Basic Reviews of the Earth

Plate tectonics is the theory that the Earth's rigid outer shell, the lithosphere, is broken into a number of large and small plates that float on the plastic asthenosphere and move relative to each other at a few centimetres per year. Most earthquakes, volcanoes and mountain building occur along plate edges.

Types of plate boundaries

1. Divergent (constructive) boundary

• Plates move apart; magma rises and forms new crust.
• Examples: Mid-Atlantic Ridge, East African Rift.
• Features: mid-ocean ridges, rift valleys, shallow earthquakes, basaltic volcanism.

2. Convergent (destructive) boundary

• Plates move towards each other; one may sink (subduct) under the other.
• Types:
  • Ocean-ocean: island arcs and trenches (Japan, Mariana).
  • Ocean-continent: trench and volcanic mountain chain (Andes).
  • Continent-continent: collision and folded mountains (Himalaya, Alps).
• Features: deep earthquakes, volcanoes, trenches, fold mountains.

3. Transform (conservative) boundary

• Plates slide horizontally past each other; crust is neither created nor destroyed.
• Example: San Andreas Fault.
• Features: strike-slip faults, shallow strong earthquakes.

 Divergent      Convergent       Transform
 <-- | -->      --> | <--       ^  |
   ridge          trench         | v

The Earth is divided into three main concentric layers, crust, mantle and core, on the basis of seismic wave behaviour. Its radius is about 6371 km.

        _______________
      /   Crust (5-70)  \
     /  ______________   \
    /  /   Mantle      \   \
   |  |   (to 2900 km)  |   |
   |  |  __________     |   |
   |  | / Outer core \  |   |
   |  | | (liquid)   |  |   |
   |  | |  Inner     |  |   |
   |  | |  core      |  |   |
   |  | \__(solid)__/   |   |
    \  \______________/   /
     \___________________/

1. Crust

• Outermost layer, thickness about 5-10 km under oceans and 30-70 km under continents.
• Continental crust: granitic (sial), density about 2.7 g/cm³.
• Oceanic crust: basaltic (sima), density about 3.0 g/cm³.
• Separated from the mantle by the Mohorovicic (Moho) discontinuity.

2. Mantle

• Extends from the Moho to 2900 km depth; about 84% of Earth's volume.
• Made of ultramafic rock (peridotite), density 3.3-5.5 g/cm³.
• Upper mantle includes the partly molten asthenosphere; lithosphere = crust + top of upper mantle.
• Separated from the core by the Gutenberg discontinuity.

3. Core

• From 2900 km to 6371 km; mostly iron and nickel (nife).
• Outer core (2900-5150 km): liquid, since S-waves do not pass through it.
• Inner core (5150-6371 km): solid due to high pressure; density about 13 g/cm³.
• Temperature reaches about 5000-6000 °C at the centre.

The structure is known from the speed and path of earthquake (seismic) waves.

The Himalaya is a young fold mountain belt formed by the collision of the Indian plate with the Eurasian (Tibetan) plate.

Stages of formation

1. Gondwana stage (about 200 Ma): India was part of the southern supercontinent Gondwanaland. It separated, and the Indian plate began to move northwards at a few cm per year.
2. Tethys Sea: Between India and Eurasia lay the Tethys Ocean. Thick marine sediments collected on its floor and margins.
3. Subduction (about 100-65 Ma): Oceanic crust of Tethys was subducted beneath Eurasia, causing volcanism and uplift on the Tibetan side.
4. Collision (about 50-55 Ma): The Indian continental crust met Eurasia. Being light, it could not sink fully, so the sediments were squeezed, folded, thrust and uplifted.
5. Main thrusting and uplift: Large thrust faults formed in sequence southwards, the Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT).
6. Continuing movement: India still moves north at about 4-5 cm per year; the Himalaya rises about 5 mm per year, causing earthquakes.

 Before:   INDIA  ~~Tethys~~  EURASIA
            ->      sea         
 After:     Himalaya  |  Tibet
            /\/\/\ folds, thrusts

Result

• Divides into Siwalik (south), Lesser Himalaya, Higher (Greater) Himalaya and Tethys Himalaya (north), separated by thrusts.
• Marine fossils on Everest show the rocks were once sea floor.

The Earth's surface shows large-scale relief features, called landforms.

