NDA Geography · Earth's Structure, Landforms and Geological Time
Rocks, Minerals and Geological Time
Three rock families — igneous, sedimentary, metamorphic — endlessly transform into one another through the rock cycle, and the eras of geological time order the whole story of Earth history.
Why this matters
15 PYQs, ~29% HARD — one of the chapter's most-tested and trickiest subtopics. The marks come from (1) sorting a named rock into the right family, (2) the parent → metamorphic pairs (sandstone → quartzite, limestone → marble), and (3) the order of geological eras. Learn the rock cycle as a story and the identifications stop being random.
Concept 1 of 6: The three rock families and the rock cycle
Every rock belongs to one of three families, defined by HOW it formed. IGNEOUS rock freezes from molten magma or lava. SEDIMENTARY rock forms when loose sediment is compacted and cemented (lithified). METAMORPHIC rock is an older rock transformed in place by heat and pressure. And none of these is permanent — melting, weathering and metamorphism keep converting one into another. That endless conversion is the rock cycle.
Definition
- Igneous — solidified from molten magma/lava (the 'primary' rock).
- Sedimentary — loose sediments compacted + cemented. The process that turns loose sediment into solid sedimentary rock is lithification.
- Metamorphic — an existing rock changed by heat + pressure (without melting).
- Rock cycle — rocks do not stay in one form for long: weathering, erosion, melting and metamorphism continually transform igneous ↔ sedimentary ↔ metamorphic.
Worked example
By what process are loose sediments turned into solid sedimentary rock?
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Apr 2026 · Q106Easy
Example 1 · Earth's Structure, Landforms and Geological Time · Rocks, Minerals and Geological Time
Loose sediments are transformed into sedimentary rocks by the process of
Concept 2 of 6: Identifying igneous rocks
Igneous rocks are the 'fire-born' rocks. If magma cools SLOWLY deep underground, big crystals grow → coarse rocks like granite and gabbro (intrusive/plutonic). If lava cools FAST at the surface, crystals stay tiny → fine rocks like basalt (extrusive/volcanic). The exam trick is to spot the impostor — a sedimentary or metamorphic rock hiding in an igneous list.
Definition
- Intrusive / plutonic (slow cooling, coarse): granite, gabbro, pegmatite.
- Extrusive / volcanic (fast cooling, fine): basalt, rhyolite, obsidian, pumice.
- Common impostors that are NOT igneous: dolomite and limestone (sedimentary), slate and marble (metamorphic).
| Rock | Igneous? | Actually |
|---|---|---|
| Granite, Gabbro, Basalt | Yes | Igneous |
| Dolomite | No | Sedimentary NDA 2017 — dolomite is the odd one out in an igneous list. |
| Slate | No | Metamorphic NDA 2023 — slate is metamorphic, not igneous. |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Apr 2023 · Q57Easy
Example 2 · Earth's Structure, Landforms and Geological Time · Rocks, Minerals and Geological Time
Which one of the following is not an igneous rock?
Slate and dolomite are the classic impostors
In 'which is NOT igneous' questions the trap answer is almost always slate (metamorphic) or dolomite/limestone (sedimentary) sitting among granite, basalt and gabbro.
Concept 3 of 6: Sedimentary rocks: mechanical, chemical, organic
Sedimentary rocks form from settled sediment, but the sediment can arrive three ways. MECHANICALLY formed rocks are made of broken fragments (sandstone, shale). CHEMICALLY formed rocks precipitate out of solution (chert, rock salt, gypsum). ORGANICALLY formed rocks are built from the remains of living things (limestone, chalk, coal).
Definition
Three modes of formation:
- Mechanically formed (clastic) — compacted fragments: sandstone, shale, conglomerate.
- Chemically formed — precipitated from solution: chert, halite (rock salt), gypsum, geyserite.
- Organically formed — from organic remains: limestone, chalk, coal.
| Mode | Examples |
|---|---|
| Mechanical (clastic) | Sandstone, shale, conglomerate |
| Chemical | Chert, halite, gypsum, geyserite |
| Organic | Limestone, chalk, coal NDA 2019 — chalk is an organically-formed sedimentary rock. |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Sep 2019 · Q113Moderate
Example 3 · Earth's Structure, Landforms and Geological Time · Rocks, Minerals and Geological Time
Which one of the following sedimentary rocks is organically formed?
Shale is mechanical, chert is chemical
Match the mode carefully: shale = mechanically formed (compacted mud), chert = chemically formed (precipitated silica), geyserite = chemically formed (siliceous sinter, NOT organic).
Concept 4 of 6: Metamorphic rocks and their parents
Metamorphism takes an existing rock and, under heat and pressure, recrystallises it into something denser and harder — without melting it. The exam staple is the parent → product pair: sandstone becomes quartzite, limestone becomes marble, shale becomes slate. Strong directed pressure also lines the minerals up into bands or sheets — that layering is called foliation.
Definition
Parent → metamorphic product:
- Sandstone → quartzite; limestone → marble; shale → slate → schist → gneiss; coal → anthracite; granite → gneiss.
- Agents of metamorphism: heat, pressure (compression), chemically active fluids/solution — but NOT 'decomposition'.
- Foliation — wavy bands or platy layers formed by recrystallisation under directed pressure. Broad mineral bands → very hard rock; thin foliation → the rock flakes apart.
- A rock can be transitional: gneissoid is regarded as both igneous and metamorphic.
| Parent rock | Metamorphic product |
|---|---|
| Sandstone | Quartzite NDA 2026 — quartzite is metamorphosed sandstone. |
| Limestone | Marble |
| Shale | Slate → schist → gneiss |
| Coal | Anthracite |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Apr 2026 · Q112Easy
Example 4 · Earth's Structure, Landforms and Geological Time · Rocks, Minerals and Geological Time
Quartzite is metamorphosed from which one of the following rocks?
