Edexcel A Level geography - Tectonics

Tectonic hazards

Earthquakes are common, and they can develop into disasters, particularly if they cause secondary hazards like landslides or tsunamis. At convergent and conservative margins there is more risk from earthquakes, as divergent margins are often in the sea, and also lead to less powerful quakes. The four types of plate boundaries present differing levels and types of risk.

  • The explosivity of a volcano depends on the amount of dissolved gases in the magma and how easily they can escape

Volcanoes appear most commonly at subduction zones, as magma plumes rise through weaknesses in the crust

Theoretical frameworks and plate movements

Two types of crust:

  • Oceanic - basalt

  • Continental - granite

At spreading ridges, magma is gap-filling, and it locks onto the earth’s magnetic field as it cools, and the process of this can be dated using paleomagnetism, which can detect periods of increased and decreased tectonic activity historically.

At subduction zones, the friction of a locked fault can exceed a threshold and cause a megathrust earthquake

Tectonic plate - massive irregular slab of solid rock, generally composed of both oceanic and continental lithosphere

Fracture - any separation in a geological formation, like a joint or fault that divides the rock into multiple pieces

At a convergent plate boundary, the hypocentre is along the Benioff zone, and it can be at a depth of anywhere between 0-700km

Continental crust is not created as it is more buoyant than oceanic, and is therefore not created or destroyed at plate boundaries

Palaeomagnetism

  • Study of earth’s magnetic field in rocks

  • The magnetic field of the earth is constantly changing, and this can be over a timescale of milliseconds to millemia.

    • Short term - pulsations, fluctuations, daily magnetic variation

    • Long term - secular variation, geomagnetic excursions and geomagnetic reversals

  • Because of these variations, rocks’ magnetic fields end up pointing different directions as they have locked on to the magnetic field at different points.

Plate margins

  • Alfred Wegener’s continental drift theory:

    • Continents fit well together

    • Fossils of some species (ie. cynognathus which could not swim) found in different continents separated by a body of water

    • Mountains and volcanoes made of the same rock in different areas

CONSERVATIVE MARGINS:

  • Two plates slide past each other, caused by convection currents in the mantle, resulting in a break in the crust as they move called a fault.

    • If on a large scale, this fault is called a transform fault

  • Very powerful earthquakes, as plates are jagged not smooth, and thus create lots of friction. This builds up until the plates can no longer deal with the stress, at which point the fault slips, rocks jolt and a thrust earthquake is caused

  • No volcanoes as no lithosphere is created or destroyed

  • Example: San Andreas Fault - lots of earthquakes

CONVERGENT (o+c)

  • Oceanic crust is denser than continental, and so oceanic crust gets subducted beneath continental into the mantle through slab pull, melting it

  • Deep ocean trenches are found at the point of subduction

  • Fold mountains are found here, as when these two plates collide, continental is crumpled and pushed up

  • These mountains are volcanoes as magma from the melting subducted plate pushes up through faults, creating explosive eruptions of andesitic lava

  • Friction between colliding plates and their subduction causes intermediate and deep focus earthquakes along the Benioff zone, of moderate magnitude and therefore of medium risk

  • Example - West of South America, where the Nazca plate is subducted, creating Andesitic lava and the Andes mountains.

CONVERGENT (o+o)

  • When two oceanic plates collide, the denser one is subducted beneath the other, forming deep ocean trenches

  • There are earthquakes here with hypocentres all along the Benioff Zone - the 2004 Indian Ocean Tsunami was caused by an earthquake at one of these faults

  • Magma from the subducting plate can rise to form underwater volcanoes, sometimes being so big that they rise above the sea level and form island arcs, like the Aleutian islands in Alaska

CONVERGENT (c+c)

  • When two continental plates collide, a collision margin occurs. As both have similar density, neither is subducted

  • Instead, sediments from both plates crumple and are forced downwards to push the lithosphere down, but also upwards to create fold mountains.

  • There are some earthquakes here, with a shallow focus as there is no Benioff zone - i.e. the 2015 Nepal earthquakes. Shallow focus earthquakes cause more ground shaking as the energy is more concentrated near ground level

  • Small amounts of magma may rise through faults, but volcanoes are not often found there

  • Some subduction happens there when compressed denser sediments result in plate subduction

DIVERGENT (o)

  • Plates move apart thanks to convection currents within the asthenosphere, creating mid-ocean spreading ridges, which extend for up to 60,000km across the ocean floor

  • Transform faults can occur here, as ridges spread at different rates

  • Magma rising through the ridge creates submarine volcanoes, which can grow above sea level and create islands (i.e. Iceland)

DIVERGENT (c)

  • Here, the crust breaks into sets of faults, and the land between them collapses and forms rift valleys

  • Same hazards as oceanic divergent

Why do plates move?