Major physical features

1. Continents and ocean basins: the two first-order features. Continents cover 29% of the surface and oceans 71%.
2. Mountains: high, steep land of great relief.
   • Fold mountains (Himalaya, Alps, Andes).
   • Block (fault) mountains (Black Forest).
   • Volcanic mountains (Mount Fuji).
   • Residual mountains (Aravalli).
3. Plateaus: broad, high, flat-topped land (Tibetan Plateau, Deccan Plateau).
4. Plains: wide, low, flat land (Gangetic plain, Terai).
5. Valleys: low land between hills, cut by rivers (V-shaped) or glaciers (U-shaped).
6. Hills: small, rounded elevations lower than mountains.
7. Deserts, glaciers and river deltas.

Ocean-floor features

• Continental shelf and slope.
• Abyssal plains.
• Mid-ocean ridges.
• Deep ocean trenches (Mariana Trench).

Features of Nepal: Terai plain, Siwalik (Churia) hills, Mahabharat range, mid-hills valleys and the Himalaya.

A mountain system is a group of mountain ranges related in origin, age and structure, forming a long belt, for example the Alpine-Himalayan system.

Fold mountains

• Formed when sediments deposited in a long, sinking basin (geosyncline) are compressed by plate collision, causing them to fold, fault and rise.
• Characterised by long, narrow belts of folds (anticlines and synclines), thrust faults, metamorphism and igneous intrusions.
• Young fold mountains: Himalaya, Alps, Andes, Rockies. Old ones: Appalachians, Aravalli.

   /\    /\    /\      anticlines
  /  \  /  \  /  \
 /    \/    \/    \    synclines

Importance

• They hold major rivers' source areas, minerals and hydropower sites.
• They are tectonically active, with earthquakes and landslides, so need careful engineering geological study.

Internal structure

The Earth consists of three concentric shells: crust, mantle and core.

  Crust   : 5-70 km    (granitic/basaltic)
  ---------- Moho ----------
  Mantle  : to 2900 km (peridotite)
  ------- Gutenberg -------
  Outer core: 2900-5150 km (liquid Fe-Ni)
  Inner core: 5150-6371 km (solid Fe-Ni)

• Crust: thin outer rock layer; continental crust is granitic and thick (30-70 km), oceanic crust is basaltic and thin (5-10 km).
• Mantle: thick layer, about 2900 km; ultramafic rocks; the upper part (asthenosphere) is plastic and allows plate movement.
• Core: iron-nickel; outer core liquid, inner core solid; highest density and temperature.

Bases of study

The interior cannot be seen directly, so it is studied by:

1. Seismic waves: change of speed and direction at boundaries; S-waves stop at the outer core (liquid), P-waves are refracted and make shadow zones.
2. Density of Earth (5.5 g/cm³) is higher than surface rocks (2.7), so the interior must be heavier.
3. Meteorites: believed to represent the material of the Earth's interior.
4. Volcanic eruptions and xenoliths: samples from the mantle.
5. Deep drilling and mines: show temperature rise with depth (geothermal gradient).
6. Magnetism and gravity: the magnetic field indicates a liquid iron core.

  Surface
  +-----------------------------+
  | Crust    5-70 km            |
  |-----------Moho--------------|
  | Mantle   to 2900 km         |
  |---------Gutenberg-----------|
  | Outer core  (liquid)        |
  | Inner core  (solid)         |
  +-----------------------------+

Variations

• Composition: changes from light silicate rocks outside to heavy metals inside.
• Density: increases with depth due to compression and heavier materials, from 2.7 to about 13 g/cm³; the mean is 5.5.
• Temperature: rises with depth. In the crust the geothermal gradient is about 25-30 °C per km; the rate drops below the crust, reaching about 5000-6000 °C at the centre.

The crust is the outermost, thin, rigid, solid layer of the Earth, lying above the Mohorovicic (Moho) discontinuity. It makes up less than 1% of Earth's volume.

Types

• Continental crust: thickness 30-70 km (thickest below the Himalaya); mainly granitic (rich in silica and aluminium, called sial); density about 2.7 g/cm³; old rocks, up to 4 billion years.
• Oceanic crust: thickness 5-10 km; basaltic (silica and magnesium, sima); density about 3.0 g/cm³; young, less than 200 million years.

