Types of plate boundaries - WordPress.com · forming a raised ridge called a Mid-Ocean Ridge. The magma also spreads outward; forming new ocean floor and new oceanic crust. New crust

Teacher: Oral Johnson

CXC Geography

Module 1: Natural Systems

School: Hydel Group of Schools

Types of plate boundaries

Divergent plate boundaries

Question

At divergent boundaries, tectonic plates are moving away from each other. But if these

huge masses of crust are moving apart, what happens in the space left between them?

Ocean-Ocean Divergent plate Boundary

Divergent plate boundaries also known as constructive plate boundaries occur when two plates

move away from each other. As plates made of oceanic crust pull apart, a crack in the ocean

floor appears. Magma then oozes up from the mantle to fill in the space between the plates,

forming a raised ridge called a Mid-Ocean Ridge. The magma also spreads outward; forming

new ocean floor and new oceanic crust. New crust is therefore form at the boundary as magma

moves up from the mantle below. Oceanic ridges rise a kilometer or so above the ocean floor and

form a global network tens of thousands of miles long. The major mid-ocean ridges are

East pacific,

Pacific-Antarctic

Atlantic- Indian

Carlsberg

Mid-Atlantic

Mid-Indian

Where plates move apart in areas of oceanic crust oceanic ridges are created

When this happens underwater, it is described as Sea Floor spreading. Sea Floor spreading over

the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of

Water between the continents of Europe, Africa and the Americas into the vast ocean that exists

today. Sea Floor spreading occurs at a number of places around the world.

Perhaps the best known example is the Mid-Atlantic Ridge. This submerged mountain range,

which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of

the global mid-ocean ridge system that encircles the Earth and has been formed from magma

coming up from the mantle. It occurs where the North and South American plates are pulled

apart from the Eurasian and African plate by convection cells. As the plates diverge molten rock

or magma rises from the mantle to fill in any possible gaps between them and in doing so creates

new oceanic crust. The lava had an unusual rounded shape and is called pillow lava. As it oozes

out along the plate boundary it cooks quickly on the ocean bed. Over time these submarine

volcanoes may become large enough to reach the surface

Mid-Atlantic Ridge

This has happened in Iceland in the North Atlantic Ocean. The main island of Iceland was

formed a long time ago but in recent times two new small islands have appeared below the sea.

Surtsey was formed between 1963 and 1967 and Heimaey in 1973. Where there is lateral

movement around the mid ocean ridges, large cracks called transform faults are produce at right

angles to the plate boundary. The largest visible product of constructive divergent plate is

Iceland where one-third of the lava enjoyed into the earth's surface in the last 500 years can be

found.

Other islands along the Mid Atlantic Ridge include the Azores, Ascension Island and Tristan da

Cunha. Because of sea floor spreading the Atlantic Ocean is being widened by 2-5cm a year.

Almost three-quarters of the lava that pours out unto the earth’s surface each year is found in mid

Atlantic Ocean ridges.

Continental –Continental Divergent plate Boundary

Rift Valleys

Rift valleys and faults occur when the lithosphere is under tensional stress. Convection currents

rise and diverge away from each other, high temperatures are created which cause updoming of

the crust and tensional forces can pull the crust apart.

Where plates move apart in areas of continental crust they produce rift valleys

When plates are pulled apart; it typically breaks along parallel faults that tilt slightly outward

from each other. As the plates are pulled apart the block between the faults cracks and drops

down into the soft plastic interior (the Asthenosphere). The sinking of the block forms a central

valley called a rift. The rift is basically a dropped zone where the plates are pulling apart.

Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the

boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the

surface

.

In areas of continental crust the lithosphere can fracture as sections of it move apart, and

areas of crust drop down between parallel faults to form rift valleys

Where two continental plates diverge plates move apart on the land to form rift valleys. As the

crust widens and thins, valleys form in and around the area, as do volcanoes, which may become

increasingly active. Early in the rift formation, streams and rivers flow into the low valleys and

long, narrow lakes can be created. Eventually, the widening crust along the boundary may

become thin enough that a piece of the continent breaks off; forming a new tectonic plate. The

rift valleys which are formed on land are typically 30 to 50 kilometers wide. In east Africa the

African plate is splitting to form the great African Rift Valley. It extends for 4000km from the

Red Sea to Mozambique. Its width varies between 10 and 50 km and its sides are up to 600

meters above the floor. This Rift Valley is possibly the start of the formation of a new ocean as

east Africa splits away from the rest I the continent. Another Example include the Rio Grande

rift in New Mexico.

