TECTONICS

geog year 1

Movement of plates:Mantle convection

Heat produced by decay of radioactive elements in the Earth’s core heats the lower mantle , creating convection currents.

Movement of plates:Slab pull

Newly formed oceanic crust at mid-ocean ridges becomes denser and thicker as it cools. This causes it to sink into the mantle under its own weight-pulling the rest of the plate down with it.

Movement of plates: Subduction

(the reason why the earth is NOT expanding despite new crust being made)

Two oceanic plates ( or an oceanic and continental) move towards each other, one slides beneath the other into the mantle where it melts in an area called the subduction zone.

Movement of plates:Seafloor Spreading

Hot magma (molten rock) is forced up from the asthenosphere and hardens-forming new oceanic crust. This new crust pushes plates apart.

Palaeomagnetism

The study of past changes in the Earth’s magnetic field (determined from rocks, sediment or archaeological records).

Reasoning for palaeomagnetism

Approx. every 400,000 years the Earth’s magnetic fields change direction-causing the magnetic north and south poles to swap.

When lava cools and becomes rock, minerals inside the rock line up with the Earth’s magnetic direction (polarity) at the time.

Arrangement of these minerals can be analysed to determine the Earth’s past magnetic fields.

Structure of the Earth:The inner core

The very centre of the Earth, and the hottest ( about 6000 degrees). Solid and consists mostly of iron.

Structure of the Earth:The outer core

Semi-molten

Consists mainly of liquid iron and nickel.

Temps range 4500-6000

Structure of the Earth:The mantle

Surrounds the core, and is the widest layer making up the Earth. Upper part is mainly solid, but below the rock is semi-molten (forming the asthenosphere)

Structure of the Earth:The crust

Forms the outer shell of the Earth. There are two types of crust:

Oceanic- A thin, dense layer (6-10 km thick), which lines ocean floors.

Continental-An older, thicker layer(45-50 km thick) which makes up the Earth’s landmasses.

Three types of plate boundary

CONVERGENT-2 plates collide (destructive)

DIVERGENT-2 plates move apart(constructive)

CONSERVATIVE-2 plates slide past each other (transform)

Destructive plates margins:Oceanic meets continental

Oceanic sinks beneath continental

Deep ocean trenches mark place of subduction

Subduction leads to formation of fold mountains

Friction between colliding plates causes intermediate and deep earthquakes in the Benioff zone. Volcanic eruptions are also generated (explosive).

Destructive plates margins:oceanic meets oceanic

The denser/faster plate subducts beneath the other.

Subducted plate forms magma, which rises up from the Benioff zone to form underwater volcanoes.

Eventually, these volcanoes grow/rise above sea level, to form island volcanoes (usually found in curved lines called island arcs).

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2004 pacific ocean tsunami

Destructive plates margins:Continental meets continental

Collision margin occurs (both plates have about the same density, and are less dense than the asthenosphere beneath, so neither subducts).

They collide and sediment between them is crumpled to form fold mountains, like the Himalayas.

Constructive plate margins

Two plates moving apart= formation of new crust. In oceans, the divergence causes mid-ocean ridges and on continents it forms rift valleys.

Mid ocean ridges:

\--Mid ocean ridges of underwater mountains extend for over 60,000 km across the world’s ocean floors. Regular breaks (transform faults) cut across these ridges as they spread at different rates. Shallow focus earthquakes and regular volcanic eruptions ( sometimes creating submarine volcanoes) occur.

Rift valleys:

\-When plates move apart on continents, the crust stretches and breaks into sets of parallel cracks (faults). The land between these faults then collapses, forming steep sided valleys called rift valleys.

Conservative plate margins

There is major breakage in the crust between the plates as they move. The break itself is called a fault ( on a large scale: transform fault). No crust is made or destroyed (no volcanic activity), but this type of plate is very tectonically active-- powerful earthquakes.

The two plates sometimes stick together as they move, causing stress and pressure to build up, which is suddenly released as a shallow-focus earthquake.

Seismic waves

An earthquakes energy which radiates (as waves) from the focus. 3 main types of waves:

Primary waves (P waves) ‘push’ through the Crust, mantle and core. - body waves

Secondary waves(S waves) ‘shake’ through the crust and mantle only.- body waves

Love waves (L waves) move side to side (surface waves)

Measuring earthquakes

Magnitude and Intensity

MAGNITUDE(amnt of energy released at epicentre)

Moment magnitude scale= measures the total energy released by earthquake at the moment it occurs using the: size of seismic waves, amount of slippage/rock movement, area of fault surface broken, resistance of affected rocks.