Features

• Composed mainly of igneous, sedimentary and metamorphic rocks.
• Most abundant elements: oxygen (46.6%), silicon (27.7%), aluminium (8.1%), iron (5%), calcium, sodium, potassium, magnesium.
• Temperature rises about 25-30 °C per km depth.
• Together with the top of the upper mantle, it forms the lithosphere, which is broken into plates.

The crust is the part on which all civil engineering works are built, and it is the layer where earthquakes, folding and faulting occur.

The mantle is the thick layer of the Earth between the crust and the core. It lies below the Mohorovicic discontinuity (about 5-70 km) and extends to 2900 km, where the Gutenberg discontinuity separates it from the core. It forms about 84% of Earth's volume.

• Composition: dense ultramafic silicate rock, mainly peridotite, rich in magnesium and iron (olivine, pyroxene).
• Density: about 3.3 g/cm³ at the top to 5.5 g/cm³ at the base.
• Temperature: about 1000 °C at the top to over 3000 °C at the base.
• Divisions: upper mantle (to about 700 km, including the plastic asthenosphere) and lower mantle (700-2900 km).
• Slow convection currents in the mantle are believed to move the lithospheric plates.
• Both P and S waves pass through it, so it is solid, though it flows very slowly.

The nebular hypothesis was proposed by Immanuel Kant (1755) and developed by Laplace (1796).

• The solar system began as a huge, hot, rotating cloud of gas and dust called a nebula.
• As it cooled, it contracted and, to conserve angular momentum, rotated faster and flattened into a disc.
• When the centrifugal force at the edge equalled gravity, a ring of gas was thrown off. This repeated several times, forming rings.
• Each ring condensed into a planet; the central mass became the Sun. Rings around planets formed satellites.
• The Earth cooled from a hot gaseous ball into a liquid and then formed a solid crust.

Limitation: it cannot explain why the Sun has only 2% of the angular momentum of the solar system, or why some satellites move backwards.

The tidal hypothesis (Jeans and Jeffreys, 1919) says that a huge star passed near the Sun, and its gravity pulled a long, cigar-shaped filament of hot gas out of the Sun. This filament broke into pieces as the star moved away, and the pieces condensed into the planets, which revolve around the Sun. The Earth thus formed from a hot gaseous mass that gradually cooled.

Constructive versus destructive boundary

Formation of the Himalaya

The Indian plate separated from Gondwanaland and moved north across the Tethys Sea. Tethys oceanic crust was subducted under Eurasia. About 50 million years ago India collided with Eurasia. Because both are light continental crust, neither sank, so the thick Tethys sediments were folded, thrust and uplifted, forming the Himalaya along thrusts such as the MCT, MBT and MFT. The movement continues at about 4-5 cm per year, so the range still rises and has earthquakes.

Plate boundaries

The lithospheric plates meet at three types of boundary.

• Divergent: plates move apart; new crust forms (mid-ocean ridges, rift valleys).
• Convergent: plates move together; subduction or collision (trenches, island arcs, fold mountains).
• Transform: plates slide past each other (San Andreas Fault).

Formation of mountains

Mountains are mainly formed at convergent boundaries.

1. Thick sediments collect in a long, sinking trough (geosyncline) at a plate margin.
2. Plates move together and compress the sediments.
3. The layers are folded, faulted and thrust upwards, with metamorphism and igneous intrusion.
4. Uplift and erosion then shape the range.

Examples: Himalaya (continent-continent collision), Andes (ocean-continent subduction). Other mountains are formed by volcanism (Fuji) and block faulting.

Formation of the Himalaya

About 200 million years ago the Indian plate broke from Gondwanaland and drifted north. The Tethys Sea between India and Eurasia narrowed as its floor was subducted under Eurasia. Around 50 million years ago the Indian continental plate collided with Eurasia. The light continental crust resisted subduction, so the thick Tethys sediments were compressed, folded and thrust southwards along the MCT, MBT and MFT, and uplifted to form the Himalaya. Movement continues at about 4-5 cm per year.

Evidences of plate tectonics

• The coastlines of continents fit together, such as South America and Africa.
• Matching rock types, mountain belts and fossils (Glossopteris, Mesosaurus) on separated continents.
• Sea-floor spreading shown by magnetic stripes symmetrical about mid-ocean ridges and young age of ocean floor near ridges.
• Distribution of earthquakes and volcanoes along plate boundaries.
• Palaeomagnetism and evidence of past glaciation in now-tropical lands.

