Plate margins

There are different types:

• Oceanic/Oceanic
• Oceanic/Continental
• Continental/Continental

Oceanic/Continental - Example: Peru-Chile trench

As the oceanic plate descends the increase in pressure can trigger major earthquakes.

As the oceanic plate further descends increasing heat causes the plate to melt in the Benioff zone. The resulting magma is less dense than the surrounding Asthenosphere and begins to rise, eventually making volcanoes on the earths surface.

The lava tends to be andesitic (viscous, creates composit, explosive volcanoes). 

Accumulated sediments on the continental shelf are deformed by folding and faulting. these uplift to form fold mountains.

Oceanic/Oceanic

Subduction still occurs here as the older/slightly denser plate is subducted.

The vulcanic activity that this subduction causes leads to the formation of a chain of vulcanic islands known as island arc.

The Mariana Islands have been found to have been formed in this way as a result of the convergence of the Pacific plate and the Philipine plate.

Continental/Continental - Collision Margin

No subduction occurs as both plates are buoyant and of low density.

Intervening Oceanic sediments trapped between the two converging plates are heaved upwards, resulting in the formation of major fold mountain ranges.

No volcanic activity is found as no crust has been destroyed

Earthquakes do occur however they are often deep beneath the earth so have limited impact.

San Andreas Fault

Examples:

• San Francisco 1989
• LA 1994

Length: 800 miles

The pacific plate moves North west at aproximatly 5-9 cm a year

The North American plate is moving in the same direction at 2-3 cm per year.

There are aproximatly 20 small earthquakes per day

1906, there was an earthquake which lasted 45 seconds - caused fires which lasted 4 days - damaged 28,000 buildings - 1/10th of the city destroyed - aprox 300 dead- catalyst for scientific research into the prediction of earthquakes.

Hawaii was formed by a localised hotspot within the Pacific plate.

Concentrated radioactive elements in the mantle may cause a hotspot to develop.

From this a plume of magma rises to the bottom of the Lithosphere and melts through to the top of the plate.

The hot spot is staionary and the plate moves over it causing an island arc.

Vulcanicity is usually associated with plate margins but the center of the pacific plate is the most volcanic place on Earth.

Hawaiin Island Arc

The Pacific plate has been moving over the hotspot for 70 million years and a succession of islands and underwater volcanoes have formed.

As the plate moves the volcanoes are moved away from the hotpsot in a NW direction.

The oldest volcanoes have sank back into the crust or been eroded by waves. These are called seamounts or guyotes.

Hawaiin Island Arc

The Pacific plate has been moving over the hotspot for 70 million years and a succession of islands and underwater volcanoes have formed.

As the plate moves the volcanoes are moved away from the hotpsot in a NW direction.

The oldest volcanoes have sank back into the crust or been eroded by waves. These are called seamounts or guyotes.

The sea is 5 km deep and Mauna Loa is 9 km tall.

Currently a new volcano is erupting 35 km south east of Hawaii. Loihi is only 3,000 m tall and at the moment needs to rise 2,000 m to be above sea level. It may reach the surface within 10,000 years.

The Indo-Australian plate is moving northwards at a rate of 5cm per year.

It collides with the Eurasian plate which is moving slowly South eastwards at a slower rate. 

Prior to their collision the two continental land masses were seperated by the Tethys Sea which originated at the time of the break up of Pangea 

As the two plates collided the Himalayas, a range of fold mountains were thrust upwards to a height of 9000 metres 

Volcanoes are openings in the Earths crust through which lava, ash and gases erupt.

Molten rock beneath the earths surface is reffered to as magma. Once above the surface it is called lava

At depth beneath the earths surface, enourmous pressure is exerted upon hot rocks in the mantle keeping them in a semi solid state. 

Fissures and faults in the crust create low pressure areas that allow some material beneath the crust to become molten and rise.

Dartmore is a big magma chamber which has become a batholith and solidified

If these molten rocks reach the surface they are called extrusive. If they solidify before reaching the surface they are called intrusive.

The distribution of world vulcanoes is closely linked with the plate boundaries

e.g. the vulcanoes along the West coast of North America are a result of the Juan de Fuca plate converging with the North American plate

Vulcanoes can occur in narrow linear belts and are mostly found along destructive boundaries with large numbers found in the Pacific ring of fire.

