3.3+Characteristics,+distribution+and+causes+of+tectonic+events

3. What are the distribution, characteristics and causes of tectonic events?

 * Tectonic events** are the group name for such occurrences as earthquakes, volcanoes and tsunamis. They are called tectonic because they are all associated with earth movements of one kind or another.

In other words:
 * Where do you find them and why are they there?
 * What are they like?
 * What makes them happen?

Where do you find them and why are they there?
Now this is not so easy! There are few things you need to understand before we can go into that!

What is the structure of the Earth?
Imagine the Earth is almost a sphere that is a bit flattened at the poles, that has a radius from the core to the crust of up to 6000km. The **core** is made up of molten nickel and iron. The outer portion is liquid while the inner portion is solid. The **inner core** is in the centre of the earth and is the hottest part of the earth.. It is made up of iron and nickel with temperatures of up to 5500°C. With its immense heat energy, the inner core is like the engine room of the Earth. The **outer core** is the layer surrounding the inner core. It is still extremely hot here, with temperatures similar to the inner core The **mantle** extends almost halfway to the centre and is hot and dense It has a diameter of approximately 2900km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down, nearer the inner core, the rock is soft and beginning to melt. The **crust** is thin. It varies from between 10-100km deep and is not very dense. This is the general picture, but we need a bit more about the structure of the earth to begin to understand the why you find tectonic events where you do. See the egg? Its been hard boiled and is just about to be peeled for a salad! That is similar to what the crust is like. Some pieces are small but some others are quite large. They are called **plates.** These plates consist of two different types of crust - **continental crust** and **oceanic crust**. There are important differences between the two types. Oceanic crust is constantly being created and destroyed. It is therefore younger than continental crust and its higher density means that it can be **subducted** (can be pushed underneath) by the continental crust, back into the mantle, where it is destroyed. As continental crust is lighter with a lower average density, it is permanent and cannot sink, and in consequence is older. It is also thicker, reaching up to 70km under mountains.

What happens to the plates?
Now these plates are always on the move, for example the Atlantic is getting wider at about the speed your finger nails grow! This is because there at 2 oceanic plates that have edges right down the middle of the Atlantic. These 2 plates are separating, being forced apart as magma from the mantle is forced up between them, forming new volcanoes. Most of this mountain range is deep under the oceans, but some erupts to such an extent, that they reach the surface as new land. An example of this the Island of Surtsey rose out of the ocean on the November 14th 1963, and by the middle of 1964, it was obvious that there was sufficient mass for it to remain a permanent new island.

But how are the plates moving?
The magma in the mantle is warmed by the core at the centre of the earth. This causes convection currents, similar to those you see when jam is being made and is bubbling away in the pan. The heated matter expands and so is less dense. It rises to the surface, pushing that already there sideways. The matter on the surface cools slowly and is sucked down to replace the heated matter that has risen to the surface – a bit like the lava lamp in the picture. As you can see from the diagram, the magma is moving in a circular motion. In the process, the plates on top are slowly dragged along with it. Hence the 2 plates under the Atlantic are being dragged apart. But no gap is ever left between plates. As a gap begins to form, the magma from beneath pushes upwards to fill it. So new plates is formed by the volcanoes erupting. This is one particular type of **plate margin** – it is called either **divergent** (because they are pushing apart) or **constructive** (because it is building new rock) - either term may be used. Looking at the original diagram, you can see another plate in the diagram, where Pacific ocean plate appears to be sliding under the South American continental plate. In the process, the Pacific plate is being reabsorbed back into the mantle – the process is called subduction. The two names for this plate are **destructive** (because the Pacific oceanic plate is being lost) or convergent (because the 2 plates are moving towards each other). There is another kind of convergent plate, one where 2 pieces of continental crust are approaching each other. As noted before, continental crust is too light to be subducted, so as they move towards each other the curl up at the edges. Imagine a head-on car cash – look what happened to the 2 bonnets. This why the Himalayas are forming now and rising at the rate of 1 cm a year!

Not all plates, converge or diverge, some merely slide past each other. No plate is created, nor is any lost in this process, so these are called conservative (neither made nor destroyed) or transform ( move past) plate margin. The most well known of these is the San Andreas fault line, which passes through San Francisco and down the coast of California.

