Rocks are naturally occurring aggregates of minerals and are classified into three main types based on their formation processes:
Igneous Rocks – Formed from the solidification of molten magma or lava.
Sedimentary Rocks – Formed from the accumulation and lithification of sediment.
Metamorphic Rocks – Formed when existing rocks are altered by heat, pressure, or chemical processes.
Understanding rock types is crucial in geology, paleontology, civil engineering, and environmental science, and aids in interpreting Earth’s history and processes.
1. Igneous Rocks
Formation: Originates from the cooling and solidification of magma (below ground) or lava (on surface).
Two types based on origin:
Intrusive (Plutonic) – Slow cooling underground → coarse-grained texture
Extrusive (Volcanic) – Rapid cooling on surface → fine-grained or glassy
Common Igneous Rocks:
| Rock Type | Texture | Minerals Present | Formation |
| Granite | Coarse | Quartz, Feldspar, Mica | Intrusive |
| Basalt | Fine | Pyroxene, Plagioclase | Extrusive |
| Diorite | Coarse | Amphibole, Feldspar | Intrusive |
| Obsidian | Glassy | Volcanic Glass | Extrusive |
| Pumice | Vesicular | Glass and gas bubbles | Extrusive |
Identification Features:
Texture (grain size)
Color and mineral content
Presence of crystals or vesicles (holes)
2. Sedimentary Rocks
Formation: Created by weathering, erosion, transportation, deposition, and lithification of sediments. Commonly found in layers or strata with fossils.
Three subtypes:
Clastic (from fragments of other rocks)
Chemical (from precipitation of minerals)
Organic (from biological material like shells or plant debris)
Common Sedimentary Rocks:
| Rock Type | Composition | Grain Size | Example of Formation |
| Sandstone | Quartz grains | Medium | River/lake beds |
| Shale | Clay particles | Very fine | Deep marine or lake |
| Limestone | Calcite (CaCO₃) | Varies | Marine organism shells |
| Conglomerate | Rounded pebbles | Coarse | River beds |
| Coal | Plant material | Varies | Swamps |
Identification Features:
Layering or stratification
Fossils
Rounded vs. angular grains
Reaction to dilute HCl (for limestone)
3. Metamorphic Rocks
Formation: Result from the alteration of pre-existing rocks (igneous, sedimentary, or metamorphic) under heat, pressure, and chemically active fluids.
Two types:
Foliated – Have layered or banded appearance (due to pressure)
Non-foliated – No layering; usually formed from contact metamorphism
Common Metamorphic Rocks:
| Rock Type | Parent Rock | Texture | Metamorphism Type |
| Slate | Shale | Foliated | Low-grade regional |
| Schist | Mudstone/Shale | Foliated | Medium-grade |
| Gneiss | Granite | Banded | High-grade |
| Marble | Limestone | Non-foliated | Contact |
| Quartzite | Sandstone | Non-foliated | Regional/Contact |
Identification Features:
Foliation or banding (gneissic structure)
Mineral alignment
Hardness and crystalline appearance
Reaction to acid (marble fizzes with HCl).
Comparison of Rock Types
| Property | Igneous | Sedimentary | Metamorphic |
| Formation | Cooling of magma/lava | Compaction of sediments | Recrystallization under pressure/heat |
| Texture | Crystalline, glassy | Layered, clastic | Foliated or massive |
| Fossils | Rare | Common | Rare or deformed |
| Examples | Granite, Basalt | Sandstone, Limestone | Slate, Marble |
Identification Techniques
Hand Sample Observation: Examine grain size, layering, crystal presence, color.
Streak Test: Check mineral color on a porcelain plate.
Acid Test: Use dilute HCl to test for carbonate rocks.
Hardness Test: Use Mohs hardness scale (e.g., quartz = 7, calcite = 3).
Thin Section Microscopy: Study rock slices under a microscope to identify minerals.