and erupt, using Paricutin as an example, and examines the geological factors and processes behind volcanic eruptions.
The Formation of Volcanoes
- Paricutin volcano in Mexico began as a crack emitting gas and rocks, eventually covering 200 km² with lava and ash.
- Volcanoes form where magma (molten rock) accumulates beneath the Earth’s surface, often due to ocean water lowering the mantle’s melting point.
Forces Governing Magma and Eruptions
- Magma is held beneath the surface by a balance of three forces: lithostatic pressure, magmastatic pressure, and rock strength.
- Lithostatic pressure is the weight of the crust pressing down on magma.
- Magmastatic pressure is the upward force of magma pushing against the crust.
- The rock strength of the Earth’s crust resists these pressures to keep magma contained.
Causes of Eruptions
- Eruptions are often triggered when magmastatic pressure increases due to gas bubbles forming in magma.
- High concentrations of dissolved gases like water or sulfur become bubbles, which lower magma density and increase its buoyancy.
- Gas bubbles can form when new, gas-rich magma rises or as magma cools and minerals crystallize.
- Eruptions can also occur if lithostatic pressure drops, such as through landslides, erosion, or melting glaciers (unloading).
- Weakened crust, due to hydrothermal alteration or tectonic activity, can also lead to eruptions.
Predicting and Studying Eruptions
- Measuring changes in magmastatic pressure is challenging due to the depth and heat of magma chambers.
- Scientists use thermal imaging, spectrometers, and lasers to detect magma movement, gas emissions, and surface deformation.
Key Terms & Definitions
- Magma — molten rock beneath the Earth’s surface.
- Lithostatic pressure — downward force from Earth’s crust on magma.
- Magmastatic pressure — upward force magma exerts on the crust.
- Rock strength — the crust’s ability to resist magma and pressure.
- Unloading — reduction of lithostatic pressure due to loss of overlying rock.
- Hydrothermal alteration — weakening of rock by acidic gases and heat from magma.
Action Items / Next Steps
- Review lecture notes on volcano formation and eruption triggers.
- Read assigned textbook chapter on volcanic processes and hazards.