World Geography
🔒 Log in to trackEarth's interior, earthquakes, rocks and volcanoes
🔒 Log in to trackWe know the inside of the Earth from seismic waves, not from digging. The Earth has three layers: crust, mantle and core. Earthquakes, volcanoes and mountains show how the crust moves. Rocks are grouped into three families by how they form.
Layers of the Earth
Our knowledge of the interior comes from seismic waves and mining.
| Layer | Depth (about) | Key facts |
|---|---|---|
| Crust | 5-10 km (ocean floors), 30-70 km (continents) | Thinnest layer. Continental crust is SiAl, lighter. Oceanic crust is SiMa, denser |
| Mantle | Up to about 2,900 km | Mostly solid rock. Its upper, partly molten part (asthenosphere) feeds magma |
| Core | 2,900 km to the centre (6,371 km) | NiFe, nickel and iron. Outer core liquid, inner core solid |
SiAl means silica + aluminium. SiMa means silica + magnesium.
Discontinuities are surfaces where wave speed jumps. From the top down:
- Conrad: inside the crust.
- Moho (Mohorovicic, 1909): crust and mantle.
- Repetti: inside the mantle.
- Gutenberg: mantle and outer core, about 2,900 km down.
- Lehmann: outer core and inner core.
Tip: Moho is crust to mantle. Gutenberg is mantle to core. Lehmann divides the two cores.
Earthquakes
An earthquake is a sudden shake as stored energy is released along faults.
- The starting point inside the Earth is the focus (hypocentre).
- The point on the surface right above it is the epicentre. The shock is strongest there.
- A seismograph records the waves.
| Wave | Speed | Special fact |
|---|---|---|
| P (primary) | Fastest | Passes through solids and liquids |
| S (secondary) | Slower | Cannot pass through liquids, which proves the outer core is liquid |
| L (surface) | Slowest | Arrives last and does the most damage |
The Richter scale measures magnitude. The Mercalli scale measures intensity, which means damage.
About 80% of earthquakes strike the Circum-Pacific belt, the Ring of Fire. A giant sea wave from an undersea quake is a tsunami. The word is Japanese for "harbour wave". The Indian Ocean tsunami came in 2004.
Volcanoes
A volcano is an opening where magma, ash and gases escape.
- Active: Barren Island (India's only active volcano), Stromboli (the "Lighthouse of the Mediterranean"), Etna, and Vesuvius (it buried Pompeii in 79 AD).
- Dormant: sleeping, such as Fuji.
- Extinct: no record of an eruption.
By shape:
- Shield: broad, with runny lava. Example: Mauna Loa, Hawaii.
- Composite (strato): steep, with layers of lava and ash. Examples: Fuji, Vesuvius.
- Caldera: a collapsed crater.
Volcanoes cluster in the Ring of Fire and along mid-ocean ridges. Geysers are related. Old Faithful in Yellowstone, USA, is one.
Rocks in three families
| Family | How it forms | Examples |
|---|---|---|
| Igneous | Magma or lava cools. Oldest, "primary" rocks | Granite (intrusive, big crystals), basalt (extrusive, ocean floors) |
| Sedimentary | Layers of sediment press together. Only rocks with fossils. Cover most of the land | Sandstone, shale, limestone, coal |
| Metamorphic | Heat and pressure change an existing rock | See the pairs below |
Metamorphic pairs (parent to new rock):
- Limestone to marble
- Sandstone to quartzite
- Shale to slate
- Granite to gneiss
The endless change of one rock type into another is the rock cycle. Coal, oil and limestone sit in sedimentary rocks.
Watch: Granite is an igneous rock. It is not sedimentary.
Mountains, plateaus and plains
- Fold mountains: crust squeezed into folds by plate collision. The young ones (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: sunken blocks between faults (Rhine, East Africa).
- Volcanic mountains: built by eruptions (Fuji, Kilimanjaro).
- Plateaus: raised flatlands. Tibet is the highest and largest. The Deccan is another.
- Plains: low, flat, fertile and the most densely populated.
Plates and drifting continents
About two-thirds of active volcanoes lie on the Pacific Ring of Fire.
- Fold mountains 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. The ocean around it was Panthalassa.
- Krakatoa (Indonesia) erupted in 1883. Mauna Loa (Hawaii) is the largest active volcano.
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
'Boundary between crust and mantle', 'layer of nickel and iron', SiAl / SiMa / NiFe, order of the discontinuities.
Write the layers: crust, mantle, core.
Attach the make-up: SiAl or SiMa, then NiFe.
List discontinuities top down: Conrad, Moho, Repetti, Gutenberg, Lehmann.
Match each name to the layers it separates.
Each discontinuity has a fixed position, so one ordered list answers every question.