Three evidences of plate tectonics:

1. Fit of continents: the coastlines of South America and Africa fit like a jigsaw; the match is even better at the edge of the continental shelves.
2. Fossil and rock similarity: identical fossils (Glossopteris plant, Mesosaurus reptile) and matching rock types and mountain belts are found on continents now far apart, meaning they were joined.
3. Sea-floor spreading and magnetic stripes: rocks on both sides of mid-ocean ridges show symmetrical bands of normal and reversed magnetism, and get older away from the ridge, showing new crust forms at the ridge and moves outward.

(Other evidences: earthquake and volcano belts matching plate edges; ancient glacial deposits in tropical areas.)

An earthquake is a sudden shaking of the ground caused by the release of stored strain energy in the Earth's crust.

Elastic rebound theory (H. F. Reid, 1906)

1. Tectonic forces (plate movements) act slowly on rocks along a fault.
2. Friction locks the fault, so the rocks bend and strain energy is stored, like a stretched spring.
3. When the stress exceeds the strength of the rock or the friction, the rock suddenly breaks or slips.
4. The rocks spring back (rebound) to a less strained position and the stored energy is released as seismic waves.
5. The waves travel outward and shake the ground. After the main shock, readjustment causes aftershocks.

 Before        Strain         After slip
 |  |  |       \  \  \        |  |  |
 |  |  |        \  \  \       |__|__|
 straight       bent         rebound,
                             fault slip

Related terms

• Focus (hypocentre): point of rupture inside the Earth.
• Epicentre: point on the surface directly above the focus.

Most earthquakes occur on plate boundaries, such as the Himalayan collision zone, which produced the 1934 Nepal-Bihar and 2015 Gorkha earthquakes.

Seismic waves

Seismic waves are elastic waves that carry the energy released by an earthquake through and along the Earth. They are of two main groups.

A. Body waves (travel through the interior)

• P-waves (primary, compressional): particles vibrate parallel to the direction of travel (push-pull). Fastest (about 6-8 km/s in crust), arrive first, pass through solids, liquids and gases.
• S-waves (secondary, shear): particles vibrate perpendicular to the direction of travel. Slower (about 3.5-4.5 km/s), arrive second, pass only through solids; they stop at the liquid outer core.

B. Surface waves (travel along the surface)

• Love waves: horizontal side-to-side shear motion.
• Rayleigh waves: elliptical rolling motion, like ocean waves.
• Slowest but largest, and cause most damage.

 P: >>><<<>>><<<  (compression)
 S: ~~~~~~~~~~~~  (shear, up-down)

Mechanism of earthquake origination

Based on the elastic rebound theory:

1. Plate movement applies stress on rocks along a fault.
2. Friction prevents slip, so rocks deform elastically and store strain energy.
3. When stress exceeds rock strength, the rock ruptures suddenly at the focus.
4. The rock rebounds and the stored energy is released as seismic waves, which reach the surface first at the epicentre.
5. Smaller aftershocks follow as the crust adjusts.

Causes are mostly tectonic; others are volcanic activity, landslides, and reservoir loading or explosions.

Earthquake terminology

        Epicentre (E)
 ground ------*--------------------
              |  \  seismic waves
              |   \
              |    \
  depth h     |     \
              * Focus (F)

• Focus (hypocentre): the point inside the Earth where the rock first ruptures and energy is released.
• Epicentre: the point on the surface vertically above the focus.
• Focal depth: vertical distance from focus to epicentre. Shallow (0-70 km), intermediate (70-300 km), deep (300-700 km).
• Epicentral distance: distance from the observation station to the epicentre.
• Magnitude: measure of energy released (Richter scale), a single value.
• Intensity: measure of effects and damage at a place (Modified Mercalli scale, I to XII).
• Isoseismal lines: lines joining points of equal intensity.
• Seismograph / seismogram: instrument and record of ground motion.
• Foreshocks and aftershocks: smaller shocks before and after the main shock.
• Seismic waves: P, S, Love and Rayleigh waves.

