Tectonic Hazards
Distribution of earthquakes and volcanoes
What is the earths structure?
The earths internal structure is made up of 5 main layers
The lower mantle (sold but can flow slowly)
The crust (part of the lithosphere)(solid)(very thin)
The inner core (solid because under high pressure)
The outer core (liquid)
The upper mantle (asthenosphere) (can be deformed like plastic)
There are 2 types of crust: continental and oceanic which differ in chemical composition thickness and density
lithosphere
made up of upper mantle and crust
Broken into fragments called tectonic plates
Moves sowly over the upper mantle
Movement of plates can be tracked by GPS
Plate margin
This is where 2 plates meet
Three different types which describe the way plates are moving
Destructive
Conservative
Constructive
The interaction of diffrent tectonic plates and the mantle below leads to the triggering of earthquakes and volcanic activity
How do tectonic plates move?
Scientist are still exploring what makes tectonic plates move
Convection
The cores temperature is around 6,000 degrees Celsius
This causes magma to rise in the mantle and sink towards the core once it had cooled
Currents flow beneth the lithosphere building up lateral pressure and carying the plates with them
Only limited evidence of convection currents has been found so far
Ridge push and slab pull
Ocean ridges form high above the sea floor at constructive margins
Beneath these ridges the mantle melts
the molten magma rises as the plates move apart and cools down to form new plate material.
As the lithphere cools it becomes denser so starts to slide down away from the ridge.
Which causes the plates to move away from each other
This is called ridge push
At destructive plate margins the denser plate sinks back into the mantle under the influence of gravity
This pulls the rest of the plate under behind it
This is called slab pull
Where are earthquakes and volcanoes located?
Eathquakes and volcanoes are not randomly distributed
They occur near plate margins
Eathquakes are found at all types of plate boundaries
Volcanoes are only found at constructive and destructive plate margins
They are found both on land and at sea
e.g. a chain of volcanoes which run along the west coast of north and south america
The large band of volcanoes and earthquakes that circles the Pacific Ocean is known as The Ring of Fire
They can also be found in the middle of plates
These are known as hotspots
Occur where the earths crust is particularly thin
e.g. Hawaii has formed due to volcanic eruptions at a hot spot that is located far from the edge of the pacific plate
Plate Margins
Constructive Plate Margin
Constructive plate margins occur when tectonic plates move apart from each other
Usally move apart by a few CM a year
Over time this can cause whole continents to move
e.g. The Eurasian and north american plate are moving away from each other at a rate of 5cm a year
Why are volcanoes found at constructive plate margins?
- At constructive plate margins, the upper part of the mantle melts and hot molten magma rises
- As the plates move apart molten magma rises in between, cools and forms solid rock. this forms part of the oceanic plate. The new solid plate sometimes fractures as it is moved, causing earthquakes. These shallow earthquakes are usually small and not violent.
- Much of the magma never reaches the surface but is buoyant enough to push up against the crust to form ridge and rift features.
The magma is thin and runny so spreads out to from a shield volcano
If it is thicker it forms a cone volcano
On land constructive plate margins form steep sided valleys known as rift valleys where land drops as the plates move apart
An example of this is the Mid Atlantic Ridge
Eurasian and North American plates moving apart
Iceland formed (Volcanic Island)
Iceland is being split in 2 by the two plates moving apart
Hundreds of small earthquakes occur on a weekly basis
Destructive plate margins
How do plates move at destructive plate margins?
If 2 continental plates collide they are both buoyant so cannot sink into the mantle as a result, compression between the 2 plates causes mountains to form
Occur where tectonic plates move towards each other and collide
If an oceanic and continental plate collide the denser oceanic plate is subducted and sinks under the continental plate into the mantle where it melts
Earthquakes, volcanoes and fold mountains occur
Why are earthquakes and volcanoes found at destructive plate margins?
The pressure and strain of oceanic and continental plates pushing together causes the earths crust to crumple and form fold mountains
As plates converge pressure builds up
the rock eventuallty fractures and this causes an earthquake
Sub-ducting plate melts and causes a build up of magma which causes part of the mantle to melt and then can burst out of the crust forming a line of volcanoes to form. This process forms steep sided cone volcanoes as the lava is thick and sticky these have more violent eruptions that shield volcanoes
Japan
Is prone to earthquakes and volcanic eruptions
118 Active volcanoes
Lies of the margins of 4 plates
10% of global total
more than anywhere else in the world
Part of the ring of fire
Surrounds the pacific ocean
Eurasian
North american
Pacific
Philippine
Sub-ducts bellow the NA and Philippine plates
Sub-ducts below Eurasian plate
Many parts of japan have experianced earthquakes due to to pressure build up in the plates as they move at this destructive plate margin
Formed at the destructive plate margin between Pacific and Philippine plates is the Mariana Trench
10,994 M deep
deeper than the height of mount Everest (8,848 M)
Deepest known part of all earths oceans
Conservative plate margins
Occurs where tectonic plates move parallel to each other
Plates may move in same or opposing directions and at same or different speeds
Why are earthquakes found at conservative plate margins?
One theory is that pressure builds up at the margin of tectonic plates as they move along.
Plates get stuck as they move passed each other
Pressure builds up till the rock breaks
release of energy when rock breaks results in an earthquake
No volcanoes formed as there is no gap between the plates for magma to escape and there is no plate being sub-ducted to cause a build up of magma
No land made or destroyed
San Andreas Fault
800 Km long
In California USA
Margin between the NA and Pacific plates
Both moving NW but at different speeds
NA moves 6cm per year
Pacific moves 10cm per year
15 - 20 million years ago Los angels would of been where San Diego is now
Hollywood and Los Angels will be adjacent to the golden gate bridge in San Francisco in 20 million years time
California experiences thousands of small earthquakes every year
One of the biggest earthquakes to hit California was in San Francisco in 1906. - 7.8 on the Richter Scale
700 people died and 500 million US $ worth of damage was caused