This animation shows 3 of the 4 plate movements really well - but not the collision plate margins. You may need to click on it to start it?
media type="file" key="tectonic plates.swf" width="510" height="510"

What has this got to do with earthquakes and why?
While we have mentioned that on average the Atlantic Ocean is widening at about the speed your finger nails grow – about 25mm a year. But this is on average – sometimes it as little as 10mm and sometimes as much as 100mm in one year This is because it is not a slow continuous widening, but one that occurs in fits and starts. Imagine pushing a heavy rough wooden packing case, over an equally rough piece of concrete. You begin to push and nothing happens. Increase the pressure by getting one more person and then another to help you and eventually it moves, often not just by a little but by quite a lot. This is more or less the same idea the movement of plates. The pressure exerted by the moving magma beneath increases over time until it forces the plate above to judder – that is you earthquake. Normally there is not just one of these, but quite often small ones before the big one and then ‘after-shocks’ come when? So wherever you have a plate margin, you could get an earthquake. So if asked where are you most likely to get earthquakes, then along any of the plate margins would be a good answer. However, you do get earthquakes, usually of a much smaller impact in other places that are not on plate margins. As the plates have been moving throughout history, the land masses have not always been the same shape (as you saw from the movement of India northwards). So for example, parts of what is now Great Britain, was not always of one piece. Many millions of years ago, the North West of Scotland for example, formed part of a land mass that was joined to Greenland and the eastern side of North America. When this part crashed into the other part of Great Britain, fractures and folds in the rock occurred, and even more faults occured when the Atlantic Ocean opened up again, detaching the North of Scotland from the land it was once part of. Because of these faults, the UK is subject to about 30 small quakes a year. See the map of the sites of those that occurred between Sept 30 and Oct 29th 2009.

None of these were more than 3 on the Richter scale (what that means will be explained later, but believe me that is not very big). But do click here [] to get more up to date figures and explore their scales yourselves.

What are earthquakes like? How often do they happen?
There are 2 types of waves in an earthquake Body waves and surface waves. Body waves travel outward in all directions, including downward, from the quake's **focus** -- that is, the particular spot where the fault first began to rupture. Surface waves, by contrast, are confined to the upper few hundred miles of the crust. They travel parallel to the surface, like ripples on the surface of a pond. They are also slower than body waves. So following an earthquake .the body waves (P-wave) strike first and are the fastest kind. People often report a sound like a train just before they feel a quake, which is the P-wave moving as an acoustic wave in the air. Then the secondary, or S-waves, arrive. A person in a building perceives the arrival of S-waves as a sudden powerful jolt, as if a giant has pounded his fist down on the roof. Finally, the surface waves strike. In very strong earthquakes, the up-and-down and back-and-forth motions caused by surface waves can make the ground appear to roll like the surface of the ocean, and can literally topple buildings over. This is Washington State 2001. media type="file" key="earth quakes.swf" width="570" height="570" media type="youtube" key="N7_ZWpOm4TQ" height="344" width="425"

Where do you get volcanoes and why?
Volcanoes are created when magma from the mantle can escape through a crack in the crust. We have already seen where the mantle is exposed on the ocean floor in the Mid-Atlantic ridge, so along this plate margin and all other constructive margins, volcanoes can occur. But these are not the only places where volcanoes can occur. First look at the map of the plates.

Now look at the one of where volcanoes exist – what is the other margin that has them? So why on these plates in particular?

As the 2 plates move towards each other, the oceanic plate is forced down into the magma, as it is denser than the continental plate which stays on the surface. To try and imagine what happens next, I want you to have a picture of a large piece of soft cloth, lying on a table. You slide your fingers across the table and push the edge forwards and you get this sort of impression: Now if that was not cloth but a Swiss roll cake before it was rolled up, then the up folds would probably crack a bit? This is the scenario I want you to keep in your head. The oceanic plate (D) is subducting under the continental plate down into the magma. The folds are being created along the margin of the continental plate but there are cracks occurring. As the oceanic plate descends, some magma becomes isolated above the sinking plate (E). The cracks at the surface release pressure allowing some of the trapped magma to erupt on the surface and add to the continental plate as a volcano. There are a few other areas, known as ‘hotspots’ where the magma reaches the surface and volcanoes that are isolated from any plates do occur. An example of this is Mona Loa on Hawaii but you do not need to know why that happens. media type="file" key="volcanoes.swf" width="510" height="510" Go to [] which is followed by [|Plate Tectonics I] and [|Plate Tectonics II] have good back up material. Also from this site is: [|Volcanoes] [|Mountain building] [|Earth's Interior] A video about the San Andreas Fault on [] @http://volcanoes.usgs.gov/hazards/ a good explanation of volcanic hazards

**Where do you find earthquakes?** Along all plates as they all move in irregular movements. Also, to a smaller extent, in other areas that contain historical faults
====**Where do you find volcanoes?** In those places where magma can escape from the mantle, so only along divergent and convergent destructive margins. In addition there are a few places, called hot spots, where the magma can get through thin pieces of crust.====