The Gutenberg discontinuity separates which two layers?
Show solutionHide solution
It lies about 2,900 km down.
That depth is the edge of the core.
The mantle and the outer core
Seismic waves and earthquake vocabulary
'Which wave cannot pass through liquid', focus vs epicentre, Richter vs Mercalli, seismograph, Ring of Fire.
P fastest, S stops in liquids, L slowest and most destructive.
Focus is inside. Epicentre is on the surface above it.
Richter is magnitude. Mercalli is intensity.
About 80% of quakes strike the Ring of Fire.
These are word pairs, and each option swaps one word of a pair.
The point inside the Earth where an earthquake starts is called the:
Show solutionHide solution
The point inside is the focus.
The epicentre is the surface point above it.
Focus (hypocentre)
Rock families and metamorphic pairs
'Marble is formed from', 'fossils are found in which rock', 'which is an igneous rock'.
Sort the rock: cooled magma is igneous.
Layered deposits with fossils are sedimentary.
Changed by heat and pressure is metamorphic.
Recall the four pairs: limestone-marble, sandstone-quartzite, shale-slate, granite-gneiss.
Three families and four pairs cover nearly every rock question.
Slate is a metamorphic rock. Which parent rock does it come from?
Show solutionHide solution
Slate is one of the four classic pairs.
The parent is shale.
Shale
Volcanoes: activity, shape and famous names
'India's only active volcano', 'Lighthouse of the Mediterranean', shield vs composite volcano, Krakatoa or Vesuvius.
By activity: active, dormant or extinct.
By shape: shield, composite or caldera.
Match names: Barren Island for India, Stromboli for the Lighthouse.
Krakatoa (1883) is the classic explosive eruption.
A short list of names and two classifications covers this topic.
Stromboli, the 'Lighthouse of the Mediterranean', is what kind of volcano?
Show solutionHide solution
Stromboli is in Italy.
It is on the list of active volcanoes.
An active volcano
Mountain and landform types
'Young fold mountains are', 'a rift valley is formed by', block mountains, highest plateau.
Fold: crust squeezed. Young folds are the highest.
Block: land raised between faults. Rift valley: land sunk between faults.
Plateau is raised flatland. Tibet is the highest.
Plain is low and flat, and the most populated.
Each landform has one cause, and the cause names the type.
The Rhine valley and the East African rifts are examples of:
Show solutionHide solution
Both are sunken blocks between faults.
Land that sinks between two faults is a rift valley.
Rift valleys
Crust types and depth figures
'Continental crust is made of', 'depth of the mantle', 'SiMa is found in', crust thickness.
Continental crust is SiAl, lighter and thicker (30-70 km).
Oceanic crust is SiMa, denser (5-10 km).
The mantle reaches about 2,900 km. The core runs to 6,371 km.
Depth and make-up figures are paired, so learn them together.
The lighter continental crust is mostly made of which pair of elements?
Show solutionHide solution
SiAl stands for silica and aluminium.
SiMa (silica and magnesium) is the oceanic crust.
Silica and aluminium (SiAl)
Shortcuts that save time
Conrad (inside crust), Moho (crust and mantle), Gutenberg (mantle and core), Lehmann (outer and inner core). Repetti sits in the mantle between Moho and Gutenberg.
Which discontinuity separates the crust from the mantle?
Show solutionHide solution
The crust-mantle surface is named after Mohorovicic.
The Moho discontinuity
Lime to Marble, Sand to Quartz(ite), Shale to Slate, Granite to Gneiss. Marble from limestone is the most asked, for example the marble of the Taj Mahal.
Mistakes to avoid
Where most students lose marks on this subtopic.
Saying S-waves pass through the outer core.
S-waves stop at liquids. That proves the outer core is liquid.
Mixing Richter and Mercalli.
Richter is magnitude. Mercalli is intensity.
Calling granite a sedimentary rock.
Granite is an intrusive igneous rock.
Looking for fossils in igneous rocks.
Only sedimentary rocks hold fossils.
Placing the Moho below the core.
Moho lies between crust and mantle. Gutenberg lies between mantle and core.
Quick revision
Read this the night before the exam.
Layers: crust (SiAl, SiMa), mantle, core (NiFe). Outer core liquid, inner core solid.
Discontinuities: Conrad, Moho, Repetti, Gutenberg, Lehmann.
S-waves stop at liquid. Richter is magnitude. Mercalli is intensity.
80% of earthquakes and most volcanoes are on the Pacific Ring of Fire. Barren Island is India's only active volcano.
Igneous: granite, basalt. Sedimentary: fossils, limestone, coal. Metamorphic: marble, slate, quartzite, gneiss.
Young fold mountains: Himalayas, Alps, Andes, Rockies. Tibet is the 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.