Plate TectonicsIntroduction

Events, hazards & disasters

A tectonic event is an occurrence caused by tectonic activity - examples include earthquakes and volcanoes

A Hazard is a major natural event which threatens both life and property. Hazards are risks to people and property. Many natural events are made much worse by what people do to the environment and where they choose to locate their buildings and activities e.g. avalanches and ski resorts.

A disaster is the realisation of this hazard. Disasters cause great loss of life and /or property and create severe disruption to people. They can be created by human actions alone e.g. road/ rail accidents or be the result of natural processes e.g. earthquake.

• Earthquake hazards include:

• ground shaking, displacement, liquefaction, tsunamis and landslides

• Volcanic hazards include:

• lava flows, pyroclastic flows, ash and lahars

• However these are not always hazardous and certainly do not always have disastrous consequences

• When such events occur in remote areas, where almost nobody lives, the events are not even hazardous let alone disastrous

Earthquake hazards• Ground shaking - most direct and obvious effect causing cracks in both land and

structures as well as falling masonry and even complete demolition

• Displacement - land may be thrust up or down relative to adjoining land. In the Boxing Day 2004 Sumatra earthquake the land was displaced about 15 metres within a few seconds

• Liquefaction -occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other. When this occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges is reduced - buildings collapse and appear to be swallowed up by the soil.

• Tsunamis -Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami (or tidal wave) can be created.

• Landslides - When land is unstable with steep gradients and unconsolidated land shaking can cause the ground to be dislodged leading to landslides. These can cause much damage and loss of life.

Volcanic hazards• Lava flows - these can be intrusive (cool slowly beneath surface) or extrusive (cool

more rapidly above surface). It is extrusive volcanic lava flows that may cause the most rapid changes to both the natural and built environment. New land forms (and it’s potentially fertile) but when towns are in the way devastation may result. In the late 1980's, the town of Kalapana in Hawai'i was destroyed by lava flows. They buried cars and burnt homes, buildings, and vegetation.

• Pyroclastic flows - a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice. These awesome features are heavier-than-air emulsions that move much like a snow avalanche, except that they are fiercely hot, contain toxic gases, and move at phenomenal, hurricane-force speeds, often over 100 km/hour. They are the most deadly of all volcanic phenomena.

• Ash - Some explosive volcanoes emit millions of tonnes of ash into the air which rains down on the land sometimes to very great depths (Pompeii was destroyed in this way in when Vesuvius erupted in AD79

• Lahars - Indonesian term for a volcanic mudflow. These lethal mixtures of water and tephra (volcanic solid fragments) have the consistency of wet concrete, yet they can flow down the slopes of volcanoes or down river valleys at rapid speeds, similar to fast-moving streams of water. These mud slurries carry debris ranging in size from ash to lapilli (pea to walnut sized solids), to boulders more than 10 meters in diameter. The maximum temperature of a lahar is 100 degrees Centigrade, but some may be cold. Lahars create enormous damage and loss of life.

Natural HazardPeople’s perception of potential hazards depend

upon ...• Magnitude - the energy released, force or strength of the

event

• Frequency - how often the event occurs

• Duration - how long the event lasts

• Areal extent - how widespread the event is

• Speed of onset - the length of lead time from knowing that such an event will occur and its occurrence

• Future probability - this is a statistical calculation but it should be noted that since people may have taken action to reduce the probability of such an event occurring again the future anticipated frequency may be less than that in the past

What does this mean?

• IF PEOPLE ARE OFTEN AFFECTED BY HIGH MAGNITUDE EVENTS THAT OCCUR OVER A WIDE AREA AND LAST A LONG TIME THEY WILL BE ALERT AND WOULD PROBABLY HAVE PREPARED THEMSELVES AS BEST THEY CAN

• ON THE OTHER HAND A RARE EVENT OF MODERATE MAGNITUDE THAT IS HIGHLY LOCALISED AND SOON OVER IS LIKELY TO FADE FROM PEOPLE’S CONSCIOUSNESS SO THAT THEY DO NOT PREPARE AND MAY EVEN BECOME COMPLACENT

Tectonic Hazards are not evenly spread throughout the world

Compare the UK with western USA, Japan and New Zealand

• Does the UK experience earthquakes?

