Living with Tectonic Hazards – Risk or opportunity? Flashcards
oceanic-oceanic plate divergence
When two oceanic plates diverge, fractures are formed at the plate boundary.
Sea-floor spreading takes place as magma rises from the mantle, cools and solidifies to form new sea-floor and the mid-oceanic ridge is formed.
Along various points on the ridge, magma builds up and solidifies to form undersea volcanoes.
When these volcanoes grow above sea level, they are known as volcanic islands.
o-o plate divergence : example
Example:
North-American plate & Eurasian plate diverge forming the Mid-Atlantic Ridge & Azores found in the Northern Atlantic Ocean
continental- continental plate divergence
Two unique landforms found at divergent continental-continental plate boundaries are block mountains and rift valleys.
At such boundaries, tensional forces result in parts of the crust being fractured in a process known as faulting.
Rift valleys form when a central block of land subsides between a pair of parallel faults.
Block mountains form when land masses surrounding a block of land subsides leaving a block of land with steep stones left standing.
c-c plate divergence : example
Examples:
block mountain ➢ the Black Forest in Germany
rift valley ➢ East African Rift Valley
⤷ Nubian boundary of African Plate
⤷ Somalian boundary of African Plate
oceanic-oceanic plate convergence
When two oceanic plates converge, the denser one subducts under the less dense plate
A subduction zone forms creating a deep depression known as an oceanic trench
The subduction of the oceanic plate causes the solid mantle material to partially melt and magma is formed
The magma rises through the mantle and cracks on the ocean floor to emerge as volcanoes
Eventually a chain of arc of islands called and island arc is formed
Earthquakes may also occur as such boundaries
o-o plate convergence : example
Example:
Convergence of Pacific & Philippine plates.
Pacific Plate subducts.
Forms the Mariana Island & Mariana Trench.
continental-continental plate convergence
Plates made largely of continental crust may converge with other plates made largely of continental crust.
However, both plates are too thick and buoyant, and have similar densities. Therefore, they resist subduction.
Instead, the plates break, slide along fractures/faults in the crust and fold, forming fold mountains
Earthquakes may occur at such boundaries
c-c plate convergence
Example:
Convergence of Indian & Eurasian Plate
Himalayas is formed
oceanic-continental plate convergence
When an oceanic plate meets a continental plate, the denser oceanic plate subducts under the less dense continental plate.
A subduction zone forms, creating a deep oceanic trench along the plate boundary.
The subduction of the oceanic plate causes the solid mantle material to partially melt and magma is formed.
The magma rises through the mantle and cracks in the continental crust to emerge as volcanoes in fold mountains
The edge of the thick continental plate folds and buckles to form fold mountains.
Earthquakes may also occur.
o-c plate convergence : example
Example:
Formation of the Sunda Trench & Barisan Mountains
Australian plate (oceanic) converge with Eurasian plate (continental)
Trench - Sunda
Fold mountains - Barisan Mountains
transforming plate boundary
Plates slide past each other.
There is frictional force.
As they do so, tremendous stress builds up because the boundary isn’t smooth.
This stress is eventually released suddenly, often as a violent earthquake
transforming plate boundary : example
Example:
North Anatolian Fault
Anatolian Plate & Eurasian Plate
Stratovolcanoes
Not as hot - flows like honey High viscosity - high silica content Doesn’t spread quickly Accumulate and grows in height Steep slopes - steeper at top, gentler at base Traps gas - may develop secondary cones Explosive eruption
Stratovolcanoes : Example
Example:
Mount Pinatubo, Philippines in 1991 emitted 10 million tonnes of lava, 20 million tonnes of sulphur dioxide and a million tonnes of ash
Shield Volcanoes
Very hot - flows almost like water Low viscosity - low silica content Spreads over large area quickly Does not grow in height quickly gentle sloping sides, broad summit Does not trap gas Non-explosive eruption
Shield Volcanoes : Example
Example:
Mount Washington in the USA
Risks : Massive destruction by volcanic materials
Volcanic materials produced by volcanic eruptions include lava and pyroclasts that consist of rash, rock fragments and volcanic bombs.
A pyroclastic flow can destroy everything in its path with hot rock fragments travelling at speeds greater than 200 m/s. Inhaling the hot ash and gases can also result in serious injury.
Risks : Massive destruction by volcanic materials [example]
Example:
The eruption of Nerado del Ruiz in the Andes Mountains of South America in 1985 released a pyroclastic flow. The mixing of pyroclasts and glacial ice triggered lahars which are flows of wet volcanic debris on the side of a volcano. The lahar engulfed the town of Amero and killed more than 20 000 people.
Risks : Respiratory Systems
Volcanic eruptions can release thick plumes of ash and gases such as carbon monoxide and sulphur dioxide which may be harmful to people.
Risks : Respiratory System - example
Example:
In May 2016, thousands of residents near Mount Sinabung in Northern Sumatra suffered respiratory diseases after inhaling volcanic ash.
Risks : Flight disruptions
Volcanic eruptions can produce extensive ash clouds containing tiny particles of abrasive glass, sand and rock that pose a serious danger to aircraft engines.
Risks : Flight disruption - example
Example:
In 2010, the eruption of Mount Eyjafjallajokull in Iceland led to a closure of airspace over Europe.
This caused delays to 1.2 million passengers and cost the airline industry US $1.8 billion.
Benefits : Fertile Soil
Lava and ash from volcanic eruptions break down to form fertile volcanic soil which are very favourable to agriculture.
Benefits : Fertile Soil - example
Example:
The volcanic soils of Java and Bali in Indonesia support the cultivations of crops such as tea, coffee and rice. (Indonesia, Philippines turn to solid because they poor)
