Magma and Volcanism Overview

Study Notes

Earth's internal heat drives geological processes, including volcanism and plate tectonics
Rocks on Earth's surface are typically solid, but rocks exist in a liquid state deep within the Earth
Molten rocks beneath the Earth's surface are called magma
Magma is less dense than surrounding solid rock, enabling it to rise towards the surface
When magma reaches the surface, it is called lava
Magma's ability to flow is called viscosity
Viscosity is impacted by silica content; higher silica equals higher viscosity
Higher temperature magma has lower viscosity, flowing more readily
Magmas form deep inside the crust and upper mantle when conditions are right for pre-existing solid rock to melt
Three ways magma is generated:
- Decompression melting: Temperature stays the same, but pressure decreases
- Flux melting: Addition of volatile (gaseous) substances to hot solid rock
- Heat transfer melting: Very hot magma brings additional heat to surrounding rock, causing it to melt

Rocks melt over a range of temperatures
The melting temperature of rock (eutectic temperature) is lower than its constituent minerals
Partial melting occurs when only certain minerals in a rock melt
The rock completely melts when temperature reaches a sufficient level

If magma separates from rock that hasn't fully melted, the magma will contain components that melted at lower temperatures, while the remaining rock will contain components with higher melting temperatures

Volcanoes are visible manifestations of rock formation
A volcano's vent acts as a conduit for lava/molten rock to reach the surface
The funnel-shaped depression where materials are ejected is called a crater
Eruptions are events when lava spews out of a volcano
Two types of eruptions:
- Effusive: Dominated by lava flow and lava fountains
- Explosive: Ejects ash and pyroclastic fragments, forming ash clouds
Lava composition impacts eruption type
- Low silica = less viscous, effusive eruptions
- High silica = more viscous, explosive eruptions

Shield volcanoes: Broad, dome-shaped with gentle slopes, covering wide areas
Cinder cone volcanoes: Conical hills composed of granular materials formed by lava fountains erupted from a single vent
Stratovolcanoes (composite volcanoes): Steep-sided, cone-shaped; formed by viscous, sticky lava

Metamorphism: The process where a rock's form changes into a new one without melting
Metamorphism implies change occurs in a solid state.
Metamorphism involves processes over thousands to millions of years
Three main processes:
- Recrystallization: Changes in shape and size of minerals without changing the mineral's identity
- Phase change: Transformation of one mineral grain into another with the same composition but different crystal structure (e.g., coesite from quartz)
- Neocrystallization: Growth of new minerals differing from those in the protolith (original rock)
- Pressure solution: Mineral grains dissolve under pressure in the presence of water
- Plastic deformation: Mineral grains become flattened or elongated without altering composition or structure due to high temperature and pressure

High temperatures cause atoms to vibrate rapidly, stretching and bending chemical bonds
Bonds breaking allows atoms to detach and form new bonds with other atoms, leading to rearrangement within the mineral or migration into other minerals
Recrystallization and neocrystallization are consequences of atomic rearrangement

Contact metamorphism: Rock minerals and textures changed by heat due to contact with magma
Regional metamorphism: Rock minerals and textures changed by pressure over a wide area or region (e.g., from converging tectonic plates)