🌋Volcanology Unit 2 – Magma Generation and Properties

What's Magma Anyway?

• Molten rock beneath Earth's surface consists of liquid rock, crystals, and dissolved gases
• Magma forms from the melting of existing rock in the Earth's mantle or crust
• Primary magmas originate from the partial melting of mantle rocks (peridotite)
• Magma composition varies depending on the source rock and degree of partial melting
• Magma is less dense than the surrounding solid rock enabling it to rise through the crust
• As magma cools and crystallizes, it forms igneous rocks (granite, basalt)
• Dissolved gases in magma play a crucial role in driving volcanic eruptions

Where Does Magma Come From?

• Magma primarily originates in the Earth's upper mantle and lower crust
• Partial melting of mantle peridotite produces primary magmas
  • Occurs at depths between 60-100 km
  • Triggered by a decrease in pressure or an increase in temperature
• Decompression melting happens when hot mantle rock rises and experiences a decrease in pressure
• Heat transfer from hot mantle plumes or upwelling asthenosphere can induce melting
• Subduction zones generate magma through fluid-induced melting and decompression
• Magma can also form by the melting of existing crustal rocks (anatexis)
• The asthenosphere, a layer of the upper mantle, is a primary source region for magma generation

Magma Composition and Types

• Magma composition depends on the source rock and degree of partial melting
• Mafic magmas (basaltic) are rich in magnesium and iron, originating from the mantle
  • Low silica content (45-52%), high temperature, low viscosity
• Felsic magmas (rhyolitic) are rich in silica, aluminum, and alkali metals, derived from the crust
  • High silica content (>63%), lower temperature, high viscosity
• Intermediate magmas (andesitic) have compositions between mafic and felsic
• Ultramafic magmas (komatiitic) are extremely rich in magnesium, rare in modern Earth
• Alkaline magmas are enriched in alkali metals (sodium, potassium) relative to silica
• Magma composition influences its physical properties, crystallization behavior, and eruption style

Temperature and Pressure Effects

• Temperature and pressure significantly influence magma properties and behavior
• Higher temperatures lead to lower magma viscosity and increased fluidity
  • Mafic magmas have higher temperatures (900-1200°C) compared to felsic magmas (700-850°C)
• Increasing pressure enhances the solubility of dissolved gases in magma
  • Decreasing pressure during magma ascent causes gas exsolution and bubble formation
• Pressure changes affect the crystallization sequence and mineral assemblages in magma
  • Higher pressures favor the crystallization of high-pressure minerals (olivine, pyroxene)
  • Lower pressures promote the crystallization of low-pressure minerals (quartz, feldspars)
• Decompression during magma ascent can trigger further melting and magma evolution
• The geothermal gradient and lithostatic pressure control the depth of magma generation and storage

How Magma Moves and Evolves

• Magma moves through the Earth's crust by exploiting pre-existing fractures or creating new ones
• Buoyancy is the primary driving force for magma ascent
  • Magma is less dense than the surrounding solid rock
• Magma can rise through dikes (vertical or inclined sheet-like intrusions) or conduits
• As magma ascends, it undergoes decompression, causing gas exsolution and potential vesiculation
• Magmatic differentiation processes modify the composition of magma during its ascent and storage
  • Fractional crystallization involves the separation of crystals from the melt
  • Assimilation incorporates wall rock material into the magma
  • Magma mixing occurs when two or more magmas of different compositions interact
• Magma evolution trends (Bowen's reaction series) describe the sequence of mineral crystallization
• The ascent rate and path of magma influence its final composition and eruptive behavior

Magma Storage and Chambers

• Magma can accumulate and reside in storage regions called magma chambers
• Magma chambers are located at various depths within the Earth's crust
• Plutons are large, solidified magma chambers that form intrusive igneous bodies (batholiths, stocks)
• Sills are horizontally oriented magma intrusions that can feed magma chambers
• Layered intrusions (Skaergaard intrusion) showcase magmatic differentiation processes
• Magma chambers can be replenished by new pulses of magma, leading to magma mixing and rejuvenation
• The size, shape, and depth of magma chambers influence the style and frequency of volcanic eruptions
• Magma storage regions can be detected using geophysical methods (seismic tomography, gravity anomalies)

Magma's Role in Volcanic Eruptions

• Magma properties and composition significantly influence the style and intensity of volcanic eruptions
• Mafic magmas tend to produce effusive eruptions with fluid lava flows (Kilauea, Hawaii)
  • Low viscosity allows for the efficient release of dissolved gases
• Felsic magmas are associated with explosive eruptions and pyroclastic density currents (Mount St. Helens, USA)
  • High viscosity hinders gas escape, leading to pressure buildup and violent ejection of magma and rock fragments
• The gas content and bubble nucleation in magma affect the explosivity of eruptions
• Magma interaction with groundwater can trigger phreatomagmatic eruptions (Taal Volcano, Philippines)
• The ascent rate of magma influences the eruptive style and the formation of lava domes or spines
• Magma chamber overpressurization and wall rock failure can initiate volcanic eruptions
• The magma supply rate and volume determine the duration and magnitude of volcanic eruptions

Real-World Examples and Case Studies

• Kilauea Volcano, Hawaii: Effusive eruptions of basaltic magma, lava fountains, and lava lakes
• Mount Vesuvius, Italy: Explosive eruption in 79 AD, buried the cities of Pompeii and Herculaneum
• Yellowstone Caldera, USA: Supervolcano with a large magma chamber, potential for future eruptions
• Eyjafjallajökull, Iceland: 2010 eruption disrupted air travel due to ash dispersal in the atmosphere
• Mount Pinatubo, Philippines: 1991 eruption, significant global cooling due to sulfur dioxide emissions
• Ol Doinyo Lengai, Tanzania: Unique carbonatite magma composition, effusive natrocarbonatite lava flows
• Stromboli, Italy: Continuous small-scale eruptions, "Strombolian" eruptive style named after this volcano
• Novarupta, Alaska: 1912 eruption, largest volcanic eruption of the 20th century, formed the Valley of Ten Thousand Smokes