How do we know what is inside? Direct and indirect sources
The deepest hole ever drilled (Kola, in the Arctic) went only about 12 km. The Earth's radius is about 6,370 km. So most knowledge is indirect.
| Direct sources | Indirect sources |
|---|---|
| Rocks from mines (a few km deep) | Temperature and pressure rise with depth; density rises too |
| Deep drilling projects in the ocean floor and on land | Meteors: made of material like the Earth's interior |
| Lava and gases from volcanoes | Gravity differences (gravity anomaly) show where heavy or light rocks lie |
| Magnetic surveys show magnetic rocks | |
| Earthquake (seismic) waves: the most important clue |
Earthquakes and earthquake waves
An earthquake is the shaking of the Earth when energy stored in rocks is suddenly released, usually along a fault (a crack where rocks move). The point inside where energy is released is the focus (hypocentre). The point on the surface right above it is the epicentre.
Body waves (travel through the inside)
- P-waves (primary): fastest, arrive first. They push and pull the rock in the same direction as they travel, like a slinky squeezed from one end. They pass through solids, liquids and gases.
- S-waves (secondary): slower. They shake rock at right angles to their travel, like a rope flicked up and down. They pass only through solids.
Surface waves
When body waves reach the surface they make surface waves. These are slowest but move the ground most, so they cause the most damage.
Waves change speed and bend (refract) or bounce (reflect) when they enter material of different density. By timing waves at many seismographs (instruments that record shaking), scientists map the inside.
Shadow zones
A shadow zone is an area on the surface where a particular wave does not arrive.
- Seismographs within about 105° of the epicentre record both P and S waves.
- Beyond 105°, S-waves never arrive. Since S-waves cannot pass through liquid, this shows the outer core is liquid. The S-wave shadow covers more than 40% of the Earth.
- Between about 105° and 145° no direct P or S waves arrive: this belt is the shadow zone. P-waves bend in the core and reappear beyond 145°.
Types of earthquakes, scales and effects
Types
- Tectonic: rocks slip along faults (most common).
- Volcanic: linked with active volcanoes.
- Collapse: roofs of underground mines cave in.
- Explosion: caused by nuclear or chemical explosions.
- Reservoir-induced: in areas of large dams, from the weight of water.
Scales
| Magnitude (Richter scale) | Intensity (Mercalli scale) |
|---|---|
| Energy released | Damage and what people feel |
| 0 to 10 | 1 to 12 |
| One number per quake | Different at different places |
Each +1 in magnitude means about 32 times more energy.
Effects
Ground shaking, ground cracks and sinking, landslides, soil turning to mud (liquefaction), avalanches, floods from broken dams, fires, building collapse and, under the sea, tsunamis.
Try it at home: stack blocks or books on a tray and shake it gently, then hard. Which stack falls first: tall-thin or short-wide? This is why building design matters.
Structure of the Earth
| Layer | Depth / thickness | Facts |
|---|---|---|
| Crust | About 5 km under oceans; about 30 km on average under continents, up to 70 km under big mountains | Brittle, solid. Oceanic crust is basalt (heavier); continental crust is granite-like (lighter). Density about 3 g/cm³. |
| Mantle | From the Moho to 2,900 km | Upper part (to about 400 km) has the soft asthenosphere, the main source of magma. Crust + top of mantle = lithosphere (10–200 km). Density about 3.4 g/cm³ at top. |
| Outer core | 2,900 to 5,150 km | Liquid (S-waves stop here). Made of nickel and iron (nife). |
| Inner core | 5,150 to about 6,370 km | Solid due to huge pressure. Density about 13 g/cm³. |
The boundary between crust and mantle is the Mohorovičić discontinuity (Moho); between mantle and core is the Gutenberg discontinuity.
Volcanoes and volcanic landforms
A volcano is a place where magma, gases and ash come out of the ground. Magma under the ground becomes lava at the surface.
Types of volcanoes
- Shield volcano: runny basalt lava spreads far, giving gentle slopes (Hawaii). Largest volcanoes.
- Composite volcano: thick, sticky lava with ash, erupts explosively; layers make a steep cone.
- Caldera: most explosive; the top collapses into a huge hollow after emptying the magma chamber.
- Flood basalt province: very runny lava pours from cracks and covers huge areas in layers, like the Deccan Traps of India.
- Mid-ocean ridge volcanoes: long chains of eruptions along ridges under the oceans.
Intrusive landforms (magma cools inside the crust)
| Landform | Shape |
|---|---|
| Batholith | Huge dome-shaped granite body deep down, seen when rock above wears away |
| Laccolith | Dome with a flat base, fed by a pipe from below |
| Lopolith | Saucer-shaped (bends downward) |
| Phacolith | Wavy body in the crests or troughs of folded rock |
| Sill / sheet | Flat, horizontal layer (thin = sheet) |
| Dyke | Wall-like, nearly vertical; feeds lava to the surface |
Key formulas and definitions
- Direct sources: mines, drilling, lava. Indirect: temperature/pressure/density with depth, meteors, gravity, magnetism, seismic waves
- Focus (hypocentre) = start point inside; Epicentre = point on the surface above it
- P-waves: fastest, push–pull, through solid + liquid + gas
- S-waves: slower, side-to-side, through solids only
- S-wave shadow: beyond 105°; P-wave shadow zone: 105° to 145°
- Magnitude (Richter 0–10) = energy; Intensity (Mercalli 1–12) = damage; +1 magnitude ≈ 32× energy
- Crust ≈ 5 km (ocean) / 30 km (continent); Mantle to 2,900 km; Outer core liquid; Inner core solid
- Moho = crust|mantle; Gutenberg = mantle|core
- Lithosphere = crust + upper mantle; Asthenosphere = soft layer (magma source)
Worked examples
1. A seismograph 120° away from the epicentre records no direct waves. Why?
It lies in the shadow zone (105°–145°). S-waves are stopped by the liquid outer core and P-waves are bent away by the core.
2. An earthquake of magnitude 6 releases how many times more energy than a magnitude 5?
About 32 times, because each step of 1 on the magnitude scale means about 32 times more energy.
3. How many times more energy is a magnitude 7 than a magnitude 5?
32 × 32 ≈ 1,024, so about 1,000 times more.
4. Why do S-waves prove the outer core is liquid?
S-waves cannot pass through liquids. They never arrive beyond about 105°, meaning a liquid layer blocks them. That layer is the outer core.
5. Identify: a wall-like body of cooled magma cutting straight up through rock layers.
A dyke. (A flat horizontal one would be a sill.)
6. The Deccan Traps are made of many flat layers of dark basalt. Which volcanic form is this?
A flood basalt province: very runny lava poured from cracks and covered a huge area layer by layer.
Common mistakes
- Saying the epicentre is deep inside. The focus is inside; the epicentre is on the surface.
- Thinking S-waves are faster. P (primary) waves are faster and arrive first.
- Mixing magnitude and intensity. Magnitude = energy (one number); intensity = damage (varies by place).
- Calling the mantle liquid. The mantle is solid but soft near the top (asthenosphere). Only the outer core is liquid.