Types of Rocks

What are the Three Basic Rocks: List of Major Rock Types and Their Subtypes

1. Igneous Rocks

Formed through the cooling and solidification of magma or lava, igneous rocks are the hardest and heaviest. The term ‘Igneous’ is in fact derived from the Latin word ‘igneus’, meaning ‘fire’. They are the ‘new’ rocks on earth.

Properties & Characteristics: Dense, hard, and crystalline; rocky streamlined flow structures with no layers; not affected by chemical corrosion

Examples: Granite, basalt, andesite, gabbro, obsidian, diorite, scoria, tuff, pumice

Uses: In construction of buildings, flooring, interior decoration (granite); as abrasive (pumice); in production of concrete (basalt)

Examples: Granite, gabbro, diorite

Magma comes out through a volcano as lava, cools rapidly and forms small rock crystals

Examples: Basalt, andesite, obsidian, pumice, rhyolite

2. Sedimentary Rocks

Formed by the accumulation of sediments, like fragments of older rocks, organic remains (shell, bones), and minerals that eventually compact and harden to rocks over a long period.  These sediments are broken down by erosion and transported by rivers and streams to accumulate in layers on earth’s surface or at the bottom of lakes, and oceans. These layers can be identified in the rocks by differences in colors, particle size, and distribution of sediments. They are the ‘recycler’ rocks.

Properties & Characteristics: Crumbles easily, can be scratched by fingernails; grainy texture due to layers of sand, silt or gravel; varying sizes of crystals

Examples: Sandstone, conglomerate, chalk, claystone, breccias, shale, calcite, siltstone

Uses: As a construction material (sandstone); production of chalk, lime, industrial carbon dioxide, cement, and in glassmaking (limestone); brick making and as base material under roads (shale)

Examples: Sandstone, conglomerate

Examples: Rock salt, iron ore, halite (common salt)

Examples: Coal, limestone

3. Metamorphic Rocks

Formed when any rock type is exposed to elevated temperature, pressure, or chemically active solution deep inside the earth’s surface. Eventually, the original rock metamorphoses or changes its structure to grow new crystals, like limestone changes into marble. These are the ‘change-makers’ of rocks.

Properties & Characteristics: Lightweight, glassy appearance; relatively hard, but still easy enough to break; layered or foliated structure made up of crystals of the same size

Examples: Gneiss (from granite), Amphibolite (from basalt), Quartzite (from quartz)

Uses: Writing board (slate); flooring and sculpture material (marble); ballast in railroad construction (amphibolite)

Examples: Gneiss, phyllite, schist, and slate

Examples: Marble, hornfels and quartzite