Modified Mercalli Intensity Scale=takes observations from people who experienced the earthquake to rate on a scale of I (hardly noticed) to X (catastrophic).

Primary effects (direct result of earthquake)

Ground shaking- causes buildings, bridges,roads and infrastructure to collapse.

Crustal fracturing-When energy released during an earthquake causes the earth’s crust to crack.

Secondary effects

liquefaction- violent shaking during earthquake causes surface rock to lose strength and become more liquid than solid. Subsoil loses ability to hold building foundations, so buildings+ roads tilt/sink.

Landslides and avalanches- Ground shaking puts stress on slopes, so they fail.

Tsunami-underwater earthquakes generate tsunami.

Volcanoes

Openings in the Earths’ crust through which lava, ash and gases erupt. When magma reaches the earth’s surface,it is called lava. When lava cools, it forms rock. So, as a volcano continues to erupt over time, it grows.

Primary hazards of volcanoes

Lava flows- Streams of lava that have erupted from a volcano onto the Earth’s surface. Take years to cool completely.Non threatening (very slow), but still destructive.

Pyroclastic flows- Mixture of dense hot rock, lava, ash and gases ejected from a volcano which move very quickly across the Earth’s surface. Fast and destructive (up to 700 degrees, 100km/h).

Tephra and ash falls-Tephra are pieces of volcanic rock and ash that blast into the air during a volcanic eruption. Larger pieces tend to fall near the volcano and smaller pieces such as ash can travel for thousands of km. Ash falls are very disruptive (poor visibility, slippery roads).

Gas eruptions-Magma contains dissolved gases that are released into the atmosphere during a volcanic eruption (potential hazard, can travel thousands of km)

Secondary hazards of volcanoes

Lahars- masses of rock, mud and water that travel quickly down the sides of a volcano.They are caused when an eruption melts snow or ice quickly or when heavy rainfall before/ after eruption erodes loose soil and rock, causing it to surge downslope.

Jokulhaup- The heat of volcanic eruption can melt snow and ice in a glacier, causing heavy and sudden floods (dangerous).

Volcanic Explosivity index (VEI)

Describes and compares the size+ magnitude of volcanic eruptions.

Scale of 0 (non-explosive) to 8 (extremely large).

Several factors used: Amount and height of volcanic material,length of eruption, qualitative descriptive terms (gentle, explosive etc)

Volcanoes Case Study 1: Montserrat

PSD: Montserrat is part of an island arc in the Caribbean sea (formed where the Atlantic plate sinks beneath the Caribbean plate). Only 16km long and 10km wide, consisting almost entirely of volcanic rock. 18th July, 1995, the Soufriere volcano in the south of the island began to erupt huge clouds of ash. Over the next 5 years, these eruptions continued- with pyroclastic flows also affecting a large part of the island.

EFFECTS:

\-Dozens dead, 7000 moved (over half of original 11,000).

\-Capital, Plymouth, destroyed.

\-2/3 housing and 3/4 infrastructure destroyed.

\-Lots of farmland destroyed or abandoned.

\-Top heavy population pyramid: young people no longer saw a future on the island.

Volcano Case Study 2:Eyjafallajokull

2010, flights over large parts of Europe were cancelled for a week, as a spreading ash cloud from the eruption of Eyjafallajokull in Iceland threatened to clog aircraft engines and stop them from working.

-100,000 commercial flights cancelled worldwide.

-European economy lost US$5 billion

Predicting volcanic eruptions

Using equipment placed on the volcano as well as remote equipment (GPS and satellite based radar)signs a volcano might erupt can be monitored. These signs include:

\-small earthquakes:as the magma rises to the surface, it breaks rocks, causing small earthquakes that scientists can often detect using seismograms.

\-Changes to the surface of the volcano: as it pushes upwards, magma builds pressure (causing the surface of the volcano to swell).

\-Changes to the ‘tilt’ of the volcano: as the magma moves inside the volcano, it changes the slope angle or ‘tilt’ of the volcano.

How do Tsunami form?

Most are caused by large underwater earthquakes along subduction zones. Energy released during the earthquake causes the seafloor to uplift-displacing the water column above. This displaced water forms Tsunami waves.

