📖 PART 1: Minerals and Rocks
A mineral is a naturally occurring, inorganic, homogeneous solid with a definite chemical
composition and crystalline structure (e.g., quartz, feldspar, mica, calcite). A rock is an
aggregate (mixture) of one or more minerals — it does not have one fixed chemical composition.
- Rocks form the solid outer crust of the Earth.
- Three main types of rocks: Igneous, Sedimentary, Metamorphic.
🔥 PART 2: Igneous Rocks (Fire Rocks)
Igneous rocks are formed by the solidification (cooling and crystallisation) of
magma (molten rock underground) or lava (magma that reaches the surface). The word
"igneous" comes from Latin ignis = fire.
Types of Igneous Rocks
| Type |
Formation |
Crystal Size |
Examples |
| Intrusive (Plutonic) Rocks |
Magma cools SLOWLY deep underground → large crystals form (slow cooling = more time for crystals
to grow) |
Large / Coarse-grained |
Granite (most common), Diorite, Gabbro, Syenite |
| Extrusive (Volcanic) Rocks |
Lava cools RAPIDLY on Earth's surface (or underwater) → tiny/no crystals (rapid cooling = no
time for crystal growth) |
Fine-grained or Glassy |
Basalt (most common extrusive), Obsidian (volcanic glass — no crystals), Pumice (porous — gas
bubbles trapped) |
Characteristics of Igneous Rocks
- No fossils (formed at very high temperatures — no organisms survive).
- No layers/strata (solidified from molten state).
- Very hard and crystalline (except obsidian and pumice).
- Economically important: granite (construction/monuments), basalt (road making), obsidian (ancient
tools).
🗂️ PART 3: Sedimentary Rocks (Layered Rocks)
Sedimentary rocks are formed from the accumulation, compaction, and cementation of
sediments (fragments of other rocks, mineral particles, organic remains) deposited by water, wind, or ice.
Also called stratified rocks because they occur in distinct layers (strata).
Types of Sedimentary Rocks
| Type |
Formation |
Examples |
| Mechanically Formed |
Physical weathering breaks rocks into fragments (sand, gravel, silt, clay) → compacted and
cemented |
Sandstone, Conglomerate, Shale, Grit |
| Chemically Formed |
Minerals dissolved in water precipitate/deposit when water evaporates |
Rock Salt, Gypsum, Dolomite, Limestone (partly) |
| Organically Formed |
Remains of organisms (shells, corals, plant matter) accumulate and solidify |
Limestone (coral/shell), Coal (compressed plant matter), Chalk (micro-organisms) |
Characteristics of Sedimentary Rocks
- Fossils are found in sedimentary rocks — organisms get buried in sediment layers and
preserved.
- Occur in layers (strata) — distinct horizontal bands.
- Porous (water can percolate through) — important for groundwater.
- Softer than igneous rocks; can be eroded more easily.
- Economically important: coal, petroleum (found in sedimentary strata); limestone (cement); sandstone
(construction).
🔄 PART 4: Metamorphic Rocks (Changed Rocks)
Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or other
metamorphic rocks) are subjected to extreme heat and/or pressure deep within the Earth —
this changes their mineral composition and structure without them melting completely.
| Original Rock |
→ Metamorphic Rock |
| Limestone |
Marble (used in Taj Mahal; sculptures) |
| Shale / Clay |
Slate (used for roofing, writing slates) |
| Sandstone |
Quartzite (very hard; construction) |
| Coal |
Graphite, then Diamond (extreme pressure) |
| Granite (igneous) |
Gneiss (banded metamorphic rock) |
Characteristics of Metamorphic Rocks
- Very hard and compact due to heat and pressure recrystallisation.
- May have a banded/foliated texture (minerals align in parallel bands — e.g., gneiss).
- Fossils destroyed (high heat/pressure destroys organic remains).
♻️ PART 5: The Rock Cycle
The Rock Cycle is the continuous process by which rocks are formed, broken down, and
reformed. No rock type is permanent — over geological time, any rock can be transformed into another type.
- Magma cools → Igneous rock forms.
- Igneous (or any) rock is weathered and eroded → sediments → transported and deposited →
compacted and cemented → Sedimentary rock forms.
- Sedimentary (or igneous) rock subjected to heat and pressure → Metamorphic rock
forms.
- Metamorphic rock subjected to extreme heat → melts → becomes Magma →
cycle continues.
📌 Chapter Summary
- Igneous: formed from cooling magma/lava. Intrusive (slow cooling → large crystals → Granite);
Extrusive (rapid cooling → fine-grained → Basalt). NO fossils, NO layers.
- Sedimentary: formed from sediment compaction. Types: mechanical (sandstone, shale), chemical (rock
salt, gypsum), organic (coal, limestone, chalk). HAS fossils; has LAYERS (strata).
- Metamorphic: formed by heat+pressure on existing rocks. Limestone→Marble; Shale→Slate;
Sandstone→Quartzite; Granite→Gneiss. Very hard; NO fossils.
- Rock Cycle: Magma→Igneous→(weathering)→Sedimentary→(heat+pressure)→Metamorphic→(extreme heat)→Magma.
Continuous, cyclical process.