geog tectonics

ground shaking example

2010 Haiti earthquake (Mw7.0)

• surr areas arnd factories polluted, debris polluted rivers

• more than 220,000 ppl killed

• more than 250,000 houses collapsed due to violent vibrations

• water pipes ruptured, causing water shortages

2010 Christchurch, New Zealand earthquakes

• many trees damaged and more than 300 had to be removed, reduced availability of habitats for terrestrial species

soil liquefaction examples

2010-2011 Christchurch, New Zealand earthquakes

• triggered severe liquefaction, bc city lies on a former swamp area where soils are loose and saturated

• more than 60,000 residential buildings and infrastructure covering about 1/3 of the city area were damaged

• liquified soil entered rivers, and untreated sewage from broken sewage pipes polluted rivers —> reduction of some species such as caddisflies

landslides examples

2018 Papua New Guinea earthquake (Mw7.5)

• triggered landslides that caused huge amts of debris to enter the rivers

• caused flooding

• destroyed forests

• polluted waters and killed fish

2008 Great Sichuan China earthquake (Mw 7.9)

• triggered more than 15,000 landslides

• destroyed many buildings and infrastructure

• caused nearly 20,000 deaths

Tsunamis examples

20l4 Indian Ocean Tsunami

• triggered by 9.1 Mw undersea earthquake near Sumatra, Indonesia

• Tsunami waves slammed into coasts of 11 Indian ocean countries, from Indonesia to Somalia in Africa

• more than 230,000 people killed, entire coastlines and habitats were destroyed

2011 Tohoku, Japan earthquakes (Mw 9.0)

• triggered tsunami up to 40m in height

• waves destroyed homes and infrastructure

• Tsunami waves carried debris inland, flooding and polluting large areas of land

• coastal city of Sendai suffered extensive damage, half its population killed

• Iwate, 70,000 pine trees knocked down, resulting in forest and biodiversity loss

• the tsunami hit the Pacific Islands, devastating the ecosystems

• entire bird nesting sites were lost, more than 100,000 Layson Albatross Chicks were killed, and thousands of fish were washed ashore, where they suffocated

what are tsunamis?

• Tsunamis are a series of ocean waves which can be caused by undersea earthquakes

• Tsunamis can travel over thousands of kilometres and devastate huge areas of coastline

When/How do tsunamis occur?

1. An undersea earthquake causes the seabed to be displaced

2. A large volume if water is lifted, forming waves of great wavelength and low height (less than 1 metre)

3. The waves travel towards land at high speeds, arnd 800km/h

4. On approaching the coast, greater friction with the shallower seabed slows the waves down

5. The waves get closer to each other and increase in height, and can reach a height of up to 15m or more, travel at speeds of 30-50km/h, and devastate shorelines the waves hit

6. Before a tsunami occurs, the sea may recede from the shore bc the sea water fills in the void caused by displacement of the seaabed.

categories of how we evaluate and anaylse tectonic hazards

• destroys ecosystems (natural systems)

• destroys properties and infrastructure (human systems)

• disrupts services (human systems)

• causes injuries and fatalities (human systems)

how ground shaking destroys ecosystems (natural systems)

• ruptures oil and chemical factories, polluting land and water

• fractures and uproots trees, cause widespread tree injury, death and damages wildlife habitats

how ground shaking destroys properties and infrastructure (human systems)

• weakens buildings, bridges, roads and railways, causing them to collapse, making it difficult to rescue people or to supply emergency aid

how ground shaking disrupts service (human systems)

• snaps water and gas pipes, resulting in water shortages and disruptions to gas supply

• electricity and communication cables break, affecting important communication services such as tsunami warnings and television broadcasts

how ground shaking causes injuries and fatalities (human systems)

• people may get trapped under collapsed buildings and infrastructure, resulting in injuries and loss of lives

how does soil liquefaction destroy ecosystems (natural systems)

• trees on liquefied soil may sink in and tip over, damaging wildlife habitats and cause forest and biodiversity loss

• liquefied soil may enter rivers and smother aquatic plants, causing them to die

• sewage pipes may be broken and untreated waste material may pollute rivers, killing aquatic species

how soil liquefaction destroys properties and infrastructure (human systems)

