Geomorphology: Earth Interior and Plate Tectonics

Core concept and process

How we know the interior: seismic waves

We cannot dig to the centre, so the interior is read indirectly through earthquake (seismic) waves recorded on seismographs, plus volcanic material and meteorite evidence. Earthquakes generate body waves (travelling through the interior) and surface waves (travelling along the surface).

• Primary (P) waves: longitudinal or compressional, fastest, arrive first, and pass through solids, liquids and gases. They cause particles to vibrate in the direction of travel (push-pull).
• Secondary (S) waves: transverse, slower, arrive second, and pass only through solids. They cannot travel through liquids, which is the single most important seismological fact: because S-waves vanish on the far side of the earth, the outer core must be liquid. This is the S-wave shadow zone.
• Surface (L) waves: long-period, slowest, travel along the crust, and are the most destructive to structures.

The P-wave shadow zone (between about 105 and 145° from the epicentre) and the S-wave shadow zone (beyond about 105°) together fix the size and state of the core.

The layers and their discontinuities

From outside in, the earth is layered by composition and state, separated by sharp boundaries (discontinuities) where wave velocity changes:

• Crust: thin and rigid. Continental crust averages about 30 km (thicker, up to 70 km under mountains) and is granitic, called sial (silica and aluminium), lighter. Oceanic crust is about 5 km thick, basaltic, called sima (silica and magnesium), denser. The Conrad discontinuity separates sial from sima within the crust.
• Moho (Mohorovicic) discontinuity: the crust-mantle boundary.
• Mantle: extends to about 2,900 km. Its upper rigid part plus the crust forms the lithosphere (the rigid plates). Below lies the asthenosphere (upper mantle), plastic and capable of slow flow, on which the plates ride. The Repetti discontinuity separates the upper and lower mantle.
• Gutenberg (Wiechert-Gutenberg) discontinuity: the mantle-core boundary at about 2,900 km.
• Core: the outer core is liquid (creates the S-wave shadow); the inner core is solid (kept solid by extreme pressure). The Lehmann discontinuity separates the outer and inner core. The core is largely iron and nickel (called nife), and its motion generates the earth's magnetic field. The centre lies at about 6,371 km depth.

Continental drift to plate tectonics

Alfred Wegener (1912) proposed continental drift: a single supercontinent, Pangaea, surrounded by the ocean Panthalassa, split into Laurasia (north) and Gondwana (south), which then drifted apart. His evidence was the jigsaw fit of coastlines, matching fossils (Mesosaurus, Glossopteris), matching rock and mountain belts across oceans, and ancient glacial deposits in now-tropical lands. He could not explain the driving force, which weakened the theory.

Harry Hess (1960s) proposed sea-floor spreading: new ocean crust forms at mid-ocean ridges, spreads outward, and is destroyed at trenches. Proof came from palaeomagnetism (symmetrical magnetic stripes either side of ridges recording reversals of the earth's field) and from the youngest ocean rocks lying at the ridge and the oldest near the continents.

Plate tectonics (1960s) unified all of this: the lithosphere is broken into rigid plates that move over the asthenosphere, driven by mantle convection currents, aided by ridge push and slab pull. There are seven major plates and several minor ones (Nazca, Cocos, Caribbean, Philippine, Arabian, Scotia, Juan de Fuca).

The three plate boundaries

• Divergent (constructive): plates move apart, magma rises, new crust forms. Examples: the Mid-Atlantic Ridge (Eurasian and North American plates pulling apart, with Iceland astride it), and the East African Rift Valley (a continent splitting).
• Convergent (destructive): plates collide and crust is consumed by subduction.
  • Oceanic-continental: the denser oceanic plate subducts under the continental, building fold mountains and a coastal volcanic arc, for example the Andes (Nazca under South American plate).
  • Oceanic-oceanic: one ocean plate subducts under the other, building deep trenches and volcanic island arcs, for example Japan, the Philippines, the Mariana arc.
  • Continental-continental: neither subducts, so crust crumples upward into the highest fold mountains, for example the Himalaya (Indian plate into Eurasian plate). No volcanism here.
• Transform (conservative): plates slide horizontally past each other and crust is neither created nor destroyed, only fractured, for example the San Andreas Fault, California (Pacific and North American plates).

