Magmatic Rocks: Formation, Properties, and Their Relation to Plate Tectonics
Magmatic Rock Formation and Transformation
Magmatic rocks, through erosion, sedimentation, and transport, become sediment. Sediment, through diagenesis, becomes sedimentary rock. Sedimentary rock, through metamorphism, becomes metamorphic rock. Metamorphic rock, through anatexis, becomes magma. Magma, through solidification, becomes magmatic rock. Sedimentary rocks can also undergo erosion, transport, and anatexis to become metamorphic rocks, which can then transform into magmatic rocks through anatexis.
Magma Composition and Properties
Magma is a molten rocky material, primarily composed of silicates. It exists in three states:
- Solid Phase: Formed by solidified minerals within the melt.
- Liquid Phase: Resulting from the melting of various minerals.
- Gas Phase: Composed of gases dissolved in the melt.
Rocks melt due to factors like temperature, pressure, and the presence of water. Temperature directly influences rock fusion; each mineral within a rock has specific pressure and temperature conditions at which it melts. Pressure inversely influences temperature; higher pressure requires higher temperature for melting. Water lowers the melting point of minerals, facilitating fusion at a given pressure.
Magmas are classified as acidic, intermediate, basic, and ultrabasic. Key physical properties include temperature and viscosity. Acidic magmas form at 900-1200°C and are more viscous due to their tectosilicate structure. Basic magmas form above 1200°C and are more fluid due to their nesosilicate structure.
Magma Consolidation Phases
- Orthomagmatic Phase: Occurs from the onset of magma crystallization to 500°C.
- Pegmatitic-Pneumatolytic Phase: Occurs between 500°C and 250°C. Residual liquid, rich in gas, penetrates surrounding rocks, forming dikes and veins.
- Hydrothermal Stage: Occurs below 250°C. Residual liquid consists of water and volatile elements.
Magmatism and Plate Tectonics
Magma originates from the partial melting of crustal and upper mantle rocks at depths of 70-200 km. Magma genesis is linked to plate tectonics, primarily occurring at plate boundaries and specific intraplate locations.
At constructive plate boundaries (mid-ocean ridges), plate separation causes decompression melting of mantle material, generating primarily basic magma. At destructive plate boundaries (subduction zones), the subducting oceanic lithosphere, along with hydrated marine sediments, generates intermediate or acidic magmas.
Intraplate magmatism occurs within continental or oceanic plates, such as in the Canary Islands and Hawaii (oceanic) and Yellowstone and Honolulu (continental).
Volcanic Activity
Volcanic activity encompasses geological phenomena related to magma ascent and eruption. An eruption involves the expulsion of magmatic products.
Products of Eruptive Activity
- Gases: Released from magma as pressure decreases during ascent, facilitating magma movement. Viscous acidic magmas trap gases, leading to explosive eruptions. Less viscous basic magmas allow gas release, resulting in effusive eruptions.
- Lava Flows: Volatile-depleted magma flowing on the Earth’s surface at 900-1200°C. Acidic lava is viscous and solidifies quickly, while basic lava is fluid and can travel long distances. Lava morphologies include pahoehoe, pillow, and wrinkled.
- Pyroclastic Fragments: Magmatic material ejected during explosive eruptions.
Igneous Rock Types
- Plutonic Rocks: Formed from slow magma cooling beneath the Earth’s surface (e.g., granite, syenite, gabbro, peridotite). They have a granular texture.
- Volcanic Rocks: Formed from rapid cooling of lava and pyroclastic material at the surface (e.g., rhyolite, andesite, basalt). They have glassy or porphyritic textures.
- Hypabyssal (Subvolcanic) or Filonian Rocks: Formed from magma solidifying in cracks or fractures (e.g., pegmatites). They often exhibit porphyritic texture.