• buildings and infrastructure can sink in and tip over, and the damage makes its difficult to rescue people or supply emergency aid

how does soil liquefaction disrupt services (human systems)

• electricity and communication cables, water and gas pipes can sink in and snap, disrupting supply of these services

• roads and railways above liquified soil can sink in and get damaged, making it difficult to rescue people or supply emergency aid

how ground shaking and soil liquefaction cause injuries and fatalities (human systems)

• people can get trapped under collapsed buildings and infrastructure, resulting in injuries and loss of lives

how landslides destroy ecosystems (natural systems)

• fast-moving debris can bury huge areas of forest and wetlands

• rivers can be polluted with debris, killing aquatic life

• rivers can be blocked, causing floods that can damage nearby ecosystems and properties

how landslides destroy properties and infrastructure (human systems)

• debris can bury villages and farms, destroying properties and infrastructure

how landslides disrupt services (human systems)

• debris can snap electricity and communication cables, water and gas pipes, disrupting supply of these services

• roads and railways can be blocked by debris, making it difficult to rescue people or supply emergency aid

how landslides cause injuries and fatalities (human services)

• debris can bury people or hit them, causing injuries and fatalities

• debris can block rivers, resulting in floods which can drown people

how tsunamis destroy ecosystems (natural systems)

• seawater can flood huge areas of coastal wetlands and forests, damaging habitats

• large amounts of debris carried in by the waves can pollute these areas, damaging ecosystems and killing wildlife

how tsunamis destroy property and infrastructure (human systems)

• fast-moving waters and large amounts of debris carried in by the waves can sweep asay buildings and infrastructure, destroying them

how tsunamis disrupt services (human systems)

• fast moving waters and the large amount of debris carried in by the waves can snap electricity and communication cables, disrupting supply of these services

• fast moving water can sweep away roads and railways, making it difficult to rescue people or supply emergency aid

how tsunamis cause injuries and fatalities (human systems)

• sweeping waters can drown people

• large amounts of debris carried in by the waves can hit and kill people

plate tectonic theory (4 points)

• earth has a three-layered structure

• earth’s lithosphere is broken up into huge pieces called tectonic plates

• tectonic plates are constantly moving

• plate movements result in formation of landforms (eg. mountain ranges, oceanic trenches) and phenomena (eg. earthquakes, tsunamis)

layers of the earth

crust, mantle, core

lithosphere: crust, uppermost mantle (solid part of the earth, broken up into tectonic plates)

asthenosphere: upper part of mantle but below the uppermost mantle cuz thats the lithosphere (semi-solid)

forces responsible for plate movements (2)

1. convection currents

2. slab-pull force

convection currents

1. heat from earth’s core causes mantle material to become less dense

2. mantle material rises towards the surface

3. the rising mantle material then spreads beneath the plates can drags them apart, causing divergent plate movement

4. the mantle material then loses heat and sinks towards the core

5. convergent plate movement occurs where the convection currents collide

6. the materials then get heated up again and the process repeats. This rising and sinking of the mantle material forms convection currents

slab-pull force

• when 2 plates converge, the denser oceanic crust is pulled down by gravity as it subducts beneath the less dense crust

• the denser oceanic crust sinks deeper into mantle under its own weight, pulling the rest of the plate with it, contributing to further convergence

seafloor spreading

1. seafloor spreading occurs where 2 oceanic plates move away from each other at divergent plate boundaries

2. magma from deep within the earth rises through the mid-ocean ridge, cools and solidifies

3. new oceanic crust is formed

why evidence of seafloor spreading supports the plate tectonic theory

1. the age if rocks at the seabed shows a pattern

• rocks nearer to the crest (centre) of the mid-ocean ridge are younger

• rocks further away from the ridge are progressively older

—> shows how new oceanic crust is created at divergent boundaries and then moves laterally on both sides of the mid-ocean ridge as seafloor spreading continues

2. little sediment accumulation is found at oceanic trenches, as older oceanic crust is being destroyed at oceanic trenches

• as a result, oceanic crusts are usually younger than continental crusts

• this proves that new crust is continually formed at divergent boundaries at the mid-ocean ridges, and as plates move, older crust is destroyed further away at oceanic tenches

what is magnetic striping

• the zebra-like pattern where there are strips of normal polarity rocks alternating alongside strips of reversed polarity rocks on the seafloor

