Geography · Lesson 2

Structure of the earth, rocks, earth movements & vulcanicity

JAMBWAECNECO

What the syllabus asks

JAMB: Physical Geography B–C: structure of the earth (crust, mantle, core; lithosphere, hydrosphere, atmosphere, biosphere); rocks; tectonic forces (tension, compression); mountains, plateaus, plains, rift valleys; vulcanicity, intrusive and extrusive landforms, types of volcanoes; earthquakes

WAEC: The earth’s structure: internal and external; rocks: types, characteristics, modes of formation and uses; earth movements and resulting landforms; vulcanicity and earthquakes

NECO: The earth’s structure: internal and external; rocks: types, characteristics, modes of formation and uses; earth movements and resulting landforms; vulcanicity and earthquakes

What you will learn

Describe the internal structure of the earth and the four spheres.
Classify rocks into igneous, sedimentary and metamorphic, with examples and modes of formation.
State characteristics and economic uses of rocks, with Nigerian examples.
Explain the rock cycle.
Explain plate tectonics in simple terms.
Explain how tension and compression produce faults, folds, rift valleys and block and fold mountains.
Describe the formation of plateaus and plains.
Describe intrusive and extrusive volcanic features and types of volcanoes.
Explain the causes, measurement and effects of earthquakes and name examples.

Key points

The earth has a crust (5–70 km), mantle (to ~2,900 km) and core (liquid outer, solid inner) of nickel and iron.
Continental crust is sial (granitic, lighter); oceanic crust is sima (basaltic, denser).
The lithosphere, hydrosphere, atmosphere and biosphere interact at the surface.
Igneous rocks form from cooled magma/lava; slow cooling gives large crystals.
Sedimentary rocks form in layers, may contain fossils, and hold oil, coal and water.
Metamorphic rocks form by heat and pressure: limestone→marble, shale→slate, sandstone→quartzite, granite→gneiss.
The rock cycle links all three families.
Plates meet at constructive, destructive and conservative boundaries.
Tension gives normal faults and rift valleys; compression gives folds and fold mountains.
Intrusive features: batholith, dyke, sill, laccolith, lopolith; extrusive: volcanoes, lava plateaus, geysers, hot springs.
Volcano types: ash cone, acid dome, shield, composite, caldera.
Earthquakes: focus and epicentre; measured by seismograph; magnitude and Mercalli intensity.

Start here: The structure of the earth

Imagine cutting a boiled egg in half. You see a thin shell, a thick white and a yolk in the centre. The earth is built in a similar way: a thin crust, a thick mantle and a central core. Nobody has drilled through the crust (the deepest borehole, in Russia, reached only about 12 km), so scientists learn about the inside of the earth by studying how earthquake (seismic) waves travel through it.

The crust

The crust is the outermost solid layer. It is very thin compared with the whole earth: about 5–10 km under the oceans and about 30–70 km under the continents (thickest under high mountains).

Layer of crust	Other name	Main minerals	Typical rock	Density
Continental crust	Sial	Silica + aluminium	Granite	Lighter (about 2.7 g/cm³)
Oceanic crust (and lower crust)	Sima	Silica + magnesium	Basalt	Heavier (about 3.0 g/cm³)

Because sial is lighter, the continents “float” higher on the sima beneath, like a block of wood on water.

The mantle

Below the crust is the mantle, which extends to a depth of about 2,900 km. It is mostly solid rock, but it is so hot that parts of it can flow very slowly over millions of years. The upper part, called the asthenosphere, is soft and partly molten; it is on this layer that the plates move. The boundary between crust and mantle is the Mohorovičić discontinuity (Moho).

The core

The core is made mainly of iron (Fe) and nickel (Ni), so it is sometimes called nife. The outer core (about 2,900–5,100 km deep) is liquid; the inner core (from about 5,100 km to the centre at about 6,370 km) is solid because of the enormous pressure, although it is extremely hot (estimated at about 5,000–6,000 °C). The boundary between mantle and core is the Gutenberg discontinuity.

Lithosphere = the crust + the rigid uppermost mantle (about 100 km thick). It is broken into plates that move on the soft asthenosphere.

Worked example 2.1

State two differences between sial and sima.

Composition: sial is rich in silica and aluminium; sima is rich in silica and magnesium.
Density: sial is lighter; sima is denser.
Location: sial forms the continents; sima forms the ocean floors and underlies the sial.

Any two of composition, density, location (with both sides stated)

⚠️ Common mistake: writing that the whole mantle is “liquid magma”. The mantle is mostly solid; only small parts melt to form magma. The outer core is the liquid layer.

Try it yourself. Why is the inner core solid even though it is hotter than the outer core?

Show solution

Because the pressure at the centre is so great that the iron and nickel cannot melt.

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Practice questions

1. The outermost solid layer of the earth is the

crust
mantle
outer core
inner core

Show answer

A: crust
The crust is the thinnest layer.

2. The layer of the crust under the oceans, rich in silica and magnesium, is called

sial
nife
lithosphere only
sima

Show answer

D: sima
Sial (silica + aluminium) forms the continents.

3. The core of the earth is made mainly of

granite
limestone
iron and nickel
silica and aluminium

Show answer

C: iron and nickel
Hence the name nife.

4. Rocks formed from cooled magma are

organic rocks
igneous rocks
sedimentary rocks
metamorphic rocks

Show answer

B: igneous rocks
Granite and basalt are examples.

5. Granite is an example of

an intrusive igneous rock
an extrusive igneous rock
a sedimentary rock
a metamorphic rock

Show answer

A: an intrusive igneous rock
It cools slowly underground, so it has large crystals.

Key words

Crust: The thin, solid outermost layer of the earth.
Sial: The lighter continental crust, rich in silica and aluminium (granitic).
Sima: The denser oceanic crust and lower crust, rich in silica and magnesium (basaltic).
Mantle: The thick layer between crust and core, mostly solid but able to flow very slowly.
Moho: The Mohorovičić discontinuity: the boundary between crust and mantle.
Core: The central part of the earth, mainly iron and nickel (nife).
Lithosphere: The rigid outer shell: crust plus the uppermost mantle.
Plate: A large rigid piece of the lithosphere that moves slowly.
Magma: Molten rock beneath the surface.
Lava: Molten rock that has reached the surface.
Igneous rock: Rock formed when magma or lava cools and solidifies.
Sedimentary rock: Rock formed from deposited layers (strata) of sediment, compacted and cemented.

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