Tectonic Change

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Tectonic Change beneath the other. Most subduction zones include
oceanic crust. As one plate descends under Fold Mountain Chains:
Plate Boundaries: another, it carries the oceanic crust back into the The world's largest mountain ranges, including
There are four plate boundaries. These are;- mantle. Subduction zones are marked by the Himalayas, Andes and the Alps are located
- Constructive margin. This is where two trenches at the surface. Example- the Aleutian around subduction zones.
plates move apart and oceanic crust is Trench and the Peru-Chile Trench. Where an oceanic plate and continental plate
formed. Creates mid-ocean ridges and The descending oceanic plate melts at around converge, an island arc may collide with the
volcanic activity is common. An example 100-300km and breaks up completely by 700km. continental plate and contribute to mountain
would be the Mid Atlantic Ridge. the amount of subduction roughly equals the building. Meanwhile, sedimentary rocks formed
- Destructive margin. The oceanic curst is amount of oceans sea floor spreading; otherwise on the continental shelf and continental slope, are
subducted into the mantle. Creates deep- the Earth would be getting bigger. However, squeezed between the island arc and the
sea trenches and island arcs, volcanic some areas are getting larger while others are continent, crumble to form mountain ranges.
activity is common. An example would be getting smaller. The Pacific Ocean crust is being Subduction of the oceanic plate may produce
the Nazca plate subducted under the reduced by subduction while the Atlantic is large intrusions of magma beneath the
South American plate. growing by spreading ridges. Parallel to the mountains, which creates further uplift.
ocean trenches and along the overhanging plate, The Himalayas were formed by two continental
- Collision zones. Two continental plates a line of volcanoes occurs due to the melted plate plates converging. The Australo-Indian plate and
move towards each other, and become material being forced back to the surface via the Eurasian plate. As the two plates converged,
Fold Mountains as neither can sink. An pressure. Example- Andean Volcanoes. the Tethys Sea narrowed until its sea floor
example would be the Himalayas, where If the subduction zone is beneath the ocean, as sediments were thrust upwards into Fold
the Indian plate collided with the Eurasian most are, this forms trench associated island Mountains. In south Asia two continental
plate. arcs such as Japan and the Philippines that masses, the Eurasian and Indian plate collided.
- Conservative margins. Two plates eventually may be carried to become part of the The collision was welded the continents together,
move sideways past each other, but and continental margin. and because there is no subduction there is no
is neither created nor destroyed. An volcanic activity in the Himalayas.
example would be the San Andreas Fault Destructive Plate Boundaries:
in California. This is where oceanic crust is destroyed. Ocean Trenches:
The descent of the subducted plate into the Narrow trenches, hundreds of km long and up to
General Information: mantle occurs first. Then the melting of the 11km deep, occur on the ocean floor parallel to
At diverging junctions (Constructive Plate subducted plate occurs at 100km or so below the island arcs and fold mountain chains. Ocean
Margins) the plates move apart and new material surface. The melt (magma) than rises through trenches are areas of subduction where oceanic
(magma) is constantly rising up from the Mantle. weaknesses in the surrounding rocks to the crust descends into the mantle. As it does so, the
This type of junction is found under the sea and is surface because it is less dense. Finally, the overhanging plate is buckled to form a trench.
responsible for the creation of ocean floors eruption of lava, gases and ash at the surface
which are continually growing. As a result of this through volcanoes and fissures. Conservative Boundaries:
lateral movement, the continents on either side of At conservative margins two plates slide past
the ridge move progressively apart. Example- Subduction occurs when two plates converge; each other with a shearing movement, which can
America is moving away from Europe and Africa the younger, denser plate is subducted. If two be violent, giving raise to severe earthquakes.
by 2cm/year. oceanic plates converge the subduction may give However, volcanoes are usually absent. In
New ocean floor, basalt, is built up into the mid- rise to island arcs such as the Kuril Islands in Southern and Central California, the boundary of
ocean ridges. Examples- the Mid-Atlantic Ridge North Pacific. the Pacific and North American plate forms a
and East Pacific Rise. Fresh magma rises from The subduction of an oceanic plate underneath a conservative margin called the San Andreas
the mantle to the Earth's surface along these continental plate often produced fold mountain Fault.
ridges, which causes volcanism and creates chains. The Andes and Western Cordillera along
islands, such as the Azores, the Ascension the Pacific coast of America formed in this way.
islands and Hawaii. Destructive plate boundaries are also the location
At converging junctions or subduction zones of volcanoes, earthquakes and ocean
the Earth's crust is destroyed when one dives trenches.


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