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.
Detailed notes
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.
Quick revision
- 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.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Layers of the Earth and discontinuitiesvery common3 practice Q
'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.
Example: The Gutenberg discontinuity separates which two layers of the Earth?
The mantle from the outer core (about 2,900 km deep). Moho separates crust from mantle; Lehmann divides the two cores.
Type 2: Seismic waves and earthquake vocabularyvery common4 practice Q
'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'.
Example: The point inside the Earth where an earthquake originates is called the:
Focus (hypocentre). The point on the surface directly above it - where the shock is felt first and strongest - is the epicentre.
Type 3: Rock families and metamorphic pairsvery common4 practice Q
'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.
Example: Slate, used for roofing, is a metamorphic rock formed from which parent rock?
Shale. The other classic pairs: limestone-marble, sandstone-quartzite, granite-gneiss.
Type 4: Volcanoes: activity, shape and famous namescommon3 practice Q
'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.
Example: Stromboli, often called the 'Lighthouse of the Mediterranean', is a/an:
Active volcano (in the Aeolian islands, Italy) - its regular night glow guided sailors for centuries.
Type 5: Mountain and landform typescommon3 practice Q
'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.
Example: The Rhine valley and the East African rifts are examples of:
Rift valleys - long, narrow sunken blocks (graben) formed where land has dropped between parallel faults.
Shortcut tricks
⚡ Discontinuity order: 'Come Make Good Lunch'
Conrad (inside crust) → Moho (crust|mantle) → Gutenberg (mantle|core) → Lehmann (outer|inner core).
Example: Which discontinuity separates the crust from the mantle?
The Mohorovičić (Moho) discontinuity.
⚡ Metamorphic pairs: 'Lime-Marble, Sand-Quartz, Shale-Slate, Granite-Gneiss'
The first letters of the new rock are easy: Marble from Limestone is the most asked (Taj Mahal marble).
Where students lose marks
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.
Practice sets — 20 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 4 min · wrong answers go to your mistake notebook automatically.