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.

All Earth's Structure, Landforms and Geological Time notes NDA Geography strategy

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

Intuition

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

From the bank · past-year question

Example 1Earth's Structure, Landforms and Geological TimeEASY

Loose sediments are transformed into sedimentary rocks by the process of

[Q106 · Apr · 2026]

Drill 1 more on the three rock families and the rock cycle

Concept 2 of 6

Identifying igneous rocks

Intuition

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

From the bank · past-year question

Example 2Earth's Structure, Landforms and Geological TimeEASY

Which one of the following is not an igneous rock?

[Q57 · Apr · 2023]

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.

Drill 1 more on identifying igneous rocks

Concept 3 of 6

Sedimentary rocks: mechanical, chemical, organic

Intuition

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

From the bank · past-year question

Example 3Earth's Structure, Landforms and Geological TimeMODERATE

Which one of the following sedimentary rocks is organically formed?

[Q113 · Sep · 2019]

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).

Drill 1 more on sedimentary rocks: mechanical, chemical, organic

Concept 4 of 6

Metamorphic rocks and their parents

Intuition

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

From the bank · past-year question

Example 4Earth's Structure, Landforms and Geological TimeEASY

Quartzite is metamorphosed from which one of the following rocks?

[Q112 · Apr · 2026]

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.

Drill 3 more on metamorphic rocks and their parents

Concept 5 of 6

Common rock-forming minerals

Intuition

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

From the bank · past-year question

Example 5Earth's Structure, Landforms and Geological TimeMODERATE

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 ?

[Q112 · Sep · 2025]

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.

Drill 1 more on common rock-forming minerals

Concept 6 of 6

Geological time scale

Intuition

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

From the bank · past-year question

Example 6Earth's Structure, Landforms and Geological TimeMODERATE

Which one of the following is the oldest era in Geological History determining the age of various rock types?

[Q79 · Sep · 2023]

Drill 2 more on geological time scale

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

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