They are also found on constructive boundaries such as the Mid Atlantic ridge - Iceland and Surtsey

An exception to the general rule are hotspots - hawaii middle of the pacific plate

Primary Volcanic Hazards

Lava flows: a greater threat to property than human life due to the opperunity for evacuation. Lava flows are more dangerous when released quickly. They ussuallt cause localised economic losses.

Pyroclastic flows: these are hot rock fragments, lava particles ash and hot gases. they are linked with subduction zone volcanoes, and the flow moves very quickly from its source.

Ash and tephra fall: Ash is material below 2 mm in diameter whilst tephra is anything above this. It is usually formed when magma is fragmented by explosions and can stay in the atmosphere causing global variations in weather patterns. Ash fall does not cause many deaths but can lead to breathing difficulties.

Volcanic gasses: gases emmited from the volcano including water vapour, carbon dioxide, sulpher dioxide, hydrogen sulphide, helium and carbon monoxide. They rarley cause death but can be problematic as many are heavier than air.

Secondary Volcanic Hazards

Lahars: Mud flows of volcanic material due to ash and debris mixing with water. On steep slopes speeds can reach 22 meters per second.

Volcanic landslides: These are slides of rock and loose volcanic material, which are driven by gravity.

Tsunamis: A rare event but the 1883 eruption of krakatoa did cause a 30 metre high tidal wave.

There are variations in the form, frequency and type of volcanic eruption. These are related to:

• The type of plate margin
• Gas emmisions 
• lava type

Basaltic lava: low viscosity, low gas content, low silica and explosivity level.

Andesitic lava: medium viscosity, medium gas content, medium silica and explosivity levels.

Rhyolitic lava: high viscosity, high gas content, high silica and explosivity levels

Volcanic landdforms vary considrably relfecting the differnt natures of the material erupted, the nature of the eruption and the time elapsed since the last activity.

Hot (1000-12000 degrees celcius)

Low silica content

Flows readily and can travel far

Originates from the upwards movement of the mantle material

Found in sreading ridges, hotspots and rift valleys

Intermediate lava

Does not flow as readily as basaltic

Typical of destructive margins where crust is being destroyed

Takes up andesite from continental crust - Andes

Source of magma is the subducted plate

Acid lava

High is silica and erupts at low temperatures (800-1000 degrees celcius)

Results in viscous lava which flows slowly

Found at destructive margins

Pyroclastic flows are made up of ash and volcanic bombs. They result from the build up of gas beneath blocked volcanic vents.

The explossiveness of an eruption depends on the composition of the molten rock. Rhyolitic lava will tend to trap dissoved gasses. The pressure of the gasses can build up leading to an explosive eruption.

Baslatic lava allows volcanic gases to bubble up more easily, preventing great build ups of pressure.

Rock type: Balsatic

Location: Rifts (Early constructive margins)

Eruptions: gentle and persistent

Occur when elongated cracks in the crust allow lava to spill out.

Typically these are found around spreading ridges where the tension pulls the crust apart

Example: Heimaey, Iceland 1973 and Deccan Plauteau, North West India

When the Eurasian and North american plates pulled apart, existing topography was drowned in a vast lake of basaltic lava. Fragments of these rocks can be seen in the columnar basalts of the Giant's Causway in Northen Ireland.

The Deccan Plateau is composed of 29 major lava flows with 700,000 kilometres cubed of basalt.

Rock type: basaltic

Location: hotspots and constructive margins

Eruptions: gentle and persistent.

These are made up of Basaltic rock and form gentle sloping cones from layers of less viscous lava.

The largest volcano in the world (Mauna Loa) in Hawaii, stands 4,170 metres above sea level with a volume of 40,000 kilometres cubed.

In reality it is much bigger as the entire structure extends 10,099 metres from the ocean floor. making it taller than Everest.

Rock type: Andesitic

Location: Destructive margins

Eruption: Explosive and unpredictable

These are the most common volcanoes found on land.

They are created by layers of ash from initial explosive phases of eruptions and subsequent layers of lava from the main eruptions phases.

Typical examples of these classically shaped volcanoes e.g.

• Mount Etna in Sicily (Italy)
• Vesuvius in Italy
• Popcatepetl in Mexico

Rock type: Rhyolitic

Location: Continental crust

Eruption: explosive and unpredictable

These are steep sided volcanoes formed from very vicous lava. 

