Big picture
The deepest mines and boreholes reach only a small fraction of the earth's radius. Yet geographers can explain its internal layers because rocks, gravity, magnetism and especially earthquake waves reveal properties of inaccessible material. Understanding the interior requires separating direct observations from inferences.
When a building shakes in an earthquake, the motion begins with energy released within the earth. The same waves that create a hazard also act as probes of the planet. This chapter connects internal structure, seismic evidence, earthquake distribution and volcanic activity rather than treating them as unrelated lists.
Learning goals
Distinguish direct and indirect sources of information about the interior.
Explain earthquake focus, epicentre and seismic waves.
Use wave behaviour and shadow zones to infer internal layers.
Compare crust, mantle and core in composition and physical state.
Classify earthquakes, volcanoes and intrusive volcanic forms.
Interpret a simple earthquake or internal-structure diagram.
Core ideas
1. Sources of information
Direct sources include rocks obtained from mining and drilling and materials brought upwards by volcanic activity. They provide actual samples, but their depth coverage is limited. Volcanic material may also change during ascent, so it is not an unchanged sample from every internal layer.
Indirect sources include seismic waves, gravity, magnetic observations and meteorites. Meteorites help scientists infer materials associated with planetary formation, but they are not pieces drilled from the earth's core. Changes in temperature, pressure and density with depth also inform models. For example, unexpectedly high density inferred for the whole earth compared with surface rocks suggests denser material inside. No single source answers every structural question.
2. Earthquake origin and location
An earthquake is shaking caused by a sudden release of energy within the earth. Commonly, stress builds as rocks resist movement along a fault, a fracture across which displacement occurs. Sudden slip releases stored energy.
The focus, or hypocentre, is the location within the earth where rupture begins. The epicentre is the point on the surface vertically above it. A diagram should show a sloping fault, an underground focus, outward-spreading waves and the epicentre directly overhead. For example, a distant city may experience shaking although the focus lies beneath another district; the earthquake is not necessarily generated directly below every damaged building.
3. Body and surface waves
Body waves travel through the interior. P waves, or primary waves, arrive first and involve compression and expansion parallel to travel. They pass through solids, liquids and gases. S waves, or secondary waves, involve motion perpendicular to travel and require material with resistance to shear; they do not travel through liquids.
Surface waves travel along the earth's surface and commonly cause severe damage through large ground movements. Their behaviour differs from that of body waves.
Wave | Particle motion or path | Material limitation | Interpretive importance |
|---|---|---|---|
P | Compression parallel to propagation | Passes through solids and fluids | Refraction reveals changing internal properties |
S | Shear perpendicular to propagation | Cannot pass through liquids | Absence through the outer core indicates liquid material |
Surface | Motion concentrated near the surface | Travels along the surface region | Often especially destructive to structures |
For example, detecting a P arrival before an S arrival is expected, not evidence of two separate earthquakes.
4. Refraction and shadow zones
Wave speed changes with material properties, and waves bend or refract at boundaries. A shadow zone is a region where particular direct seismic waves from an earthquake are not recorded because of the earth's internal structure.
In the standard school diagram, the P-wave shadow zone extends approximately from 105° to 145° angular distance from the epicentre. S waves are not received beyond about 105° by a direct path through the core because the outer core is liquid. The P-wave gap is not explained by an inability of P waves to pass through any liquid: they can do so, but their paths change strongly at the core boundary.
Draw a circular earth cross-section and several curved ray paths rather than straight lines through all layers. Label the angular distances at the centre. These are angles on the globe, not kilometre distances or compass bearings from a local map.
5. Crust, mantle and core
The crust is the thin outer solid layer. Oceanic crust is generally thinner and denser than continental crust and is commonly basaltic; continental crust has a greater proportion of granitic material. Crustal thickness varies considerably, especially beneath mountain belts.
The mantle extends from below the crust to about 2,900 km depth. It consists mainly of silicate material rich in magnesium and iron. Much of it is solid but can deform over geological time. The core lies below the mantle and is dominated by iron with nickel and other components. Its outer portion is liquid and its inner portion solid because state depends on pressure as well as temperature.
Boundary or layer | Meaning | Example of evidence |
|---|---|---|
Mohorovičić discontinuity | Crust–mantle boundary | Change in seismic-wave velocities |
Gutenberg discontinuity | Mantle–core boundary | Strong wave refraction and S-wave exclusion |
Outer core | Liquid metallic region | S-wave behaviour |
Inner core | Solid central region | Seismic evidence of a distinct central zone |
Do not confuse compositional divisions with mechanical divisions: crust and mantle describe composition, while lithosphere and asthenosphere chiefly describe behaviour.