Rifting has been pulling Arabia away from the African continent and a new ocean is

developing in the Red Sea and the Gulf of Aden. Another rift, the East African Rift Valley,

is still in the phase where it forms a graben that is 'trying' to rip East-Africa apart from the

rest of Africa

What are the features of constructive margins?

Due to the high temperatures that cause the updoming of crust and tensional forces that pull it

apart, rift valleys systems form in areas of continental crust and mid ocean ridges are formed in

areas of oceanic crust. Continued rifting can lead to the creation of new areas of oceanic floor,

mid-oceanic ridges and areas of basaltic lava and dykes fill in the spaces left by plates pulling

apart

Convergent plate boundaries

Convergent boundaries are the place where two tectonic plates converge (i.e. two plates move

toward each other). These zones tend to be where compression stresses are active and this result

in thrust or reverse faults being common. Here crust is destroyed and recycled back into the

interior of the Earth as one plate dives under another. These are known as Subduction Zones -

mountains and volcanoes are often found where plates converge. There are 3 types of convergent

boundaries: Oceanic-Continental Convergence; Oceanic-Oceanic Convergence; and Continental-

Continental Convergence

Oceanic- continental convergent plate boundary

Subduction Boundaries occur where oceanic lithosphere is pushed beneath continental or

oceanic lithospheres. Where two plates converge at an oceanic trench a subduction boundary is

formed as the denser (cold) oceanic lithosphere is pushed back down into the mantle. This

happens because the oceanic plate is denser than the continental plate so, as they move together,

the oceanic plate is forced underneath the continental plate. In this case one plate subducts the

other, pushing it slowly downward into the mantle where it melts to form magma. The subducted

lithosphere remains cold and brittle as it descends and can fracture under compressional stress.

These fractures generate earthquakes that define a zone of quakes at increasing focal depth under

the overriding plate. This zone is called the Benioff Zone. Depths of up to 700km are reached in

the Benioff Zone.

This type of plate margin can be seen running the length of the west coast of America where the

Nazca plate is being subducted between South America. The western margin of South America

is made up of considerable thickness of low density continental crust which makes the plate

buoyant and thus causes it to override or float above the denser Nazca plate. The Nazca plate is

composed entirely of dense basaltic rocks and periodotite, and therefore when the plates are

forced together the denser Nazca plate is dragged down beneath the less dense continental plate

forming a subduction zone.

What happens?

1) A deep sea trench (the Peru-Chile trench) or subduction zone occurs where the Nazca plate is

forced downwards into the mantle. (The Peru- Chile trench is 8050 meter deeps. The deepest

trench in the world is the Mariana Trench in the west Pacific Ocean which is 11 022 meters

deep)

2) The increase in pressure along the plate boundary causes the descending plate to crack. This

can cause large earthquakes.

3) The oceanic crust breaks to and melts to form new magma as it descends to great depths. This

is due to the friction and the very high temperatures as it enters the mantle.

4) The newly formed magma is less dense (lighter) than the mantle. Being less dense than the

mantle the newly forms magma will try to rise to earth’s surface. Where the magma does reach

the surface volcanoes will occur.

5) If a lot if magma rises toward volcanoes maybe formed. Sometimes the magma rises through

the sea floor to form island arcs. The Windward and Leeward Island are good examples of Island

Arcs.

.

These volcanoes are likely to form either a long chain of fold mountains (e.g. the Andes) or if

eruption takes place offshore an island add e.g. (Japan, West Indies). Estimates claim that 80%

of the world’s present active volcanoes are located above subduction zones. Oceanic-continental

convergence sustains many of the Earth's active volcanoes, such as those in the Andes and the

Cascade Range in the Pacific Northwest. The eruptive activity is clearly associated with

subduction. As the rising magma at destructive margins is more acidic than the lava of

constructive margins, it is more vicious and flows less easily. It may solidify within the mountain

mass to form large intrusive features called Batholiths. Tsunamis are giant tidal waves

generated usually at destructive margins either by volcanic eruptions as with Krakatoa in 1833 or

by submarine earthquakes. They can travel across oceans at speed of up to 600km/hr.

The most obvious features of continental-oceanic destructive margins are the linear belts of Fold

Mountains which run along the boundaries of the overriding continental plate. The mountains are

formed when rocks and sediment on the edge of the overriding plate gets compressed and

crumpled An example of this is where the nazca plate meets the South American plate; the

overriding South American plate is being lifted up, creating the towering Andes Mountains.

Another example is the Rocky Mountain in North America.

