60-second revision
- Crust > Moho > mantle > Gutenberg (~2,900 km) > outer core (liquid) > Lehmann > inner core (solid).
- S-waves do not pass through liquids; their shadow zone proves the outer core is liquid.
- The asthenosphere (upper mantle) is the weak layer the plates ride on and the main source of magma.
- Divergent = ridges and rift valleys; convergent = trenches, fold mountains, volcanic arcs; transform = plates slide past.
- Limestone > marble, shale > slate, sandstone > quartzite, granite > gneiss.
- Richter = magnitude (energy); Mercalli = intensity (damage).
- Stalactites hang from the roof; stalagmites rise from the floor.
Layers of the Earth
We cannot drill deep into the Earth, so its structure is known indirectly — mainly from how earthquake (seismic) waves travel, bend and disappear, and from meteorites, gravity and volcanic material. The Earth is made of three main shells: crust, mantle and core, separated by sharp changes in wave speed called discontinuities.
| Layer | Key features | Boundary below it |
|---|---|---|
| Crust | Thinnest layer; about 30 km thick under continents (more under mountains), about 5 km under oceans. Continental crust is granitic (lighter), oceanic crust basaltic (denser) | Mohorovicic (Moho) discontinuity |
| Mantle | Extends to about 2,900 km. Upper part includes the asthenosphere. Crust plus the uppermost rigid mantle = lithosphere | Gutenberg discontinuity |
| Outer core | Liquid, mainly iron and nickel; its motion generates the Earth's magnetic field | Lehmann discontinuity |
| Inner core | Solid because of enormous pressure, despite very high temperature | — |
Seismic waves and shadow zones
- P-waves (primary): fastest, longitudinal, travel through solids, liquids and gases; reach the seismograph first.
- S-waves (secondary): slower, transverse, travel only through solids.
- Surface waves (Love and Rayleigh): slowest but most destructive, as they move along the surface.
- Shadow zone: between about 105 and 145 degrees from the epicentre neither wave is recorded directly; beyond 105 degrees S-waves are absent altogether.
Plate tectonics
Wegener's continental drift theory (1912) proposed that all continents once formed a single landmass, Pangaea, surrounded by the ocean Panthalassa. Later, evidence of sea-floor spreading led to the plate tectonic theory: the lithosphere is broken into large and small plates that move over the asthenosphere.
| Boundary type | Movement | Features and examples |
|---|---|---|
| Divergent (constructive) | Plates move apart; new crust forms | Mid-ocean ridges (Mid-Atlantic Ridge), rift valleys (East African Rift) |
| Convergent (destructive) | Plates collide; one may sink (subduct) | Ocean–ocean or ocean–continent: deep trenches and volcanic arcs (Pacific Ring of Fire). Continent–continent: fold mountains (Himalayas) |
| Transform (conservative) | Plates slide past each other; crust neither made nor destroyed | San Andreas Fault, California |
Rocks
- Igneous rocks form from cooled magma or lava: granite (intrusive, coarse grained), basalt (extrusive, fine grained). They contain no fossils.
- Sedimentary rocks form in layers from deposited sediment: sandstone, shale, limestone, coal. They may contain fossils.
- Metamorphic rocks form when existing rocks change under heat and pressure: limestone to marble, shale to slate, sandstone to quartzite, granite to gneiss.
Earthquakes and volcanoes
The point inside the Earth where the energy is released is the focus (hypocentre); the point directly above it on the surface is the epicentre. Magnitude is measured on the Richter scale (logarithmic: each whole number means about ten times more ground motion). Intensity, the observed damage, is measured on the Mercalli scale.
| Volcano type | Features | Example |
|---|---|---|
| Shield | Fluid basaltic lava, gentle slopes, broad cone | Mauna Loa, Hawaii |
| Composite (strato) | Alternate layers of lava and ash, explosive eruptions, steep cone | Mount Fuji, Vesuvius |
| Caldera | Large depression left after an explosive collapse | Crater Lake, USA |
| Flood basalt | Huge sheets of fluid lava over wide areas | Deccan Traps, India |
Landforms by agent
| Agent | Erosional landforms | Depositional landforms |
|---|---|---|
| Running water | V-shaped valleys, gorges, canyons, waterfalls, potholes | Alluvial fans, floodplains, levees, meanders, oxbow lakes, deltas |
| Groundwater (karst) | Sinkholes, uvalas, caves, lapies | Stalactites, stalagmites, pillars |
| Glaciers | Cirques, U-shaped valleys, hanging valleys, horns, aretes | Moraines, eskers, drumlins, outwash plains |
| Wind | Mushroom rocks, yardangs, deflation hollows, inselbergs | Barchans, seif dunes, loess |
| Waves | Cliffs, sea caves, arches, stacks | Beaches, spits, bars, lagoons |
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