World Geography
🔒 Log in to trackEarth's interior, earthquakes, rocks and volcanoes
🔒 Log in to trackInterior of the Earth
| Layer | Nature | Old name |
|---|---|---|
| Crust | Thinnest; continental crust mostly silica + aluminium | SiAl |
| Mantle | Silica + magnesium; upper part includes the asthenosphere (magma source) | SiMa |
| Core | Nickel + iron; outer core liquid, inner core solid | NiFe |
Discontinuities: Conrad (within crust), Mohorovičić / Moho (crust–mantle), Gutenberg (mantle–core), Lehmann (outer–inner core).
Earthquakes
- Point of origin inside the Earth = focus (hypocentre); point on the surface directly above = epicentre.
- P-waves (primary) are fastest and travel through solids, liquids and gases. S-waves (secondary) cannot pass through liquids — their shadow zone proved the outer core is liquid. Surface (L) waves are slowest and most destructive.
- Recorded by a seismograph; Richter scale measures magnitude (energy, logarithmic), Mercalli scale measures intensity (damage).
Rocks
| Type | Formed by | Examples |
|---|---|---|
| Igneous ('primary rocks') | Cooling of magma/lava | Granite (intrusive), basalt (extrusive) |
| Sedimentary | Layers of deposits; contain fossils | Sandstone, shale, limestone, coal |
| Metamorphic | Heat and pressure change existing rocks | Limestone → marble, sandstone → quartzite, shale → slate, granite → gneiss |
Volcanoes and plates
- About two-thirds of active volcanoes lie on the Pacific 'Ring of Fire'.
- Stromboli (Italy) is the 'Lighthouse of the Mediterranean'; Vesuvius buried Pompeii in AD 79; Krakatoa (Indonesia) erupted catastrophically in 1883; Mauna Loa (Hawaii) is the largest active volcano.
- Fold mountains (Himalayas, Alps, Andes, Rockies) form at convergent plate boundaries; mid-ocean ridges form at divergent boundaries; the San Andreas Fault is a transform boundary.
- Continental drift was proposed by Alfred Wegener (1912); the supercontinent was Pangaea, surrounded by the ocean Panthalassa.
Inside the Earth - three layers
Our knowledge of the interior comes from seismic waves and mining, not direct observation. The three layers:
| Layer | Depth (about) | Key facts |
|---|---|---|
| Crust | 5-10 km (ocean floors) to 30-70 km (continents) | Thinnest layer; continental crust = SiAl (silica + aluminium), lighter; oceanic = SiMa (silica + magnesium), denser |
| Mantle | up to about 2,900 km | Mostly solid rock; the partly-molten asthenosphere in its upper part is the source of magma |
| Core | 2,900 km to centre (6,371 km) | NiFe - nickel and iron; outer core liquid, inner core solid |
Discontinuities (surfaces where wave speeds jump) - remember the order top to bottom: Conrad (within the crust), Mohorovicic (Moho) - crust and mantle (1909), Repetti (within the mantle), Gutenberg - mantle and outer core (about 2,900 km), Lehmann - outer and inner core.
Earthquakes
An earthquake is a sudden shake caused by the release of stored energy along faults. The point inside the Earth where it starts is the focus (hypocentre); the point on the surface directly above is the epicentre (shock strongest there). Waves are recorded by a seismograph.
- P-waves (primary) - fastest, longitudinal, pass through solids and liquids.
- S-waves (secondary) - slower, transverse, cannot pass through liquids - this proves the outer core is liquid.
- L-waves (surface) - slowest, arrive last, do the most damage. Magnitude is measured on the Richter scale; damage (intensity) on the Mercalli scale. About 80% of the world's earthquakes occur in the Circum-Pacific belt - the 'Ring of Fire'. Giant sea waves set off by undersea earthquakes are tsunamis (a Japanese word meaning 'harbour wave'), as in the Indian Ocean tsunami of 2004.
Volcanoes
A volcano is an opening where magma (molten rock), ash and gases escape.
- By activity: active (Barren Island - India's only active volcano; Stromboli, the 'Lighthouse of the Mediterranean'; Etna; Vesuvius, which buried Pompeii in 79 AD), dormant (sleeping, e.g. Fuji) and extinct (no record of eruption).
- By shape: shield volcanoes (broad, runny lava - Mauna Loa, Hawaii), composite (strato) volcanoes (steep, alternating lava and ash - Fuji, Vesuvius), and calderas (collapsed craters).
- Hot springs and geysers (e.g. Old Faithful of Yellowstone, USA) are related features. Volcanoes also cluster in the Ring of Fire and along mid-ocean ridges.
Rocks - three families
| Family | Formed by | Examples |
|---|---|---|
| Igneous | Cooling of magma/lava ('fire rocks'; the oldest, primary rocks) | Granite (intrusive/plutonic, big crystals), basalt (extrusive/volcanic; ocean floors) |
| Sedimentary | Layers (strata) of sediments pressed together; the only rocks with fossils; cover most of the land surface | Sandstone, shale, limestone, coal |
| Metamorphic | Heat + pressure change an existing rock | Limestone to marble, sandstone to quartzite, shale to slate, granite to gneiss |
The endless conversion of one rock type into another is the rock cycle. Igneous and metamorphic rocks hold most minerals (gold, copper); coal, oil and limestone - the economic favourites - sit in sedimentary rocks.
Mountains, plateaus and plains - the big division
- Fold mountains - crust squeezed into folds by plate collision; the young fold mountains (Himalayas, Alps, Andes, Rockies) are the highest; older folds (Aravallis, Urals) are worn down.