• it is symmetrical on both sides of the mid-ocean ridge

why does magnetic striping occur

• basaltic rocks from the oceanic crust are volcanic rocks that form from iron-rich lava

• they contain magnetic materials

• when the iron-rich lava erupts, it cools and solidifies

• its magnetic materials point to earth’s magnetic north, recording evidence of earth’s polarity at that time

how evidence of magnetic striping supports the plate tectonic theory

• the zebra-like pattern is symmetrical on either side of mid-ocean ridges, showing this is not a random isolated occurrence

• this provides evidence that plates move, supporting the plate tectonic theory as:

1. oceanic plates move away from each other

2. iron-rich lava erupts from the centre of the ridge

3. lava cools and solidifies, and forms new oceanic crust

4. the crust is then pushed in both direstions away from the ridge when new lava erupts and solidifies as plates move apart

5. when the Earth’s polarity reverses, the rocks record the reversals

6. over time, as more lava moves away from the ridge due to plat divergence, a symmetrical zebra-like pattern forms

divergent plate boundary definitions and types (x2)

where two tectonic plates move apart

• oceanic-oceanic divergent plate boundary

• continental-continental divergent plate boundary

what do divergent plate boundary result in

• mid-ocean ridges

• volcanoes, including volcanic islands and submarine volcanoes

• rift systems (rift valleys)

• earthquakes

o-o divergent plate boundary

1. two oceanic plates move apart

2. the decrease in overlying pressure causes part of the underlying mantle to melt, forming magma

3. magma rises through weak areas in the crust to the earth’s surface and fills in the gaps caused by the spreading plates

• lava cools and solidifies to form basaltic rocks

• the rocks make up a new oceanic crust

• an extensive underwater mountain chain — the mid-ocean ridge — forms

eg. oceanic North American Plate and oceanic Eurasian Plate move apart, forming the Mid-Atlantic ridge

• at the centre of the ridge is a deep rift valley with steep sides

• magma rises through weak areas in the crust to the earth’s surface, forming submarine volcanoes

• after many eruptions, the volcanoes may break the surface of the ocean to form volcanic islands, eg. in Surtsey, Iceland

• earthquakes also occur here due to stress and tension released when plates move

c-c divergent plate boundaries

1. two continental plates move apart

2. rocks eventually fracture to form parallel faults

3. the rock between these faults collapses to form a deep rift valley with steep sides

4. As the plates move apart, the decrease in overlying pressure causes parts of the underlying mantle to melt, forming magma. Magma rises through weak areas in the earth’s crust to the earth’s surface, forming volcanoes.

• earthquakes also occur as stress and tension are released when plates move

eg. Continental Nubian Plate pulls apart from the Somalian Plate, forming the Great Rift Valley.

Volcanoes formed here are Mount Kenya and Mount Kilimanjaro

convergent plate boundary definition and types (x3)

where two tectonic plates move towards each other

• oceanic-oceanic convergent plate boundary

• continental-continental convergent plate boundary

• oceanic-continental convergent plate boundary

what do convergent plate boundaries result in

• fold mountains

• volcanoes, including submarine volcanoes

• oceanic trenches

• earthquakes

o-o convergent plate boundaries

1. two oceanic plates collide

2. the denser plate subducts beneath the other plate

3. this forms an oceanic trench — a deep depression

4. as the subducting plate sinks below the mantle, the high pressure forces water out of its oceanic crust. Water lowers the melting point of the overlying mantle, causing it to melt, forming magma.

5. Magma rises from weak points in the crust to the earth’s surface

6. this forms a chain of volcanoes known as a volcanic island

• friction along the subducting oceanic plate also causes earthquakes to occur

eg. the oceanic Pacific Plate (pacific ring of fire?) subducts beneath the oceanic Philippine Plate.

This forms the Mariana Trench, as well as volcanic islands known as the Mariana Islands.