6. Lithosphere and asthenosphere
The lithosphere comprises the crust and rigid uppermost mantle. It is divided into tectonic plates. Beneath it, the asthenosphere is a relatively weak, deformable zone in the upper mantle. It is not a global underground ocean of fully molten rock.
For example, an oceanic plate contains both oceanic crust and uppermost mantle. Calling the plate crust alone omits part of its rigid thickness. The asthenosphere's ability to deform helps accommodate plate movement, while mantle circulation and forces associated with plate boundaries contribute to tectonic motion. Later chapters use this distinction to explain earthquakes, mountain building and volcanism.
7. Earthquake types, measurement and effects
Most earthquakes are tectonic, linked to fault movement. Others are associated with volcanism, collapse of underground cavities, explosions or changes connected with large reservoirs. Reservoir-induced seismicity does not mean every reservoir produces a destructive earthquake.
Magnitude describes earthquake size from instrumental measurements; intensity describes shaking effects at a particular location. One earthquake has different intensities at different places because distance, depth, ground material and building vulnerability differ. For example, soft sediments may amplify shaking compared with nearby firm rock.
Effects include ground rupture, landslides, liquefaction, fires and infrastructure disruption. Submarine displacement can generate a tsunami, but not every undersea earthquake does so. Disaster severity therefore depends on exposure and vulnerability as well as the released energy.
8. Volcanism and surface forms
Magma is molten or partly molten rock beneath the surface; lava is magma that reaches the surface. Volcanic emissions also include gases, ash and rock fragments. Eruption style depends partly on viscosity and gas behaviour.
Shield volcanoes have broad, gentle slopes built mainly from fluid lava, as in Hawaii. Composite volcanoes commonly have steeper cones built from alternating lava and fragmental deposits; Mount Fuji is an example. A caldera is a large depression often associated with collapse after major magma withdrawal. Flood basalts, such as the Deccan volcanic province, result from extensive lava outpourings, often through fissures. These forms are not simply different names for one conical hill.
9. Intrusive forms and world distribution
Magma cooling below the surface forms intrusive bodies. Later erosion may expose them.
Form | Geometry and relationship | Imagined example |
|---|---|---|
Batholith | Very large irregular deep-seated body | A broad granite mass exposed after erosion |
Laccolith | Dome-shaped intrusion lifting overlying layers | A mushroom-like body between strata |
Lopolith | Basin- or saucer-shaped body | An intrusion sagging towards its centre |
Phacolith | Lens-shaped body associated with fold crests or troughs | Magma concentrated in a folded layer |
Sill | Generally parallel to surrounding layers | A horizontal sheet between beds |
Dyke | Cuts across surrounding layers | A steep sheet crossing sedimentary beds |
On a world map, locate the Pacific margins, the Mediterranean–Himalayan belt and the Mid-Atlantic Ridge. These major seismic or volcanic belts relate to plate boundaries, although some volcanism occurs within plates. Barren Island in the Andaman Sea is an important Indian volcanic location. Map patterns are evidence for relationships, not a claim that every point along a belt is equally active.
A reliable approach to questions
Identify whether the evidence is a sample, measurement, wave observation or surface effect.
Define wave properties before interpreting a shadow-zone diagram.
Separate compositional layers from mechanical layers and label their boundaries.
Compare volcanic forms using shape, emplacement and relation to rock layers.
For earthquake damage, explain the hazard together with local exposure and vulnerability.
Connections that matter
Origin and evolution of the earth explains why the planet differentiated into layers.
Distribution of oceans and continents links internal behaviour to moving plates.
Natural hazards examines preparedness and vulnerability in earthquake-prone regions.
Common mistakes
Calling the epicentre an underground point: the focus is underground.
Saying P waves cannot cross liquids: this restriction applies to S waves.
Describing the entire mantle as liquid: much is solid but deforms slowly.
Interchanging sill and dyke: a sill follows layers, whereas a dyke cuts across them.
Revision checklist
I can compare direct and indirect evidence.
I can explain seismic waves and shadow zones.
I can distinguish crust, mantle, core, lithosphere and asthenosphere.
I can classify volcanic landforms and intrusions.
I can separate earthquake magnitude from local intensity.