Plate TectonicsIntroduction

Prepared By:Engr. Waseem Ali Khan

Events, hazards & disastersA tectonic event is an occurrence caused by tectonic activity - examples include earthquakes and volcanoesA 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 landslidesVolcanic hazards include:lava flows, pyroclastic flows, ash and laharsHowever these are not always hazardous and certainly do not always have disastrous consequencesWhen such events occur in remote areas, where almost nobody lives, the events are not even hazardous let alone disastrous

Natural HazardPeoples perception of potential hazards depend upon ...Magnitude - the energy released, force or strength of the event Frequency - how often the event occursDuration - how long the event lastsAreal extent - how widespread the event isSpeed of onset - the length of lead time from knowing that such an event will occur and its occurrenceFuture 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 CANON THE OTHER HAND A RARE EVENT OF MODERATE MAGNITUDE THAT IS HIGHLY LOCALISED AND SOON OVER IS LIKELY TO FADE FROM PEOPLES CONSCIOUSNESS SO THAT THEY DO NOT PREPARE AND MAY EVEN BECOME COMPLACENT

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....PopulationEconomyLand use and developmentInfrastructure and critical facilitiesCultural assetsNatural resources

Areas prone to tectonic eventsthat are deemed hazardswill experience disastersif they are highly vulnerable

VULNERABILITYNATURAL HAZARDRISKOFDISASTERSudden events and chronic issues

Past recurrence intervalsFuture probabilitySpeed of onsetMagnitudeDurationAreal extentExposure, sensitivityand resilience of:

PopulationEconomyLand use and developmentInfrastructure and critical facilitiesCultural assetsNatural resources

EARTHQUAKES VENN DIAGRAMThe connection between natural hazard events and locational site vulnerability

Distribution of earthquakes & major plate boundaries

Major plate boundaries superimposed on political map

Detailed tectonics map

Tectonic events are not evenly distributed across the globeBut they do form distinctive patternsThey are closely aligned to the plate boundariesCONCLUSIONS

The structureof the EarthThe 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 Earths 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 earthquakes 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.

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.