• yes

• How frequent are they?

• very frequent - perhaps 200 to 300 per year

• Are all parts of the UK at risk?

• No. Tends to be western side where faults are more common, or in areas disturbed by subsidence because of mining

• How strong are they?

• Most go unnoticed, but every so many years there’s one that causes minor damage such as the 4.3 Kent tremor in April 2007 or the 5.2 quake at Market Rasen, Lincolnshire or the biggest ever in UK offshore from Great Yarmouth in 1931 which recorded, for UK, a massive 6.1!

• Do they cause loss or life or damage to the built environment?

• The Kent and Market Rasen earthquakes led to some superficial cracking in walls and the odd fallen chimney. The 1931 quake was over 100 kms offshore so did no damage on land. Deaths are almost unheard of; the occasional heart attack maybe!

• NOW COMPARE WITH WESTERN USA, JAPAN AND NEW ZEALAND

• This will be part of the research undertaken - but here earthquakes and volcanic events are frequent occurrences because of their position, unlike UK, on the edge of tectonic plates, with some being of very high magnitude, affecting large land masses and, in the case of volcanoes, lasting for long periods of time.

VulnerabilitySome areas are more vulnerable than others, so that the tectonic

events that do occur are not only hazardous but more likely to lead to disasters.

This depends upon the exposure, sensitivity and resilience of....

• Population

• Economy

• Land use and development

• Infrastructure and critical facilities

• Cultural assets

• Natural resources

Areas prone to tectonic eventsthat are deemed hazardswill experience disasters

if they are highly vulnerable

VULNERABILITYNATURAL HAZARD

RISKOF

DISASTER

Sudden events and chronic issues

Past recurrence intervalsFuture probabilitySpeed of onsetMagnitudeDurationAreal extent

Exposure, sensitivityand resilience of:

PopulationEconomyLand use and developmentInfrastructure and critical facilitiesCultural assetsNatural resources

Source: Geography for EDEXCEL by Digby et al (Oxford University Press)

EARTHQUAKES VENN DIAGRAM

The connection between natural hazard events and locational site vulnerability

What does this map show?

Distribution of earthquakes & major plate boundaries

Major plate boundaries superimposed on political map

Live volcanoes superimposed on political map

Detailed tectonics map

Tectonic events are not evenly distributed across the globe

But they do form distinctive patterns

They are closely aligned to the plate boundaries

CONCLUSIONS

The structureof the Earth

The Earth is made up of an inner core, outer core, mantle and crust. The crust and upper mantle form a cold, strong layer known as a lithosphere.

Many forces cause the surface of the Earth to change over time. However, the largest force is the movement of Earth's outer layer through the process of plate tectonics. This process causes mountains to push higher and oceans to grow wider.

The rigid outer layer of the Earth, called the lithosphere, is made of plates which fit together like a jigsaw puzzle. These solid but lightweight plates seem to "float" on top of a more dense, fluid layer.

Motions deep within the Earth carry heat from the hot interior to the cooler surface. These motions of material under the Earth's surface cause the plates to move very slowly across the surface of the Earth, at a rate of about 2 inches per year. There are several different hypotheses to explain exactly how these motions allow plates to move.

This image is a cross section through the Earth showing the convection cells of the mantle.

Ridge push happens at spreading centres where plates are moving apart.

Slab pull happens at subduction zones where one plate is pulled down into the mantle.

The Earth’s core is intensely hot at about 5000oC. This heat causes molten rock deep within the mantle to rise.

As it nears the surface it cools, becomes more dense and sinks back down. It is again heated so that once again it is forced to rise.This constant circular motion is called a convection cell.

