The Features and Processes associated with boundaries

What happens at divergent plate boundaries as magma rises to the surface?

Overlying rocks can be forced into a dome. The rigid lithosphere is placed under great stress and eventually fractures along parallel faults.

What is a prominent feature in Iceland?

The central rift valley is a prominent landscape feature.

What are ocean trenches?

underwater caverns submerged below the ocean. They're formed at destructive plate boundaries, where the plates are oceanic-oceanic or oceanic-continental.

Rising magma causes fractures along parallel faults, producing...

underwater rift valleys found along mid-ocean ridges.

Rift zones also occur on land and partly explain how continents can break up. The continental crust must be...

very thin for rifting to occur.

Give an example of a rift valley.

The Jordan Rift Valley stretching from the Red Sea northwards to Turkey. Here the crust has been stretched, causing faulting and forming a sunken valley known as a graben.

The relative movements of the plates at there respective boundaries are critical in...

determining the different processes operating there. In turn, distinctive landforms and landscapes develop along the plate margins.

What are divergent Plate Boundaries?

Locations where plates are diverging (moving apart) are where magma is rising through the asthenosphere and forcing its way to the surface.

The stretching process associated with rift formation is often preceded by huge volcanic eruptions which flow over large areas and are usually preserved on the flanks of the rift. What are these eruptions considered by some geologists to be?

"flood basalts" - the lava is erupted along fractures (rather than at individual volcanoes) and runs over the land in sheets like water during a flood.

Divergent plate boundaries at mid-ocean ridges consist of huge submarine mountain chains, in places rising...

3000 m above the sea bed with a combined length of 60,000 km.

What are island arcs?

A curved chain of volcanic islands located at a tectonic plate margin, typically with a deep ocean trench on the convex side.

Give an example of an island arc

Aleutian Islands are a chain of 14 large volcanic islands and 55 smaller ones belonging to the U.S. and Russia.

Describe the physical dimensions of the East African Rift Valley.

Length: 4,000km - Width: 50km - Height: 600m. From the Red Sea, it extends North to Jordan, a total distance of 5,500 km. Mt. Kilimanjaro Mount Kenya

Where do divergent plate boundaries mostly occur?

At mid-ocean ridges, probably the most spectacular relief features on the planet, at an average depth of 2.5 km below the ocean surface.

What are the landforms produced at fast spreading (up to 16.5 cm per year) divergent (constructive) plate boundaries?

Broad and smooth mountain chains; no central rift valleys. E.g East Pacific Rise (Pacific and Nazca plates)!

Give an example of a fast spreading boundary.

East Pacific Rise (Pacific and Nazca plates)

Give an example of a medium spreading boundary.

Galapagos Ridge (just south of the Nazca and Cocos plate boundary)

What will happen if spreading continues in the red sea?

If the crust continues to thin there, magma will well up to form a new spreading boundary between Africa and the Arabian Peninsula.

Eruption of magma at divergent boundaries occurs mainly underwater. What is the result of the magma cooling rapidly?

Magma erupting directly on the sea bed is cooled rapidly, forming rounded mounds called pillow lavas.

Oceanic-Continental Plate Margins Features: Ocean Trenches and Volcanoes. Examples: Peru-Chile Trench. The eastern edge of Nazca Plate being subducted under the South American Plate. Describe this plate boundary.

Max depth: 8,065m - Length: 5,900km - Width: 64km Processes - • denser oceanic plate is subducted under the continental plate when the two converge. • subduction causes a deepening of the ocean at the plate boundary and forms an ocean trench. Ocean trenches are long, narrow depressions with depths of 6000 - 11,000 m.

Give an example of a slow spreading boundary.

Mid-Atlantic Ridge (North America and Eurasian and South America and African plates)

Oceanic and continental tectonic plates may converge in one of three different combinations:

Oceanic - Continental b. Oceanic - Oceanic: destructive a. Continental - Continental: collision

Describe the key landforms at medium spreading (5 - 10 cm per year) divergent plate boundaries?

Poorly defined central rift valleys; relatively smooth mountain chains. E.g Galapagos Ridge (just south of the Nazca and Cocos plate boundary)

The Aleutian Islands are part of the...

Ring of Fire, a series of fault lines and volcanic formations along the western coast of North America and the eastern coast of Asia.

What are mid-ocean ridges frequently broken into?

Segments by transform faults. These displace ridges sideways by tens - hundreds of km.

There are 3 sub-types of plate boundary. Slow, Medium and Fast spreading. Can you describe slow spreading boundaries?

