Classification and Characteristics of Common Rock Types

Volcanic and Vitreous Rocks

• Andesite: A gray rock with a mineralogical composition similar to diorite.
• Rhyolite: A light-colored rock with a mineral composition similar to granite.
• Trachyte: A light-colored rock with a mineral composition similar to syenite.
• Obsidian: This is the name given to this volcanic glass; it has a glassy texture, may or may not have hollows, is black, and its composition is very variable.
• Pumice: The name given to these volcanic rocks; they have a vacuolar texture and light color. Its low density makes it unmistakable. Its mineral components may be similar to rhyolite or trachyte.

Philonian Rocks

Sometimes the magma does not reach the surface but remains close to it. Under these conditions, the cooling is faster than that experienced in the formation of plutonic rocks, but slower than in volcanic rocks. This originates rocks with intermediate characteristics between plutonic and volcanic rocks, called Philonian rocks.

• Aplite: A rock with a holocrystalline texture and fine homometric grain. It is light gray in color and its components are similar to granite.
• Porphyry granite: A rock with large crystals called phenocrysts. Its composition is similar to granite.

Carbonate Rocks

Carbonate rocks are formed from substances dissolved in water that have precipitated.

• Limestone: Composed of calcium carbonate. If formed by the accumulation of shells of organisms, it is called lumachelle.
• Marl: These are rocks formed by the mixture of clay and limestone; therefore, they are intermediate between detrital and carbonate rocks.

Evaporite Rocks

These are rocks formed by the precipitation of mineral salts dissolved in water that has undergone intense evaporation. These include:

• Halite: Made up of sodium chloride.
• Sylvite: Consisting of potassium chloride.
• Gypsum: Composed of hydrated calcium sulfate.

Organogenic Rocks

Organogenic rocks originate from the accumulation of remains of organisms.

• Coal: Formed from plant debris accumulated in swamps, lagoons, or deltas. Types include peat, lignite, coal, and anthracite.
• Oil: Formed from the remains of plankton accumulated in the seabed where deposits of clay or mud have been discovered.