What is a water column?

The area of seawater from the surface to the seafloor.

Predicting Tsunami

Due to the causation of tsunami mainly being earthquakes (difficult to predict), there is no way to predict Tsunami before they occur.

Instead, Tsunami early warning systems are now in place in both the Pacific and Indian Oceans. These systems use seismic sensors to detect undersea earthquakes.

In 2004, the Indian Ocean did not have such systems, allowing for more deaths in Sri Lanka where people would have otherwise had plenty of time to evacuate. Japan has the most extensive earthquake and tsunami warning systems in the world. When 9.0 magnitude earthquake struck in 2011, warning were issued. However, the system (in part) failed because the size of the earthquake was underestimated, causing people to choose not to evacuate or prepare.

Tsunami case study 1:The 2004 Indian Ocean Tsunami

An earthquake off the coast of Sumatra (Indonesia)that caused the Tsunami was estimated between 9.0 and 9.3 magnitude. Its thrust heaved the floor of the Indian Ocean towards Indonesia by about 15 metres and, in doing so, sent out shock waves

Governance and natural disasters Case study 2:Japan 2011

-Good building construction (strict building regulations =75% of buildings are built with earthquakes in mind).

-Low level of corruption

-Well-developed disaster plans.

-Education and preparedness for earthquakes and tsunami- emergency drills practised regularly in both schools and businesses.

(despite this investment in disaster planning, Japan failed to take into account the impact of a tsunami on a nuclear power plant--contributing to the events at Fukushima)

RESPONSE:

-within 24 hours, 110,000 defense troops mobilised

-Immediately after the earthquake, all radio and TV stations switched to official earthquake coverage, advising people.

-The Bank of Japan offered US$183billion to Japanese banks to keep them operating (protecting the economy).

The hazard management cycle

The Hazard management cycle is a tool to help us understand the response, recovery, mitigation and preparedness for natural hazards.

The Park Model (hazard-response curve)

A model that shows how a country or region might respond after a hazard event. It can be used to directly compare how areas at different levels of development may recover after a hazard event.

\-The impacts of a hazard event change over time- depending on factors such as the size of the hazard, the development level of the areas affected and the aid received.

\-All hazard events have different impacts, so their curves are different. For example, hazard events that happen suddenly (such as the 2010 Haiti earthquake), and those that happen more slowly (Montserrat 1995+ a few years)have different responses.

\-Wealthier countries have different curves than developing countries, because they will be able to recover much faster.

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Hazard mitigation?

Hazard adaptation?

Hazard mitigation= strategies meant to avoid, delay or prevent hazard events.

Hazard adaptation=Strategies designed to reduce the impacts of hazard events

Hazard mitigation strategies:

1)Land use zoning (eg/ in high risk areas, settlements and essential structures would be limited).

2)Diverting Lava flows (with consideration of which areas are more profitable or important)

3)Hazard resistant design and engineering defenses.

Hazard adaptation strategies

1)High tech monitoring (GIS, Early warning systems, satellite communication technology, mobile phone technology)

2)Crisis mapping(using crows sourced info and other methods such as satellite mapping, areas struck by disasters can be accurately mapped for recovery efforts).

3)Public education

Risk equation

Risk(R)= Hazard(H)X Vulnerability(V)

Capacity to cope(C)

HAITI

• Jan 2010 7.0 earthquake struck near capital.

• Shallow focus (13km)which increased ground shaking, Liquefaction on looser soil caused building foundations to sink, epicentre was only 24 km from Port-Au-Prince (country’s capital and most populated city).

• Haiti is a developing country-poor,and limited resources are spent on more immediate issues such as disease instead of earthquake prevention.

• Lack of building controls and regulations+ disaster preparation.

  • Over a quarter of government officials killed, poor infrastructure (1 airport, several ports and a few main roads). Oct 2010, outbreak of cholera.

Haiti recovery

* By 2015, Haiti was still recovering. $13billion aid donated, but most of it lay in the hands of international governments and organisations(With Haitian governments and organisations controlling less than 10%).
* Concerns about mismanagement and corruption meant that many organisations were unwilling to channel money through the Haitian government- hampering Haiti’s ability to become self-sufficient.
* Some improvements: 2012 (hurricane Sandy)government warned citizens and told them to get to higher ground, saving lives+ after the storm the government took a leading role in organising international aid.