Magma and Igneous Rocks: Formation, Types, and Mineral Deposits
Magma and Igneous Rocks
Magma: is a portion of earth material in a liquid state, which can contain components in gas or solid state. The solidification process of magma results in magmatic rocks.
Plutonic: Magma that solidifies slowly inside the crust allows minerals to crystallize slowly.
Filonian Magma: Rising magma may melt surrounding rocks.
Volcanic: These are rocks resulting from magma rising through cracks and reaching the surface.
Texture of Igneous Rocks
Grainy: The components are well differentiated from each other and have a large size.
Porphyritic: Large geometric crystals are present within a microcrystalline appearance.
Aplitic: Small minerals are uniformly sized.
Pegmatitic: Very large crystals.
Microcrystalline: Minerals are difficult to recognize, even with microscopic observation.
Placement of Igneous Rocks
Plutonic Rocks: Form slowly inside the earth. Plutonic rock is also called intrusive rock. Magmatic intrusion occurs when rising magma breaks through surrounding structures. A large mass of plutonic rock is called an intrusive batholith.
Filonian Rocks: Solidify in cracks, giving rise to:
- Dikes
- Intrusive sheets or sills
- Veins
The most abundant component of magma is silica. If it has more than 70% silica, it is felsic magma; if it is below 50%, it is mafic magma.
Volcanic Rocks: Formations newly solidified on the Earth’s surface. Materials in a liquid state coming out of a volcano are called lava:
- Block or Mass: Fluid lava with a rough crust.
- Cord: More fluid and faster than the previous type, solidifying slowly.
- Pillow Lavas: Originate in submarine eruptions.
Pyroclasts: Volcanic ash, dust, blocks, lapilli, and bombs.
Solidification of Magma
Molten magma is almost entirely liquid and subsequently solidifies. Magma is a mixture of silicate melts with a large amount of water vapor and other gases.
According to Bowen: The order of crystallization of silicates depends on the point of fusion. Bowen’s reaction series applies to the crystallization of basaltic magma.
Discontinuous Reaction Series: Olivine, pyroxene, amphibole, biotite, potassium feldspar, muscovite, quartz.
Continuous Reaction Series: Calcium plagioclase feldspar to albite.
Magmatic Phases
Orthomagmatic Phase: 1000 to 600 ºC. Crystallization starts within the magma with the formation of silicates. In a more advanced phase, olivines crystallize (basalt and diabase). Volcanoes formed at this stage are called fissural. If the temperature continues to drop, crystallization occurs (diorite, andesite, porphyry, granite, rhyolite) at 600-450 ºC.
Pegmatite Stage: The flow and pressure of magma are large, resulting in a pegmatite filonian rock.
Pneumatolytic Phase: + 374 ºC
Hydrothermal Stage: Develops at temperatures below the critical point of water, allowing it to remain in a liquid state.
Magma Evolution
If already formed crystals are separated from the magma, it can create two different magmas that separate at a point in the formation, deriving from a primary magma.
Types of Magma
Tholeiitic: Originates from mid-ocean ridges, basalt, and gabbro.
Alkali: Generated in the mantle (35-75km depth). Contains alkali metals.
Calc-alkaline: Develops in subduction zones. Andesite.
Magmatism and Tectonic Plates
Magmatism at Constructive Plate Boundaries: Materials ascend from the asthenosphere at high temperatures, causing melting due to drastically reduced pressure. The fusion mass of these minerals becomes magma, and its evolution originates tholeiitic basalts.
Magmatism at Destructive Plate Boundaries: The subduction of oceanic crust causes magmatism. In these areas, a calc-alkaline type of magmatism is created.
Magmatism at Transform Plate Boundaries: Transform faults show a small amount of volcanic activity, producing basaltic rocks.
Intraplate Magmatism: Magmatic phenomena occur inside the same plate, with the existence of a hot spot.
Mineral Deposits
These are natural deposits of minerals with high concentrations. Ore is economically viable, while gangue is not. During the orthomagmatic phase, deposits form at the same time as silicates crystallize, originating minerals such as diamond. If the mineral formed is denser than silicates, it generates a deposit of segregation.