Intra-plate earthquakes
These occur in the middle or interior of tectonic plates and are much rarer
than boundary earthquakes
Volcanic hazards
Associated with eruption
Volcano
A landform that develops around a weakness in the Earth’s crust from which molten magma, volcanic rock and gases are ejected and extruded
Seismic hazards
Generated when rocks within 700km of the Earth’s surface come under such
stress that they break and become displaced
Tectonic hazards
These include earthquakes and volcanic eruptions as well as secondary hazards such as tsunami and represent a significant risk in some parts of the
world in terms of loss of life, livelihoods and economic impact
The distribution of earthquakes reveals 3 patterns:
- The oceanic
fracture zone (OFZ) – activity found in mid-ocean ridges. E.g. the mid- Atlantic ridge. - The continental fracture zone (CFZ) – activity found in mountain ranges e.g. across the Himalayas.
- Scattered earthquakes in
continental interiors
– along old fault
lines. E.g. the Church Stretton Fault in Shropshire
where are earthquakes found facts
95% of earthquakes are found along plate boundaries.
Around 70% of all earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean
distribution of volcanoes
The majority occur on plate boundaries however the type of plate boundary can determine whether a volcano exists and what type it is.
The violence of a volcano eruption is determined by the number of dissolved gases in the magma and how easily they can escape.
They can form away from plate boundaries, on areas called hot spots.
There are around 500 active volcanoes globally and on average, 50 erupt each year.
Intra-plate earthquakes
Intra-plate earthquakes happen in the middle of plates.
Scientists think that they occur when stresses build up in ancient faults - causing them to become active again.
However, as they don’t occur in well-defined patterns along plate margins, they are harder to predict.
Hotspot Volcanoes
A volcanic hotspot is an area in the mantle from which heat rises as a hot thermal plume from deep in the Earth – often called a ‘magma plume’.
* High heat and low pressure at the base of the lithosphere enable melting of the rock.
four scientists/geologists produced the theories of plate tectonics
Harry Hess
John Tuzo Wilson
Alfred Wegener
James Hutton
James Hutton - 1785
- Presented his ‘Theory of the Earth’ in front of the Royal Society in Edinburgh in 1785.
- He realised the processes of erosion, deposition and uplift were connected and operated continuously, driven by the Earth’s internal heat.
- After studying the strata and deposits of rocks in various locations, he also concluded that the Earth was much older than people believed at the times (which was about 6,000 years old) – due to the geological processes needing much longer.
- Through his study of geology he also came up with the rock cycle, where rock particles were transported to sea, buried, solidified and then later lifted back to the Earth’s surface through tectonic processes.
Alfred Wegener - 1912
- Alfred Wegener came up with the theory of ‘continental drift’ in 1912.
- He suggested that 200 million years ago, a supercontinent called Pangaea began to break into pieces, its parts moving away from one another. The continents we see today are fragments of that supercontinent.
- He supported his theory, by pointing to matching rock formations and similar fossils across continents.
- He was greatly ridiculed for it at the time, but it has since evolved into a well accepted theory.
Fossils and plants as well as similar rock layers were found on separate continents, supported the theory that these continents were once together.
Harry Hess - 1962
- Harry Hess came up with the hypothesis of seafloor spreading.
- He published ‘The History of Ocean Basins’ in 1962 which outlined a theory which later become known as ‘seafloor s p r e a d i n g ’.
- He discovered that the oceans were shallower in the middle and identified the presence of mid-ocean ridges which were as high as 1.5km above the flat sea floor.
- He envisaged that oceans grew from their centres, with molten material (basalt) oozing up from the Earth’s mantle, creating seafloor spreading either side.
John Tuzo Wilson - 1963
- Initially sceptical of tectonic theory, he eventually became one of its most famous supporters.
- To answer the question: why do active volcanoes occur thousands of miles from plate boundaries: John proposed in 1963 that plates might move over fixed ‘hotspots’ in the mantle, forming volcanic island chains.
- In 1965, he followed this discovery with the idea of a third type of plate boundary – transform faults (conservative plate boundaries) – regarded as the missing piece in the puzzle for plate tectonic theory.
Plate-tectonic Theory
The lithosphere is broken up into seven major and several minor parts – tectonic plates.
These plates move relative to each other over the asthenosphere.
There are a number of processes which drive their movement…
Asthenosphere
the part of the mantle below the lithosphere, where the rock is semi-molten.
Mantle Convection
Heat produced by the decay of radioactive elements in the Earth’s core heats the lower mantle – creating convection currents.
These hot, liquid magma currents are thought to move in circles in the asthenosphere – thus causing the plates to move.
This argument is now less accepted as being responsible for plate movement, see other info sheet.
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.
This argument is increasingly being seen as a major driving force in plate movement.
Subduction
Subduction is the process of a plate being destroyed.
As two oceanic plates OR an oceanic and continental plate move towards each other, one slides under the other into the mantle – where it melts into an area called the subduction zone (the circle on the diagram).
Seafloor Spreading
This is the process of new crust pushing tectonic plates apart.
In the middle of many oceans there are mid-ocean ridges, or underwater mountain ranges. These are formed when hot magma (molten rock) is forced up from the asthenosphere and hardens – forming new oceanic crust.
Palaeomagnetism
In the 1950’s, studies of palaeomagnetism confirmed that the sea floor was spreading.
Every 400,000 years or so, the Earth’s magnetic fields change direction i.e. the magnetic north and south swaps.
When lava cools and becomes rock, minerals inside the rock line up with the Earth’s magnetic direction (polarity) at the time.
The development of plate movement