Hot magma rises to the surface to create spreading ridges. As new crust is formed existing crust is actively pushed out of the way. This is called ridge push.

Old parts of a plate are likely to sink down into the mantle at subduction zones because they are colder, thicker and denser than the warm mantle material underneath them. This is called slab pull.

Why do the plates move?

When plates converge as an oceanic plate meets a continental plate for example, the denser oceanic plate is drawn slowly beneath the other over a period of thousands of years - this happens at the subduction zone and destroys crust, hence it is often referred to as a destructive margin. The movement is irregular and sudden jolts can cause tremors or earthquakes. The friction caused as one plate subducts beneath the other also leads to volcanic activity.

When two continental plates collide, rock layers are forced upwards in folds creating mountains like the Himalayas, the Rockies, Andes and Alps. Powerful earthquakes are often associated with the many fault lines that run through these collision zones. However, volcanic activity is far less likely.

Other plates create transform movements, moving very slowly alongside each other. Faults are found at the edges of the plates where the crust is moving in different directions. In some places the plates become locked together and energy builds up. When the plates give, the stored energy is released in the form of an earthquake. The point of the earthquake’s origin beneath the surface is known as the focus and the point on the surface immediately above it is the epicentre.

Where plates diverge, lava emerges from the mantle and cools to form new sections of crust. These diverging plate boundaries are often found underwater as mid-ocean ridges such as in the mid Atlantic where a ridge of volcanoes has formed. This is a constructive margin.

Iceland

Ocean floor spreading

Divergent (constructive) plate margin - ocean floor spreading along the plate boundary. A volcanic ridge has developed as lava fills the gap as the plates move apart

North American Plate

Eurasian Plate

The Azores - a group of islands formed as the ocean floor volcanic activity eventually broke surface

Iceland, several thousand kms north of here was formed in a similar way. However, unlike the Azores, it is still growing because the plate boundary runs through the middle.So where in Iceland would you go to see its oldest rocks?

• The Haiti earthquake of 12 January 2010 with its magnitude of 7.0 was such an earthquake - the Caribbean plate moved suddenly eastwards along the Enriquillo Fault as the North American plate slid past it in the other direction.

• But this is not the only type of plate margin to threaten Haiti with earthquake activity. What else is happening?

• Haiti is also very close to a destructive margin along a subduction zone.

• One of the largest earthquakes ever was the one that hit Chile on 22 May 1960 with its exceptionally high magnitude of 9.5. Was this along a transform margin?

• No - the Nazca Plate is moving east towards the South American Plate to form the Peru-Chile Trench along a subduction zone.

• Another very large earthquake occurred in Lisbon on 1 November 1755, magnitude 8.7. What kind of margin cuts through Italy?

• This too is a subduction zone. Currently the Mediterranean is getting smaller as the African Plate subducts beneath the Eurasian Plate.

• Can you name any active Italian volcanoes associated with this plate margin?

• Vesuvius, Stromboli and Etna

Mariana Trench

PhilippinesPlate

PacificPlate

The western edge of the westerly spreading Pacific Plate is over 170 million years old which means it has had a huge amount of time to compact and become exceptionally dense. This has led to its great height-difference (which translates to water depth) relative to the higher-riding Mariana Plate, a small section of the larger Philippines Plate, at the point where the Pacific Plate crust is subducted (forced down beneath the other). This is the deepest part of any ocean and at over 11,000m is deeper than Mt Everest is high

The Philippines

Southern Japan

Indonesia

Malaysia

Taiwan

PapuaNew Guinea

Subduction

Plate movements have led to continental drift. 225 million years ago all the land masses were fused to form a super-continent - Pangea.

200 million years ago Pangea began to move apart to form two great land masses - Laurasia and Gondwanaland.

About 135 million years ago it was possible to make out land masses that looked more like those we see today.

For example it is possible to see how South America and Africa were drifting apart to form separate continents.

The End