Guam, an island near the Mariana Trench, commonly experiences earthquakes

c-c convergent plate boundaries

1. two continental plates collide

2. subduction does not take place because continental plates are too buoyant to subduct

3. enormous pressure causes the rocks to be uplifted and buckled to form fold mountains

• friction along the convergent plate boundary also causes earthquakes to occur

• magma does not rise to the surface, hence no volcanoes are formed

eg. continental Indo-Australian Plate (or Indian plate — part of the larger Indo-Australian Plate) converges with the continental Eurasian Plate

this forms the Himalayan Mountain Range

This area experiences many earthquakes, such as the devastating 2015 Nepal earthquake

o-c convergent plate boundaries

1. an oceanic plate collides with a continental plate

2. the denser oceanic plate subducts beneath the continental plate

3. this forms an oceanic trench in the subduction zone

4. as the subducting plate sinks into the mantle, the high pressure forces water out of its oceanic crust. Water lowers the melting point of the overlying mantle, causing it to melt, forming magma.

5. magma rises through weak areas in the crust to the earth’s surface, forming volcanoes on the continental plate

6. enormous pressure at this plate boundary causes rocks on the continental plate to be uplifted and buckled, forming fold mountains

• friction along the subducting oceanic plate causes earthquakes to occur

eg. Oceanic Nazca Plate converges with the continental South American Plate. The denser Nazca plate subducts beneath the South American plate, forming:

• the Peru-Chille Trench

• the Andes fold mountain range

• the Nelvado Del Ruiz volcano

Earthquakes are also common here, such as 2010 Chille earthquake

transform plate boundary definition

where two tectonic plates slide past each other

no crust is created or destroyed

transform plate boundaries

stress created by plate movement produces a fault — a zone of fractures

• earthquakes occur here as one plate suddenly slips past another

• magma does not rise to earth’s surface, hence there are no volcanoes

eg. Pacific plate (Pacific Ring of fire?) slides past North American plate

forms San Andreas Fault in California, USA

earthquakes are common here, such as 1989 Loma Prieta earthquake in California

what are earthquakes & where are they generally found

earthquakes refer to the shaking of the Earth’s ground due to the sudden release of energy in the Earth’s lithosphere

they are generally found along plate boundaries, which contain systems of deep fractures — faults

how do earthquakes occur

1. rock masses on either side of a fault are pushed by tectonic plates

2. friction causes them to get locked, and stress builds up

3. when the stress exceeds the strength of the fault (or rock), the rocks snap or suddenly move to a new position

4. this sudden movement causes seismic waves to be released, resulting in ground shaking

• focus: the point in earth’s ground where seismic waves are released — the origin of the earthquake

• epicenter: the point on earth’s surface directly above the focus — shaking generally felt most strongly (impact is higher) near the epicenter

how earthquakes are measured

use seismometers — sensitive instruments that detect ground vibrations and determine the magnitude of an earthquake

• the greater the seismic energy of an earthquake, the greater the magnitude

types of scales used to rate magnitudes of earthquakes (x2)

1. Richter scale (ML)

2. Moment Magnitude Scale (Mw)

Richter Scale (ML) — how does it work

• calculates earthquake magnitude using the height of the largest wave recorded on seismometer

• thus, earthquake magnitude is measured based on the maximum seismic intensity reached, rather than the total seismic energy released throughout the earthquake

• scale numbered from 1-10, with 10 being the greatest magnitude

• scale is logarithmic — earthquake of magnitude 6 releases abt 32 times more E than a magnitude 5 earthquake

Richter scale (ML) — limitations

It rates an earthquake with a single drastic spike in seismic energy as one with a higher magnitude compared to a long earthquake with many large, intense waves.

• this underestimates longer earthquakes that release more overall energy by rating them as having lower magnitudes, even though they are likely to do more damage

• hence, richter scale is no longer commonly used, except for small, local earthquakes

Moment Magnitude Scale (Mw)

• rates earthquake magnitude based on the total energy released during the earthquake

• estimates the total energy released during an earthquake instead of just the largest wave like the richter scale

• hence, its generally more accurate, especially in measuring earthquakes of magnitude 8 and above

• scientists have also adjusted the magnitudes of past earthquakes which were initially measured using the richter scale

eg. Prince Willian Sound Earthquake in Alaska, USA in 1964 was originally rated 8.6 on the richter scale, but was later adjusted to 9.2 based on the Mw scale