As the lava cannot travel far, it builds up convex cone shaped volcano. 

lava may solidify in the vent and be revealed later by erosion

Example: Puy de Domes in France

Rock Type: Andesitic

Location: Destructive margins

Eruption: Very explosive and unpredictable

These form when gases that have built up beneath a blocked volcanic vent result in a catastrophic eruption that destroys (at least) the volcanoes summit, leaving an enourmous crater where later eruptions may form smaller cones.

In the case of Crater Lake in the USA, the Caldera has been filled with water, while in the case of Krakatoa in Indonesia and Santorini in Greece the sea has inundated the broken remains of the volcano.

These are formed by lava fountains or from ash, cinders and volcanic bombs ejected from the crater.

The sides are steep and symetrical

Example: Paricutin in Mexico - Cinder cone

it was born in Feb 20, 1943 in a corn field and grew to 300 ft in 5 days.

Cinder cones are the most common and also smallest type of volcano ranging from tens to hundreds of metres tall.

They can occur as individual volcanoes, or as parasitic cones generated by flank eruptions on shield and composit volcanoes.

Icelandic - Lava is runny and basaltic, the ruptions are non-violent and look like lava plumes. 

Example: Mid Atlantic Ridge

Hawaiin - lava slowly and easily escapes from the vent. It is runny and basaltic. Gases escape easily and gentle sided shield volcanoes result. Effusive eruptions

Example: Mauna Loa

Strombolian - Gas explosions occur more frequently, small but frequent eruptions. Cone shaped volcano. Andesitic lava. Tephra, volcanic ash and cinder produced.

Example: Kula, Turkey

Vulcanian - Gas emmisions involved, more violent but less frequent. plugs of cooled lava may be ejected in a blast. Andesitic. Ash cloud. 55% silica content common.

Example: Stromboli, Itlay - tsunami and ash cloud

Vesuvian - Extremly strong explosions, often after the volcano has been dorment for a while. Gas and ash clouds which fall over a large area. Andesitic.

Example: Vesuvius

Kraktoan - Exceptionally violent. Rhyolitic

Pelean - glowing avalanch of hot volcanic ash, pyroclastic flow. Formation of lava domes. Short ash flows and pumice cones are possible.

Example:Mayon volcano, Phillipines

Plinian - usually most violent. Massive amounts of lava, gas and pyroclastic material emitted. coloumns of gss and volcanic ash into the atmosphere.Rhyolitic.

Example: Mt Pinatubo, Phillipines

Hot Springs

Water seeps down from the surface. The heat from the hotspot warms it causing it to become less dense and rise. These vents join together into springs. Hotter than 100 degrees celcius.

Fumaroles

Also known as steam vents are hot springs with more heat so the water boils away before reaching the surface.

Solfatara

when the gas given off by steam vents is sulfurous not carbon dioxide.

Boiling mud

A steam vent forms. under ground water and the chemicals it mixes with forms sulphuric acid and mud. The acid dissolves surrounding mud into fine particles of silica and clay which mix with a small amount of water to make mud.

Water arrives as rain or snow - takes 100's of years to sink to bottom and resurface

Earthquakes fracture and harden the rhyolitic lava causing fissures and fractures which can form a plumbing system.

The plumbing for a geyser needs to be specific or it would just be a hot spring.

The main requirment is that rhyolitic rock with a high silica content which is redeposited on the side of the plumbing system making the water/pressure tighter. It is called sinter or geyserite.

The plumbing needs to be tight in places to create pressure. The bubbles are too big and too many build up behing the restriction.

More water and steam build up and the bubbles move around forcing the water from below them out the top causing a rapid drop in pressure.

All the water turns to steam and gets expelled out the top of the vent.

Example: Old faithful, Iceland. (& Yellowstone NP)

Batholiths are large masses of intrusive rock that may cause a doming up of the surface they are forming under.

They are only exposed after the gradual weathering and erosion of the less resitent over lying 'country rock' 

If it comes into contact with limestone when forming it creates marble (heat causes metamorphic rock) 

Examples:

• Dartmore
• Granite batholiths beneath the Isle of Arron

Sills

Sills are intrustions that are formed parrallel to the bedding planes in the 'country rock' and often lie horrizontally.

The bedding planes provide a line of weakness along which the magma will flow before solidifying 

When the overlying rock is weathered and eroded the sill is exposed

Example: Hadrians Wall built on a sill formed 295 million years ago

Dykes

Dykes are intrusions formed perpendicular to the bedding planes and often lie vertically.

They form through fissures and fractures in the rock.

Wheathering and erosions reveals them.