Decomposition is not a metamorphic agent
Heat, pressure and chemically active solutions drive metamorphism. Decomposition (a chemical-weathering term) is the impostor in an 'agents of metamorphism' list.
Concept 5 of 6: Common rock-forming minerals
Rocks are made of minerals, and a few dominate. Feldspar is the single most abundant mineral group in the crust; quartz is the hard, glassy one; pyroxene and amphibole are the dark, iron-magnesium minerals. The NDA tests their distinguishing facts: feldspar's colour and share of the crust, pyroxene's presence in meteorites.
Definition
- Feldspar — about HALF the Earth's crust; light cream to salmon-pink. (It does NOT always contain magnesium.)
- Quartz — silica (SiO₂), very hard, and NOT soluble in water.
- Pyroxene — a dark ferromagnesian mineral; commonly found in meteorites.
- Amphibole — dark ferromagnesian mineral; forms only a small percentage of the crust (not 20%).
| Mineral | Key fact |
|---|---|
| Feldspar | ~half the crust; cream to salmon-pink NDA 2025 — feldspar is ~half the crust and cream/pink, but does NOT always contain Mg. |
| Quartz | SiO₂, hard, insoluble in water |
| Pyroxene | Dark mineral; common in meteorites NDA 2024 — pyroxene is commonly found in meteorites. |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Sep 2025 · Q112Moderate
Example 5 · Earth's Structure, Landforms and Geological Time · Rocks, Minerals and Geological Time
Consider the following statements with reference to Feldspar mineral :
1. Half of the earth's crust is composed of feldspar
2. It has light cream to salmon pink colour
3. Magnesium is a common element in all types of feldspar
Which of the statements given above is/are correct ?
Feldspar does NOT always contain magnesium
Feldspar is ~half the crust and cream-to-pink (both true), but a third claim — that magnesium is common in ALL feldspar — is false. Feldspars are mainly K/Na/Ca aluminosilicates.
Concept 6 of 6: Geological time scale
Earth's 4.6-billion-year history is sliced into eras, then periods, then epochs. The oldest and longest stretch is the Precambrian; the most recent period (the Quaternary, our Ice-Age-to-now interval) splits into the Pleistocene and the Holocene. Reconstructing these past worlds and their maps is called palaeogeography.
Definition
- Eras, oldest → youngest: Precambrian → Palaeozoic → Mesozoic → Cenozoic. Precambrian is the OLDEST (and by far the longest).
- The Quaternary period (most recent) has two epochs: Pleistocene (Ice Age) and Holocene (present).
- Palaeogeography — piecing together geological time to make historical maps of the Earth (palaeoclimatology = past climate; palaeolithology = past rocks).
| Division | Detail |
|---|---|
| Oldest era | Precambrian NDA 2023 — Precambrian is the oldest era. |
| Quaternary epochs | Pleistocene + Holocene NDA 2023 — the two epochs of the Quaternary. |
| Historical Earth maps | Palaeogeography |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
NDA · Sep 2023 · Q79Moderate
Example 6 · Earth's Structure, Landforms and Geological Time · Rocks, Minerals and Geological Time
Which one of the following is the oldest era in Geological History determining the age of various rock types?
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (5)
Identifying igneous rocks3 rows
| Rock | Igneous? | Actually |
|---|---|---|
| Granite, Gabbro, Basalt | Yes | Igneous |
| Dolomite | No | Sedimentary NDA 2017 — dolomite is the odd one out in an igneous list. |
| Slate | No | Metamorphic NDA 2023 — slate is metamorphic, not igneous. |
Sedimentary rocks: mechanical, chemical, organic3 rows
| Mode | Examples |
|---|---|
| Mechanical (clastic) | Sandstone, shale, conglomerate |
| Chemical | Chert, halite, gypsum, geyserite |
| Organic | Limestone, chalk, coal NDA 2019 — chalk is an organically-formed sedimentary rock. |
Metamorphic rocks and their parents4 rows
| Parent rock | Metamorphic product |
|---|---|
| Sandstone | Quartzite NDA 2026 — quartzite is metamorphosed sandstone. |
| Limestone | Marble |
| Shale | Slate → schist → gneiss |
| Coal | Anthracite |
Common rock-forming minerals3 rows
| Mineral | Key fact |
|---|---|
| Feldspar | ~half the crust; cream to salmon-pink NDA 2025 — feldspar is ~half the crust and cream/pink, but does NOT always contain Mg. |
| Quartz | SiO₂, hard, insoluble in water |
| Pyroxene | Dark mineral; common in meteorites NDA 2024 — pyroxene is commonly found in meteorites. |
Geological time scale3 rows
| Division | Detail |
|---|---|
| Oldest era | Precambrian NDA 2023 — Precambrian is the oldest era. |
| Quaternary epochs | Pleistocene + Holocene NDA 2023 — the two epochs of the Quaternary. |
| Historical Earth maps | Palaeogeography |
Watch out for (4)
- Slate and dolomite are the classic impostors→ Identifying igneous rocks
- Shale is mechanical, chert is chemical→ Sedimentary rocks: mechanical, chemical, organic
- Decomposition is not a metamorphic agent→ Metamorphic rocks and their parents
- Feldspar does NOT always contain magnesium→ Common rock-forming minerals
Test yourself on Earth's Structure, Landforms and Geological Time
20 past NDA questions from this chapter, timed at 20 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.