- Block mountains - land raised between two faults (Vosges, Black Forest, Sierra Nevada); rift valleys (Rhine, East Africa) are the sunken blocks between faults.
- Volcanic mountains - built by eruptions (Fuji, Kilimanjaro).
- Plateaus - raised flatlands (Tibet, the highest and largest; Deccan); plains - low, flat, fertile, the most densely populated landforms.
Question types you will see
Each type: how to recognise it, the method step by step, and one question to try.
Layers of the Earth and discontinuities
'The boundary between crust and mantle is', 'which layer is made of nickel and iron', 'SiAl/SiMa/NiFe', ordering of Conrad-Moho-Repetti-Gutenberg-Lehmann.
Layers: crust (SiAl/SiMa) - mantle - core (NiFe); outer core liquid, inner core solid.
Discontinuities top to bottom: Conrad (in crust), Moho (crust-mantle), Repetti (in mantle), Gutenberg (mantle-outer core), Lehmann (outer-inner core) - 'Come Make Good Lunch'.
Continental crust is lighter and thicker; oceanic crust is denser basalt.
The Gutenberg discontinuity separates which two layers of the Earth?
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The mantle from the outer core (about 2,900 km deep). Moho separates crust from mantle; Lehmann divides the two cores.
Seismic waves and earthquake vocabulary
'Which wave cannot pass through liquid', focus vs epicentre, Richter vs Mercalli, seismograph, 'Ring of Fire'.
Waves: P fastest (passes all matter), S stops in liquids (proves the outer core is molten), L surface waves - slowest, most destructive.
Focus = origin inside; epicentre = surface point above it; waves recorded on a seismograph.
Magnitude = Richter scale; damage intensity = Mercalli scale. About 80% of earthquakes strike the Circum-Pacific 'Ring of Fire'.
The point inside the Earth where an earthquake originates is called the:
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Focus (hypocentre). The point on the surface directly above it - where the shock is felt first and strongest - is the epicentre.
Rock families and metamorphic pairs
'Marble is formed from', 'fossils are found in which rock', 'which is an igneous rock', classify a list of rocks.
Three families: igneous (cooled magma - granite, basalt), sedimentary (strata, the only rocks with fossils - limestone, sandstone, shale, coal), metamorphic (changed by heat-pressure).
Metamorphic pairs: limestone-marble, sandstone-quartzite, shale-slate, granite-gneiss.
Igneous rocks are the oldest 'primary' rocks; sedimentary rocks cover most of the land surface and hold coal and oil.
Slate, used for roofing, is a metamorphic rock formed from which parent rock?
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Shale. The other classic pairs: limestone-marble, sandstone-quartzite, granite-gneiss.
Volcanoes: activity, shape and famous names
'India's only active volcano', 'Lighthouse of the Mediterranean', shield vs composite volcano, Krakatoa/Vesuvius events.
Activity: active (Barren Island - India's only one; Stromboli; Etna; Vesuvius), dormant (Fuji), extinct.
Shape: shield (broad, fluid lava - Mauna Loa, Hawaii), composite/strato (steep, alternating layers - Fuji, Vesuvius), caldera (collapsed crater).
Volcanoes cluster in the Ring of Fire and along mid-ocean ridges; Krakatoa (Indonesia, 1883) is the classic explosive eruption.
Stromboli, often called the 'Lighthouse of the Mediterranean', is a/an:
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Active volcano (in the Aeolian islands, Italy) - its regular night glow guided sailors for centuries.
Mountain and landform types
'Young fold mountains are', 'rift valley is formed by', block mountains with faults, highest/largest plateaus.
Fold mountains - squeezed crust; young folds (Himalaya, Alps, Andes, Rockies) are the highest, old folds (Aravallis, Urals) are worn down.
Block mountains - raised fault blocks (Vosges, Black Forest); rift valleys - sunken blocks between faults (Rhine valley, East African rift).
Plateau = raised flatland (Tibet highest and largest); plain = low, flat, fertile, most populated.
The Rhine valley and the East African rifts are examples of:
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Rift valleys - long, narrow sunken blocks (graben) formed where land has dropped between parallel faults.
Shortcuts that save time
Conrad (inside crust) → Moho (crust|mantle) → Gutenberg (mantle|core) → Lehmann (outer|inner core).
Which discontinuity separates the crust from the mantle?
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The Mohorovičić (Moho) discontinuity.
The first letters of the new rock are easy: Marble from Limestone is the most asked (Taj Mahal marble).
Mistakes to avoid
Where most students lose marks on this subtopic.
Saying S-waves travel through the outer core — they cannot pass through liquids.
Mixing Richter (magnitude) with Mercalli (intensity).
Calling granite a sedimentary rock — it is an intrusive igneous rock.
Quick revision
Read this the night before the exam.
Layers: crust (SiAl/SiMa) - mantle - core (NiFe); outer core liquid, inner solid.
Discontinuity order 'Come Make Good Lunch': Conrad, Moho, Repetti, Gutenberg, Lehmann.
S-waves stop at liquid - proof the outer core is molten; Richter = magnitude, Mercalli = intensity.
80% of earthquakes and volcanoes: Pacific Ring of Fire; Barren Island = India's only active volcano.
Igneous (granite, basalt), sedimentary (fossils: limestone, coal), metamorphic (marble, slate, quartzite, gneiss).
Young fold mountains: Himalaya, Alps, Andes, Rockies; Tibet = highest plateau.
Practice: 20 questions
Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.
Topic test · 10 questions
Suggested time 4 min · wrong answers go to your mistake notebook automatically.