Magmatism and Metamorphism: An Overview
Magmatism and Magmatic Rocks
Magmatism is the geological process by which magmas are generated, move, rise, and cool, forming magmatic rocks. It is the primary petrogenic process, as over 80% of the Earth’s crust is composed of magmatic rocks. Magmatism also plays a crucial role in crustal dynamics, including the formation of oceanic crust and seafloor spreading.
The most visible evidence of magma is volcanism, a process where magma reaches the surface as lava, cools, and forms volcanic rocks. However, about two-thirds of magma doesn’t reach the surface, instead cooling at depth to form plutonic rocks through a process called plutonism. It’s estimated that over 130 million tons of magma are generated daily, with 10% erupting as lava.
Fig. 36: Distribution of magmatic processes and their relation to plate tectonics. The figures indicate the volume of different magmas generated (in km3) annually.
What is magma? Magmas are masses of molten rock originating within the Earth. They exist at high temperatures (700-1200°C) and contain varying proportions of water and other gaseous components, which remain dissolved due to the immense pressure.
How does magma originate? Rocks are composed of various minerals, each with a specific melting point. Thus, a rock doesn’t have a single melting point but a temperature range where some parts melt while others remain solid (similar to a glass of water with ice). The temperature at which melting begins is the solidus, while the temperature at which melting completes is the liquidus. Between these points, the rock is partially molten.
1. Factors Governing Magma Formation
Increased temperature, from radioactive decay within the Earth or friction between lithospheric plates, tends to melt rocks. Each mineral has a characteristic melting temperature.
Pressure counteracts temperature’s effect, as melting increases volume. Higher pressure at depth requires higher temperatures for melting.
Adding water lowers a rock’s melting point. Even a small amount can significantly affect melting.
2. Where Magmas are Generated
Magmas form in the upper mantle and lower crust (30-200 km depth), where pressure and temperature conditions allow partial melting. Plate tectonics explains the locations of magmatic activity:
- Ocean Ridges: Lithospheric plates separate, allowing hot asthenosphere to rise. Decompression melting occurs as this mantle material rises, further aided by fracturing.
- Subduction Zones: The subducting plate releases water, lowering the melting point of the surrounding mantle and triggering melting despite lower temperatures.
- Intraplate: Mantle plumes (hot spots) cause localized melting beneath the lithosphere, as seen in the Hawaiian Islands. In continental areas, this can lead to crustal thinning and rifting.
3. Magmatic Rocks: Types
When magma cools and solidifies, it forms igneous rocks. The cooling rate and location determine the rock type:
- Plutonic (Intrusive) Rocks: Slow cooling at depth allows large crystals to form (e.g., granite). Hypabyssal rocks form in fissures and fractures.
- Volcanic (Extrusive) Rocks: Rapid cooling at the surface results in small crystals or glassy textures (e.g., basalt).
Rock Texture:
- Crystallinity: Holocrystalline (entirely crystals), Hypocrystalline (crystals and glass), Hyaline/Vitreous (mostly glass).
- Grain Size: Phaneritic (visible crystals), Aphanitic (microscopic crystals).
- Texture Types: Granular (uniform crystal size), Porphyritic (large crystals in a fine matrix), Vitreous (no crystals), Vesicular (gas bubbles).
Volcanic Manifestations
A volcano is an opening where magma reaches the surface. Eruptions can be explosive, effusive, or mixed, depending on magma viscosity and gas content.
Volcanic Structures:
- Volcanic Cone: Formed by the accumulation of volcanic material around the vent.
- Shield Volcanoes: Formed by fluid lava flows, with gentle slopes.
- Stratovolcanoes: Formed by alternating lava and pyroclastic layers.
- Domes: Formed by viscous lava accumulating around the vent.
Metamorphism
Metamorphism is the process where rocks change mineralogically and structurally due to temperature, pressure, and chemically active fluids, often obscuring the original rock’s characteristics. It’s an endogenous process occurring in the solid state.
Factors Controlling Metamorphism
- Temperature: Significant changes occur above 100-200°C, while melting begins around 700°C (though this varies).
- Pressure: Lithostatic pressure (from overlying rocks) and directed pressure (from tectonic forces) cause changes in volume and mineral orientation.
Types of Metamorphism
- Contact (Thermal) Metamorphism: Localized heating from magma intrusions.
- Dynamic Metamorphism: Pressure-dominated, often in fault zones.
- Regional (Dynamo-thermal) Metamorphism: Occurs over large areas, often in subduction zones, with both increased temperature and pressure.
- Burial Metamorphism: Occurs at the base of sedimentary basins due to high pressure.
Metamorphic Rocks
Metamorphism affects rock texture through:
- Increased Grain Size: Metamorphic rocks often have larger crystals than their parent rocks.
- Orientation of Crystals: Directed pressure can align crystals, creating foliation (e.g., gneiss, schist).