Spread rate: 2 - 3 cm a year Landforms: Steep slopes to mountain chains; clearly defined rift valleys.

How was the red sea formed?

The graben widened and more magma erupted. Eventually the rift valley sank below sea level, forming the Red Sea.

What type of activity is absent at transform faults?

Volcanic activity is absent along transform faults, but as they slip, energy is released in the form of earthquakes.

Describe the plate interactions where the Aluetian islands are located.

Where the Pacific and North American plates meet, the North American plate subducts the Pacific plate. The Aleutian Islands are just one formation created by this subduction zone. The Aleutian range of volcanoes is another by-product of this zone. The lower the plate goes, the more melting comes from the heat inside the earth.

The Pacific plate constantly moves in a N.W. direction, while the North American plate travels southward. Subduction takes place at...

approximately 6-8 cm/year at the middle and northern intersection points of the plates. This means that more of the Pacific plate heads toward the earth's core each year, creating the possibility of increased magma flow.

At mid-ocean ridges, sea water seeps into rifts and is super-heated. As it rises towards the surface it...

causes chemical changes in the basaltic rocks.

Eruption of magma at divergent boundaries occurs mainly underwater. Magma erupting directly on the sea bed is cooled rapidly, forming rounded mounds called pillow lavas. As magma rises to the surfaces, overlying rocks can be...

forced into a dome. The rigid lithosphere is placed under great stress and eventually fractures along parallel faults. This produces underwater rift valleys found along mid-ocean ridges.

The Mariana Trench is part of the Izu-Bonin-Mariana subduction system that...

forms the boundary between the oceanic Philippine and oceanic Pacific plate.

Sediment layers and rocks develop on oceanic plates adjacent to continents; these crumple, fold and are uplifted along the leading edge of the continental plate as the oceanic plate and continental plate converge. The continental crust is buckled, and uplifted, and vast amounts of molten material are injected into it. Resulting in mountain chains like the Andes on the South American Pacific coast. The descending oceanic plate undergoes...

intense pressures and friction. Faulting and fracturing occur in the Benioff zone (100 - 700 km deep), where the descending plate is at an angle close to 45º. Subduction melts the oceanic plate, releasing high energy earthquakes.

After water has seeped into fractures in the rock, sometimes super-heated...

jets of water re-emerge on the ocean floor containing metal sulphides. These features are called black smokers, supporting unique and highly specialised organisms and ecosystems.

In the North Atlantic, the extrusion of magma has been so great it has created the world's...

largest volcanic island - Iceland (where the central rift valley is a prominent landscape feature).

When an oceanic plate is subducted into the mantle, the melted material is less dense than its surroundings and rises to the surface as...

magma plutons. Huge intrusions of magma create further uplift of fold mountains. Surfacing magma forms volcanoes.

We now know the pattern of 7...

major plates and more minor ones, but the details are a subject of research.

Rift Valleys are where plates diverge beneath land, rising magma causes the continental crust to bulge and fracture, forming fault lines. As the plates diverging, the crust between...

parallel faults drops down to form a rift valley. Ocean trenches mark the subduction zone where the oceanic crust descends (between 30º and 70º) into the asthenosphere. As it does so, the leading edge of the overriding plate is buckled to form a trench.

Although the Aleutian volcanoes are active, the biggest volcanic explosion in recent history took place in 1912. Mount Katmai, on Alaska's southern...

peninsula, erupted emptying its magma chamber and eventually creating a caldera, which is now the site of a warm crater lake.

Both the Philippine and Pacific plates are moving in a northwesterly direction, but the Pacific Plate is moving faster than the Philippine Plate. The motion of these plates...

produces a convergent plate boundary because the greater speed of the Pacific Plate is causing it to collide into the Philippine Plate.

Continental drift and plate tectonics were gaining acceptance in the 50s, but little was known about tectonic plates. In the mid-60s, military subterranean nuclear tests were conducted. Releasing vast amounts of energy detected by seismometers. The maps produced from global seismic data showed most earthquakes, especially the high-magnitude ones, were spatially concentrated in narrow bands. In between were...

relatively large areas generating few earthquakes. Suggesting the rigid lithosphere and crust were broken up - tectonic plates. In some places, these slabs of crust and lithosphere were diverging, in others, converging.

The Mid-Atlantic ridge isn't a linear line. It's offset and has transform faults running off it at...

right angles. These faults are prone to lateral movement and thus cause earthquakes.

What determines the shape of mid-ocean ridges?

the rate of spreading, influenced by the amount and rate of magma brought to the surface by convection currents.