Strong destructive earthquakes and the rapid uplift of mountain ranges are common in this

region. Even though the Nazca Plate as a whole is sinking smoothly and continuously into the

trench, the deepest part of the subducting plate breaks into smaller pieces that become locked in

place for long periods of time before suddenly moving to generate large earthquakes

Subduction Trenches

On 9 June 1994, a magnitude-8.3 earthquake struck about 320 km

northeast of La Paz, Bolivia, at a depth of 636 km. This earthquake,

within the subduction zone between the Nazca Plate and the South

American Plate, was one of deepest and largest subduction earthquakes

recorded in South America. Fortunately, even though this powerful

earthquake was felt as far away as Minnesota and Toronto, Canada, it

caused no major damage because of its great depth

As aferomentioned a subduction trench marks the zone where the plates meet- an example of this

is the Peru-Chile Trench. Subduction trenches can reach depths of 4.5 km below the level of the

adjacent ocean floor. They will progressively become infilled by sediment scraped off the top of

the subducting plate, and by sediment eroded from the adjacent continental plate. The Marianas

Trench in the Western Pacific is the deepest subduction trench nearly 11000m- you could drop

Mount Everest into the Marianas Trench and the summit would still be underwater.

Oceanic-Oceanic convergent zone

Oceanic-oceanic convergence

As with oceanic-continental convergence, when two oceanic plates converge, one is usually

subducted under the other, and in the process a trench is formed. The Marianas Trench

(paralleling the Mariana Islands), for example, marks where the fast-moving Pacific Plate

converges against the slower moving Philippine Plate.

Subduction processes in oceanic-oceanic plate convergence also result in the formation of

volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor

until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are

typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which

closely parallel the trenches, are generally curved. The trenches are the key to understanding how

island arcs such as the Marianas and the Aleutian Islands have formed and why they experience

numerous strong earthquakes. Island Arcs are also formed in Japan and Philippines. Magmas

that form island arcs are produced by the partial melting of the descending plate and/or the

overlying oceanic lithosphere. The descending plate also provides a source of stress as the two

plates interact, leading to frequent moderate to strong earthquakes

OCEANIC/OCEANIC: THE CARIBBEAN ISLANDS

The South American Plate is moving westwards due to sea floor spreading at the Mid Atlantic

Ridge. Where it meets the Caribbean Plate, it descends (subducts) beneath it. This is because

the oceanic lithosphere of the South American Plate is cooler and denser than that of the

Caribbean Plate. The subduction causes low density ocean floor sediment to be scraped off the

surface of the South American Plate and thrust onto the Caribbean Islands as accretionary

wedges, in a process called obduction. The line of subduction is marked by the deep sea Puerto

Rico Trench

As the South American Plate descends, it drags against the overlying plate, causing both to

fracture and deform. This results in frequent shallow focus earthquakes that get deeper as the

ocean plate descends further, defining a zone of earthquake foci known as a Benioff zone.

Continued subduction of the South American Plate brings sea water, locked in the ocean

crust, deep into the mantle. As the plate heats up the water is liberated, lowering the melting

point of the mantle and causing partial melting. This produces magma, which rises and may

be erupted explosively as andesite at the surface.

Andesitic magma is less dense than the surrounding material, and can have a temperature of

1000oC. It is viscous, trapping gases as it rises. The water and gases in andesitic magma

account for the explosive activity of andesitic volcanoes, which typically lie dormant for many

hundreds or thousands of years. These volcanoes typically produce ash and pyroclastic flows,

as well as small amounts of andesitic lava.

The eruptions on Montserrat during the 1990s are a good example of this type of activity.

The Caribbean volcanic islands form a curved linear chain or ‘volcanic island arc’ parallel

and to the west of the Puerto Rico Trench

Collision zone

A collision boundary occurs where two plates of continental lithosphere collide resulting in Fold

Mountains. The continental crust is squashed together as the plates push together and is forced

upwards. This is called folding. Where two continental plates converge and push towards each

other fold mountains can also be formed such as the Himalayas and Alps. Fold Mountains are

also formed at Oceanic-Continental convergent plate boundary where oceanic crust is subducted

by continental crust as in the pacific. Earthquakes occur due to the thrust faulting and range in

depth from shallow to about 200 km. Examples are found along the Himalayan Belt into China,

along the Northern edge of the Mediterranean Sea through Black Sea and Caspian Sea into Iraq

and Iran.