• like the richter scale, the Mw scale is also logarithmic — an earthquake of magnitude 6 released about 32 times more energy than a magnitude 5 earthquake

what is a volcano

a landform created when lava erupts onto earth’s surface

• it may form a cone-shaped mountain over time, as more lava erupts and accumulates over time

parts of a volcano

(ref to tb if u needa see the parts)

• magma chamber

• conduit

• volcanic cone

• crater

• active vent

• depression (caldera) — forms after the volcano erupts

where volcanic eruptions occur at

1. divergent plate boundaries

2. convergent plate boundaries

volcanic eruptions at divergent plate boundaries

1. plates move apart in the upper part of the mantle, causing the crust to stretch and develop fractures

2. the decrease in overlying pressure causes parts of the underlying mantle to melt, forming magma

3. magma contains dissolved gases and is less dense than the surrounding materials

4. therefore, the magma rises through weak areas (like the fractures) in the crust to the earth’s surface to erupt as lava, causing a volcanic eruption

5. the lava cools, solidifies and accumulates over time, forming a volcano

volcanic eruptions at convergent plate boundaries

1. plates move towards each other and the denser plate subducts beneath the other

2. as the subducting plate sinks into the mantle, the high pressure forces water out of the oceanic crust, and the water lowers the melting point of the overlying mantle, causing it to melt, forming magma

3. the magma contains dissolved gases and is less dense than the surrounding materials

4. thus magma rises through weak areas in the crust to the earth’s surface to erupt at lava, causing a volcanic eruption

5. the lava cools, solidifies and accumulates over time, forming a volcano

what does explosivity of volcanic eruptions depend on

how easily dissolved gases can escape from the magma

high silica magma vs low silica magma & special cases

high silica magma:

• more viscous magma

• as magma rises towards the earth’s surface, the dissolved gases in the magma cannot escape easily

• more pressure builds up until gases escape explosively

• results in violent, explosive eruptions

• generally associated with stratovolcanoes

low silica magma:

• less viscous magma

• as magma rises towards the earth’s surface, the dissolved gases can escape easily

• less pressure builds up

• results in gentle, effusive explosions

• generally associated with shield volcanoes

special case:

some volcanoes with viscous magma may not result in explosive eruptions, as magma rises in a way that allows the gases to escape.

eg. Mount Merapi is a stratovolcano, but eruption in 2006 was not explosive as viscous magma rose in a way that allowed the dissolved gases to escape easily

when stratovolcanoes form

1. high viscosity magma rises through weak areas in the crust to the earth’s surface and erupts explosively as lava, ash and rocks

2. the ash and rocks settle on the sides of the volcano, and are later covered by lava

3. over successive eruptions, a tall volcano consisting of alternating layers of ash and lava develops

4. as the highly viscous magma travels a shorter distance before cooling and solidifying, the volcano has steep sides and a narrow summit

eg. Mount Mayon in Philippines

when do shield volcanoes form

1. low viscosity magma rises through weak areas in the crust to the earth’s surface and erupts effusively

2. over successive eruptions, a volcano consisting of layers of lava develops

3. as the less viscous lava travels a longer distance before cooling and solidifying, the volcano has gently sloping sides with a broad summit

eg. Kilauea in Hawaii, USA

how volcanic eruptions are measured

the Volcanic Explosivity Index (VEI) measures the magnitude of different volcanic eruptions based on explosivity

criteria used to determine explosivity of volcanic eruptions to measure magnitude

1. volume of ejected material

2. height of eruption cloud

3. duration of eruption

scale of VEI

measured on a scale of 0-8

• effusive eruptions are generally given a value of 0 or 1, as they are non-explosive, with less than 0.0001 cubic km of material ejected

eg. ongoing effusive eruption of Kilauea, Hawaii, which is non-explosive

• a mega-colossal explosive eruption is represented by a value of 8, can eject more than 1,000 cubic km of tephra, with an eruption cloud column height of over 25km

eg. Toba volcano in Northern Sumatra, Indonesia 74,000 years ago

→ the volcano erupted approx. 2,800 cubic km of material, which covered India and parts of SEA in over 15cm of ash

• VEI scale is logarithmic — an increase of 1 on the VEI scale indicates an eruption 10 times more powerful than the number before it