Overview
This lecture covers magma's chemical composition, types, formation processes, and how these influence volcanic activity and eruptions.
Magma: Definition & Basic Properties
- Magma is molten or semi-molten rock found beneath Earth's surface.
- When magma reaches the surface it is called lava; when it solidifies, it becomes igneous rock.
- Magma temperatures range from 700°C to 1,300°C.
- Main elements in magma: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.
- Oxygen is the most abundant element, followed by silicon.
Magma Formation Processes
- Magma forms mainly in Earth's lower crust and upper mantle.
- Decompression melting occurs when rising mantle rock enters areas of lower pressure, usually at divergent boundaries and mantle plumes (hot spots).
- Heat transfer melting occurs at convergent boundaries when hot rock intrudes and melts cooler surrounding crust.
- Flux melting happens when water or other volatiles lower the melting temperature of rock, common in subduction zones.
Types of Magma
- Basaltic (mafic) magma: Low in silica (45-55%), high in iron and magnesium, forms at mid-ocean ridges and oceanic hot spots.
- Andesitic (intermediate) magma: Intermediate silica (55-65%), forms from mixing or partial crustal melting.
- Rhyolitic (felsic/granitic) magma: High in silica (65-75%), low in iron and magnesium, forms from melting continental crust.
Magma Composition & Eruption Style
- Silica content affects magma’s viscosity (resistance to flow).
- Higher silica = higher viscosity; lower temperature also increases viscosity.
- Gas content, mostly water and some carbon dioxide, influences eruption explosiveness.
- Basaltic magma is less viscous and more fluid; rhyolitic magma is highly viscous and can lead to explosive eruptions.
Key Terms & Definitions
- Magma — Molten rock beneath Earth’s surface.
- Lava — Magma that reaches Earth’s surface.
- Igneous rock — Solidified magma or lava.
- Decompression melting — Magma formation due to reduced pressure.
- Heat transfer melting — Magma formation by heat transfer from hot to cool rock.
- Flux melting — Melting caused by volatiles lowering rock's melting point.
- Viscosity — A fluid's resistance to flow; higher viscosity means thicker fluid.
Action Items / Next Steps
- Review the chemical composition and three types of magma.
- Be able to explain the three main magma formation processes.
- Prepare for questions about how magma composition affects volcanic eruptions.