The Himalayan mountain range dramatically demonstrates one of the most visible and

spectacular consequences of plate tectonics. When two continents meet, neither is subducted

because the continental rocks are relatively light and. Instead, the crust tends to buckle and be

pushed upward or sideways. The collision of India into Asia 50 million years ago caused the

Indian and Eurasian Plates to crumple up along the collision zone. After the collision, the slow

continuous convergence of these two plates over millions of years pushed up the Himalayas and

the Tibetan Plateau to their present heights. Most of this growth occurred during the past 10

million years. The Himalayas, towering as high as 8,854 m above sea level, form the highest

continental mountains in the world. Moreover, the neighboring Tibetan Plateau, at an average

elevation of about 4,600 m, is higher than all the peaks in the Alps except for Mont Blanc and

Monte Rosa, and is well above the summits of most mountains in the United States. It is where

these continental collisions are that fold mountains form and the earth's crust is at its

thickest. The Indian plate is still moving into the Eurasian plate at 5cm a year. At times this

movement cause major earthquakes. A long time in the past sea of Tethys lay between the two

land masses. But as the land masses slowly moved together the squeezed out

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The collision between the Indian and Eurasian plates has pushed up the Himalayas and the

Tibetan Plateau. The figure below show a cross section of the two plates before and after

their collision. The reference points (small squares) show the amount of uplift of an

imaginary point in the Earth's crust during this mountain-building process

Transform plate margins

Two plates slide last each other at a transform plate margins (also known as conservative plate

boundary). Crust is either formed or destroyed nor there any volcanic activity. However major

earthquakes can occur. Usually the plates slide past each other very slowly without any impact

on the surface. But now and then the plates stick. When this happens huge pressure can build up.

If the pressure is released suddenly an earthquake occurs. The plate margin is therefore

conservative because crystal rocks are neither being destroyed or created. The boundary between

the two plates is characterized by pronounced transform Fault.

The zone between two plates sliding horizontally past one another is called a transform-fault

boundary, or simply a transform boundary. The concept of transform faults originated with

Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults or fracture

zones connect two spreading centers (divergent plate boundaries) or, less commonly, trenches

(convergent plate boundaries). Most transform faults are found on the ocean floor. They

commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally

defined by shallow earthquakes. However, a few occur on land, for example the San Andreas

Fault zone in California. This transform fault connects the East Pacific Rise, a divergent

boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, another divergent

boundary to the north The San Andreas Fault is the most notorious of several hundred known

transform faults in California.

The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many fracture

zones (transform faults) that scar the ocean floor and offset ridges (see text). The San Andreas is

one of the few transform faults exposed on land.

The San Andreas Fault – a conservative plate margin

The San Andreas Fault zone, which is about 1,300 km long and in places tens of kilometers

wide, slices through two thirds of the length of California. Along it, the Pacific Plate has been

grinding horizontally past the North American Plate for 10 million years, at an average rate of

about 5 cm/yr. Land on the west side of the fault zone (on the Pacific Plate) is moving in a

northwesterly direction relative to the land on the east side of the fault zone (on the North

American Plate

The San Andreas faults forms a junction between the North American and pacific plates.

Although both plates are moving north west the pacific plate moves faster giving the illusion

they are moving in opposite directions/ the pacific plate moves about 6cm a year , but sometimes

it sticks ( like a machine without oil) until pressure builds up enabling it to jerk forwards . The

last major movements resulted in the San Francisco earthquake of 1989. Should these plates

continue to slide past each other, it is likely that Los Angeles will eventually be on an Island of

the Canadian coast.

The major Landforms resulting from plate movements

Type of plate Boundary Description Changes Examples

Constructive margins

(spreading or divergent

plates)

Two plates move away from

each other; new oceanic crust

appears forming mid-0cean

ridges with volcanoes

Mid-Atlanic Ridge (Americas

moving away from Eurasian

and African plates)

East Pacific Rise (Nazca and

Pacific plates moving apart)

Convergent Plate

Boundaries

Subduction zones (Oceanic

crust moves towards

continental crust but, being

heavier, sinks and is destroyed

forming deep-sea trenches and

island arcs with Volcnaoes)

Nazca sinks under South

American plate (Andes)

Juan de Fuca sinks under

North American plate

(Rockies)

Island Arcs of the West Indies

and Aleutians

Collison zones (Continental-

continental collision)

Two continental crust collide

and as neither can sink and

forced up into fold mountains

Indian Plate Collided with

Eurasian Plate, Forming

Himalayas

Africa plate Collided with

Eurasian plate, forming Alps

Transform plate boundaries Two plates move sideways

past each other- land is neither

formed nor destroyed

San Andreas Fault in

California

Plate boundaries and their associated activity

Types of plate boundary

Divergent Gentle volcanic activity and earthquake

activity

Convergent Violent volcanic and earthquake activity

Collison zones Earthquake activity and (no volcanic activity)

Transform Can be violent earthquake activity (no volcanic

activity)