Points to consider for construction in seismic areas

1. Avoid sites on active faults, soft or loose saturated soils (liquefaction), steep unstable slopes and landslide areas; prefer hard rock or firm soil.
2. Follow the earthquake code (Nepal National Building Code NBC 105 and IS 1893), using the seismic zone factor.
3. Use simple, symmetrical plans and regular elevations, and avoid large overhangs and soft storeys.
4. Provide ductile detailing, tie beams, bands (plinth, lintel, roof) and vertical reinforcement in masonry.
5. Make the foundation uniform and at one level; use a raft or tie beams, and avoid mixed foundation types.
6. Use light roofing and light materials; avoid heavy parapets.
7. Provide adequate separation between adjacent buildings to prevent pounding.
8. Use good quality materials and workmanship, proper mortar and anchorage.
9. For dams and bridges, use flexible joints, adequate freeboard and proper shear keys.
10. Use base isolation or dampers for important structures.

How an earthquake occurs

Plate motion slowly loads rocks along a fault. Friction holds the fault, so strain energy is stored in the rock. When the stress exceeds the strength, the rock suddenly breaks and slips (elastic rebound theory), releasing energy as seismic waves that shake the ground.

Terminology

• Focus (hypocentre): point of rupture inside the Earth.
• Epicentre: point on the surface directly above the focus.
• Focal depth: distance from focus to epicentre.
• Epicentral distance: distance from a station to the epicentre.
• Magnitude: energy released (Richter scale).
• Intensity: damage and effects at a place (Mercalli scale).
• Isoseismal line: line joining points of equal intensity.
• Foreshock / aftershock: smaller shocks before / after the main shock.
• Seismograph: instrument recording ground motion.

   Epicentre *
             |
             |
             * Focus

Causes

1. Tectonic: movement of plates and slip on faults (most earthquakes, including Himalayan ones).
2. Volcanic: movement of magma and gases.
3. Isostatic adjustment and landslides, rockfalls and cave collapse.
4. Man-made: reservoir impoundment, deep mining, underground explosions, deep well injection.

Effects

• Ground shaking, cracks and surface rupture.
• Collapse of buildings, bridges and dams, with loss of life.
• Landslides, rockfall and avalanches in hills.
• Liquefaction of saturated sandy soil and ground settlement.
• Tsunamis along coasts and seiches in lakes.
• Fire, floods from dam failure, and disease.
• Change in river courses and groundwater level, and economic loss.

Intensity

Intensity is a measure of the effects of an earthquake on people, structures and ground at a given place. It is judged from observed damage and felt effects. It is expressed on the Modified Mercalli Intensity (MMI) scale from I (not felt) to XII (total destruction). It varies from place to place, decreasing with distance from the epicentre.

Magnitude

Magnitude is a measure of the energy released at the focus. It is calculated from seismograph amplitudes on the Richter scale (logarithmic): each unit increase means 10 times larger amplitude and about 32 times more energy. It has a single value for one earthquake. The 2015 Gorkha earthquake had magnitude about 7.8.

Mechanism of an earthquake

1. Plate motion stresses rocks along a fault.
2. Friction locks the fault, so rocks deform and store elastic strain energy.
3. When stress exceeds strength, the rock ruptures at the focus and rebounds (elastic rebound theory).
4. Stored energy is released as seismic waves.
5. Aftershocks follow as the crust readjusts.

Seismic waves

• P-waves: compressional, fastest, travel through solid, liquid and gas; first to arrive.
• S-waves: shear, slower, travel only through solids; second to arrive.
• Love waves: surface waves, horizontal shaking.
• Rayleigh waves: surface waves, rolling elliptical motion.

P and S are body waves; Love and Rayleigh are surface waves and cause the greatest damage.

Effects of an earthquake:

1. Ground shaking, causing vibration and damage to structures.
2. Surface rupture, cracks and fissures, and fault displacement.
3. Collapse of buildings, bridges, dams and lifelines (roads, pipes, power lines), leading to loss of life and injury.
4. Landslides, rockfall and avalanches on steep hill slopes.
5. Liquefaction: saturated sand loses strength, causing settlement, tilting and sand boils.
6. Ground subsidence and uplift, change of river courses and groundwater levels.
7. Tsunamis along coasts and seiches in lakes and reservoirs.
8. Fires and floods, from broken gas lines and failed dams.
9. Epidemics, economic loss and social disruption.