Volcanoes

A volcano is a vent through which magma, gases and pyroclastic material escape. Classified by activity into active (Etna, Stromboli, Kilauea), dormant (Vesuvius, Fujiyama), and extinct (Aconcagua, Deccan vents). Classified by form into shield (broad, gentle, basaltic, like Mauna Loa, Hawaii), composite or strato (steep, alternating lava and ash, like Fujiyama and Vesuvius), cinder cone (small, steep, ash), and caldera (a vast collapsed crater). Intrusive (plutonic) igneous forms include the batholith, laccolith, sill, dyke and phacolith. The Pacific Ring of Fire, a horseshoe of subduction zones, holds about two-thirds of the world's active volcanoes and most great earthquakes. Hot-spot volcanism (a fixed mantle plume) builds chains like the Hawaiian Islands away from any plate boundary.

Earthquakes

The focus (hypocentre) is the point of energy release inside the earth; the epicentre is the point on the surface directly above it. Earthquakes are caused mostly by tectonic stress released along faults, and also by volcanic activity, reservoir loading (induced seismicity behind large dams) and collapse. Magnitude (energy released) is measured on the Richter scale (logarithmic, so each whole step is about 32 times more energy; the moment magnitude scale is now preferred for large quakes). Intensity (felt effects and damage) is measured on the modified Mercalli scale, running from I (not felt) to XII (total destruction). A submarine earthquake or landslide can displace water and generate a tsunami. Most earthquakes lie in two great belts: the Circum-Pacific belt (the Ring of Fire) and the Alpine-Himalayan (Mediterranean-Himalayan) belt, both at convergent plate margins.

Rocks: the raw material of landforms

Geomorphology rests on three rock families, a frequent matching question:

• Igneous rocks: formed by the cooling of magma or lava. Intrusive or plutonic rocks cool slowly underground with coarse crystals (granite); extrusive or volcanic rocks cool fast on the surface with fine crystals (basalt, the rock of the Deccan Traps). Igneous rocks are the "primary" rocks and carry no fossils.
• Sedimentary rocks: formed by the deposition and compaction of sediment in layers (strata). They often hold fossils, coal, petroleum and limestone. Examples are sandstone, shale, limestone and conglomerate.
• Metamorphic rocks: formed when existing rocks are altered by heat and pressure. Limestone becomes marble, sandstone becomes quartzite, shale becomes slate, granite becomes gneiss, and coal becomes graphite.

The slow conversion among these three under the agents of melting, weathering, deposition and metamorphism is the rock cycle.

Endogenic versus exogenic forces

Endogenic forces act from within (tectonic movement, volcanism, earthquakes) and build relief: they are the constructive force. They include diastrophism (slow building, splitting into epeirogenic vertical movements and orogenic mountain-building movements) and sudden movements (earthquakes, volcanism). Exogenic forces act from without (weathering, mass wasting, and the erosional and depositional work of rivers, glaciers, wind, waves and groundwater) and wear relief down: they are the destructive force, ultimately powered by the sun and gravity. The net result is gradation, the levelling of the land by degradation (wearing down) and aggradation (filling up).

Weathering and mass wasting

Weathering is the in-situ breakdown of rock, with no transport; erosion is breakdown plus removal by an agent. Weathering has three modes:

• Physical (mechanical) weathering: breaks rock without changing its chemistry, through frost action (freeze-thaw), exfoliation (onion-skin peeling from heating and cooling), thermal stress and pressure release.
• Chemical weathering: alters rock chemistry through solution, carbonation (limestone dissolved by carbonic acid, the basis of karst), oxidation (rusting of iron minerals), and hydration. It dominates in warm, humid climates and produces laterite and deep soils.
• Biological weathering: by roots, burrowing animals and the acids of plants and microbes.

Mass wasting (mass movement) is the downslope movement of rock and soil under gravity: slow creep, slumps, mudflows, and rapid landslides and rockfalls. It is a major hazard in the young, steep, rain-soaked Himalaya.

Fluvial, glacial, aeolian, karst and coastal cycles

Each erosional agent works through a sequence of forms. The Davisian "cycle of erosion" carries a fluvial landscape through stages of youth (active downcutting, V-shaped valleys, waterfalls), maturity (graded slopes, meanders) and old age (broad flood plains, ox-bow lakes, deltas), ending in a near-level peneplain interrupted only by resistant monadnocks. Glaciers carve in the highlands and deposit in the lowlands; wind sculpts and drifts in deserts; groundwater dissolves limestone into karst; and waves cut and build along coasts. The landform tables below summarise the products.