Magmatic processes
Magmatic processes are the dynamic and fascinating series of events that lead to the formation, evolution, and eventual solidification of magma, the molten rock found beneath the Earth's surface. These processes play a crucial role in shaping the Earth's landscape, creating diverse rock formations, and influencing the planet's interior composition.
Melting
This is the initial step and involves the partial melting of rocks in the Earth's mantle or crust. The most common cause of melting is an increase in temperature, which can occur due to several factors:
- Increased heat flow: This can happen near plate boundaries where tectonic activity generates friction and heat.
- Decompression melting: As rocks rise towards the surface due to plate movement or mantle convection, the pressure on them decreases. This can lower the melting point and trigger partial melting.
- Addition of volatiles: Water and other volatile elements can lower the melting point of rocks, facilitating partial melting.
Melting of the mantle generates basalt or andsites; melting of the crust generates granites.
The composition of magma can vary widely depending on the types of rocks that undergo partial melting. Common components of magma include silicate minerals such as quartz, feldspar, and mica, as well as dissolved gases and volatiles.
Magma evolution: fractional crystallization
The newly formed magma isn't static and undergoes various changes as it ascends towards the surface:
Fractional crystallization: As the magma cools, different minerals crystallize out at specific temperatures. These crystals can settle to the bottom, leaving the remaining magma enriched in certain elements. This process can significantly alter the composition of the magma.
Magma mixing: Magma can also interact and mix with other magma bodies of different compositions during its ascent. This mixing can create new and unique rock types.
Interaction with surrounding rocks: Magma can react with the rocks it comes in contact with, causing chemical changes in both the magma and the surrounding rock.
- Basalts
evolve towards high-SiO₂.
- Undersaturated
basalts remain so, and move in the undersaturated field, towards olivine-
or nepheline-normative rocks (i.e., alkali series).
- Saturated
basalts remain so and evolve towards quartz-normative rocks
(basalts-andesite-dacite-rhyolite, aka BADR series) (sub-alkaline
series).
- Granites are already SiO₂ rich… and remain so (little or no evolution)
Degasssing: Magma contains dissolved gases like water vapor and carbon dioxide. As the pressure decreases during ascent, these gases come out of solution and form bubbles, which can eventually contribute to volcanic eruptions.
Magma Emplacement and Crystallization
Solidification: Eventually, the rising magma either reaches the surface and erupts as lava, or it cools and solidifies within the Earth's crust to form intrusive igneous rocks like granite or gabbro.
Crystallization: As the magma cools, further crystallization occurs, forming the various minerals that make up the final igneous rock. The size and texture of these crystals depend on the cooling rate. Slow cooling allows for the formation of larger, more coarse-grained crystals, while rapid cooling results in finer-grained or even glassy textures.
A. Plutons and batholiths
Intrusive: When magma intrudes into existing rock bodies, it cools and crystallizes slowly, often forming large, coarse-grained crystals. The resulting rock is called an intrusive igneous rock or plutonic rock. Plutons are typically found as clusters of several to many intrusions
(batholiths), corresponding to successive magma inputs, typically during a
small time period.
B.Volcanoes erupts
Extrusion: If the magma reaches the surface, it erupts as lava, forming extrusive igneous rocks like basalt, andesite, and rhyolite. The type of eruption and resulting rock characteristics depend on the magma's composition, viscosity, and the presence of gas bubbles.
Cooling and sub-solidus evolution
Once the magma cools and crystallizes, it forms igneous rock. The rate of cooling significantly impacts the final texture of the rock:
Slow cooling: Allows for larger crystals to form, resulting in a coarse-grained igneous rock (e.g., granite).
Fast cooling: Leads to the formation of smaller crystals or even glass, resulting in a fine-grained or glassy igneous rock (e.g., basalt, obsidian).
However, the rock can continue to change even after it has solidified, through a process called sub-solidus evolution.
Sub-solidus processes: Even after solidification, the rock can continue to undergo changes below the solidus temperature (the minimum temperature for melting to occur). This can involve the release of fluids, alteration of minerals, and the formation of new minerals through reactions within the solid rock.
Solidus evolution. This can involve:
Metamorphism: The rock can be exposed to high temperatures and pressures, causing it to recrystallize and form a new type of metamorphic rock.
Hydrothermal alteration: Hot fluids can circulate through the rock, dissolving and removing some minerals and depositing new ones.
Magmatic Processes Importance
Understanding these fundamental magmatic processes helps us:
Classify and interpret different types of igneous rocks: Based on their composition, texture, and occurrence, we can infer the processes that led to their formation.
Predict volcanic activity: Studying magma movement and evolution can provide valuable insights into potential volcanic eruptions.
Understand the formation of mineral deposits: Certain minerals concentrate during various stages of magmatic processes, leading to the formation of economically valuable mineral deposits.
Explore the Earth's interior: By analyzing the composition of igneous rocks, we can gain indirect knowledge about the composition and dynamics of the Earth's mantle and crust.
Magmatic processes are a fascinating and dynamic aspect of Earth's geology, constantly shaping our planet and providing valuable clues to its history and ongoing evolution.
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