Igneous, Metamorphic Rocks: Textures, Uses, and Formation
Igneous Rock Textures and Magmatism
The magmatic texture is the overall appearance of an igneous rock based on the size, shape, and arrangement of its minerals.
Plutonic Texture: All the minerals are crystallized, and the crystal size is roughly the same. This texture indicates that the magma cooled very slowly, over millions of years, allowing sufficient time for crystals to grow.
Porphyritic Texture: This texture consists of large crystals (phenocrysts) surrounded by microscopic crystals, thin and elongated (microliths). These form in an amorphous vitreous paste, crystallizing in two phases:
First phase: Slow cooling, crystallizing phenocrysts.
Second phase: Rapid cooling, crystallizing the paste and microliths.
Glassy Texture: This texture lacks crystals because the magma rose to the surface during a volcanic eruption and cooled very quickly. These rocks usually form an amorphous mass of glass with a minimal amount of crystals.
Extrusive or Volcanic Rocks
Extrusive volcanic rocks often exhibit porphyritic and glassy textures. Examples include basalt, andesite, and pumice.
Intrusive Rocks
Plutonic Rocks: Characterized by plutonic texture, as they form very slowly at great depth. Examples include granite, syenite, gabbro, and peridotite.
Hypabyssal Rocks: These rocks are intermediate between plutonic and volcanic, consolidating in the Earth’s interior but in areas relatively close to the surface. Examples include porphyry and pegmatite.
Uses of Rocks
Humans have used rocks for millennia as materials for construction, sculpting, and making useful and ornamental objects.
Marble is used in construction and sculpture.
Granite is used in the construction of buildings, monuments, and paving.
Quartzite is used in the construction of buildings, roads, walls, and floors.
Slate is used in walls and roofs of buildings.
Metamorphic Rocks
Metamorphic rocks are formed in a solid state by physical and chemical changes resulting from pressure and temperature variations inside the Earth.
The process that creates these rocks is called metamorphism and can last millions of years.
The factors controlling metamorphism, which can transform sedimentary, igneous, and other metamorphic rocks, are:
Temperature: Temperature increases when materials are close to magma or deeply buried, increasing approximately 1°C every 33 meters.
Pressure: Rocks are buried under two forces: the pressure exerted by overlying materials (lithostatic pressure) and the pressure generated by the collision of lithospheric plates (directed pressure), leading to mountain ranges.
Fluids: When pressure and temperature increase, fluids found in the pores of rocks are released, reacting with minerals and changing the rock’s composition.
Changes During Metamorphism
The effects of metamorphism are reflected in the chemical composition and texture of rocks:
Mineralogical Changes: Under increased pressure and temperature, minerals become unstable. After a series of chemical reactions, they convert into other minerals, called metamorphic minerals, which are stable under the new conditions.
Changes in Texture: With increasing pressure, the minerals in a rock reorient and align perpendicular to the direction of the force. This mineral alignment gives the rock a banded or leafy appearance, known as foliation. Foliation in fine-grained rocks is called slate, while in rocks with larger crystals, it’s called schistosity.
Types of Metamorphism
Metamorphism occurs when rocks are subjected to conditions different from those of their formation. In response, unstable rocks change gradually to reach equilibrium with the new environment.
There are three main types of metamorphism: contact metamorphism, regional metamorphism, and dynamic metamorphism.
Regional metamorphism occurs when rocks are subjected simultaneously to an increase in pressure and temperature. This occurs, for example, during mountain building.
Regional Metamorphic Origin
In large sedimentation basins, sedimentary rocks undergo progressive regional metamorphism, becoming more intense with increasing depth.
As a result, a series of metamorphic rocks form, showing a range of transformations that progressively increase. The most famous series is that of clay, which includes slate, schist, and gneiss, differing by their degree of metamorphism.
Contact Metamorphic Origin
The most important examples are marble and quartzite.