In 1957, the Tharp-Heezen map was produced. it was the most accurate picture of the sea floor at the time, and it pointed to various underfloor ridges that exist below the sea. A mid ocean ridge consists of a chain of volcanoes and valleys. The inner core is solid and magnetic, while the outer core and mantle are liquid. The mantle is made up of peridotite, and the core made of iron

  • Hot rock rises and spreads at the surface

  • It then sinks upon cooling, and this creates convection currents that make our plates move

  • Under subduction, the rock melts, CO2 is released, and it comes back out of the earth in explosive eruptions from composite volcanoes

  • Upwelling magma transfers heat from the core to the top of the mantle in a process called convection

  • As the magma reaches the main part of the asthenosphere, it heats up again, causing a convection current.

Slab pull and ridge push work in conjunction at spreading ridges, by newly formed oceanic crust

Subduction stops the planet expanding lol

How to measure earthquakes

  • 10,000 people every year die from earthquakes

  • All earthquakes are hazards but hardly any are disasters - only when they cause significant damage

  • Strike-slip faults are where the crust moves sideways at a conservative fault

  • Dip-slip faults are where the ground either drops or is pushed upwards

  • The strongest earthquakes are found at subduction zones, and at conservative boundaries, where friction is built in a locked fault.

Shield volcanoes are made at divergent boundaries on island arcs

Composite volcanoes are in the arcs above convergent plate boundaries

95% of earthquakes are on plate boundaries

Types of waves

Primary waves (P-waves, pressure waves)

  • They are a body wave

  • They are the first to reach the surface, and travel quickly through solids and liquids

  • Can sometimes be heard by animals

  • Short wavelength

  • Cause ground compressions and expansions (longitudinal)

  • Shake back and forth

  • Only damaging in the most powerful quakes

  • First to show in recording stations

Secondary waves (S-waves, shear waves)

  • Type of body wave

  • Slower (half the speed of p)

  • Travel through solids only

  • Shake up and down in right angles to direction of travel (transverse)

  • Do more damage than p

  • Stronger and travel further, so more people feel them

  • Rock particles move side-to-side

Love waves (surface waves)

  • Slowest (last to arrive)

  • Only travel on the surface of the crust

  • Larger waves (higher amplitude)

  • Moves the ground horizontally as it encroaches

  • Focuses all energy on the surface, so causes the most damage

Evidence for plate tectonic theory

  1. Palaeomagnetism

  2. Fossils - cynognathus

  3. Earthquakes - 95% of earthquakes at plate boundaries

  4. Complementary continental shelf shapes - africa and south america logically “fit together”

  • See exam answer on this 🙂

Secondary hazards of earthquakes

Vulnerability - ability to cope with, predict and manage disasters

Hazard - naturally occurring event that has the potential to cause significant damage

Indian Ocean Tsunami 2004

  • Series of waves caused by water column displacement

  • Can affect huge coastal areas

  • 9.3 magnitude earthquake

  • Affected all countries around Indian ocean - 225,000 dead, 1.7 million homeless

  • 60% of Thailand’s fishing fleet was washed away and Thailand lost 120,000 jobs

  • Ocean floor moved 15 metres upwards, waves moved at 800 kph

  • 250 dead in Phuket and damage to beaches

  • Lots of people stranded on exposed beaches due to the drawback effect

Liquefaction - Christchurch 2011

  • Happens in the immediate vicinity of the earthquake

  • Can disrupt power and gas lines, and also lead to fires

  • Violent shaking causes surface rocks to lose strength and begin to act more as a liquid than a solid

  • 6.3 magnitude shallow focus earthquake - was actually an aftershock of a nearby earthquake a month earlier

  • As it becomes more liquid, subsoil cannot support foundations so buildings begin to sink

  • Water is saturated, meaning soils get watery

  • Dipping fault hit the city centre

  • Made rescue efforts difficult, as aid had to traverse unstable roads

  • 185 dead, over half of which in the TV tower that collapsed

  • Building foundations lost so lateral spread carried foundations of buildings down hills

  • New buildings since the disaster have foundations of 10 m to reduce stress and water pressure that can build up

  • Steel cables in walls so they can rock during an earthquake

  • Some buildings now built on flexible foundations that can not experience lateral spread

  • Soil was tested for liquefaction likelihood

Landslides - Nepal 2015

  • Rarely happens below 4 magnitude, but significant issues with landslides with large magnitudes

  • Can travel several miles from their source

  • Account for a big portion of earthquake damage wherever they occur

  • 1/3 of deaths in some earthquakes

  • Earthquake 7.8 magnitude, causing an estimated 10,000 landslides - the land is now more likely to slip

  • 9,000 deaths and 800,000 buildings damages

  • Landslides made worse by unstable ground caused by monsoon rainfall - saturated and heavy ground

  • Rockslides blocked the river and caused more flooding

  • Now, researchers are waiting for signs that landslides may occur

  • Researchers track whether cracks are opening or contracting, as this affects soil saturation