Conservative Plate Boundaries - California, where the North American and Pacific Plates...

slide past each other along the San Andreas fault system. Processes - • Frictional resistance to movement along plate boundaries often causes the build-up of pressure. • Occasionally these pressures cause rocks to fracture, releasing huge amounts of energy as earthquakes.

Oceanic-Oceanic Plate Margins - Features: Island Arcs and Ocean Trenches. Examples: In the Central Atlantic, the North American plate is subducted beneath the smaller Caribbean plate, forming island arcs like the Antilles. The N.W. zone of the Pacific 'Rim of Fire' has island arcs (e.g. Aleutian Islands extending W from Alaska). Relatively nearby is...

the Challenger Deep, the deepest part of the world's oceans. Part of the Mariana Trench, it lies 11,000 m below the ocean surface and is located where the Pacific plate is subducted beneath the Philippine plate.

Continental-Continental Plate Margins - Features: Mountain ranges and Fold Mountains. Examples: The Indian subcontinent was propelled by sea floor spreading of the Indo-Australian Plate in a N.E. direction until its collision with the Eurasian Plate 40 million years ago. This collision formed...

the Himalayan mountain chain, home of Mt. Everest, which is constantly changing, as these highly faulted and folded regions don't become seismically quiet after first impact.

When was the Mariana trench formed?

the Philippine Plate (denser oceanic plate) was subducted under the Eurasian plate upon collision. The intense driving forces of the two plates has created a trench where the oceanic plate drags the edge of the continental crust down as its descends below.

Conservative Plate Boundaries - Features: Giant Tear in the Landscape The plates move past each other in a shearing motion. Volcanic activity is absent here. Active plate boundaries tear through...

the landscape. Where rocks are exposed at the surface, the extent of movement between strata may be visible. Drainage is also modified as river courses are deflected by movement along the faults.

Further north from the Red Sea is Isreal, here the rift dips below sea level, forming the Dead Sea. At -400m it's...

the lowest point on the continental surface.

Divergent Continental - When spreading occurs under the land. Heating and updoming of the crust leads to fracturing and rifting (sections of the crust drop downwards). The sides of...

the rift move apart and form central sections, called rift valleys. Active volcanoes at the sides of the valley are surface evidence of volcanic activity beneath the rift valley.

At mid-ocean ridges, water seeps into rifts and is superheated. As it rises to...

the surface it causes chemical changes in the basaltic rocks. Superheated jets of water sometimes re-emerge on the ocean floor containing metal sulphides. These features are called black smokers, supporting unique and highly specialised organisms and ecosystems.

There are 3 sub-types of plate boundary. Slow, Medium and Fast spreading. Can you describe fast spreading boundaries?

Spread rate: Up to 16.5 cm a year Landforms: Broad and smooth mountain chains; no central rift valleys.

Describe some of the main landforms at slow-spreading divergent plate boundaries?

Steep slopes to mountain chains; clearly defined rift valleys. E.g. Mid-Atlantic Ridge (North America and Eurasian and South America and African plates).

There are 3 sub-types of plate boundary. Slow, Medium and Fast spreading. Can you describe medium spreading boundaries?

Spread rate: 5 - 10 cm a year Landforms: Poorly defined central rift valleys; relatively smooth mountain chains. Example: Galapagos Ridge (just south of the Nazca and Cocos plate boundary)

Give an example of an ocean trench

The Mariana Trench is located in the western Pacific east of the Philippines. • A crescent shaped scar in the crust measuring 1,500 miles long and 43 miles wide on average. • The trench's deepest point - the Challenger Deep, lies about 200 miles S.W. of Guam is 11,000 m below sea level. The deepest point on Earth.

Divergent Continental - Along the Great African Rift valley in East Africa, the eastern section is moving north east whilst the western section is moving W and N.W. This brittle crust is thus...

fracturing and as the crust moves apart some sections have dropped downwards to form the western and eastern rift valleys. The area is also characterised by volcanoes in the east such as Kilimanjaro and Ruwenzori in the west.

Continental-Continental Plate Margins - When two continental plates converge, little if any subduction takes place because the plates have similar densities (2.7kg/m3) and thus buoyancies, creating a collision margin. At these boundaries, solid rock is crumpled and faulted. Huge slivers of...

rock, many km wide are thrust on top of one another, forming a towering mountain range. Volcanic activity associated with earlier subduction and sediments scraped off the vanishing ocean floor are mixed up and compressed to form young fold mountain chains, deep rooted in the lithosphere.

Rift zones also occur on very thin continental crust and attribute to breaking up continents. E.g. the rift from the Red Sea northwards to Turkey. Here the crust has stretched, causing faulting and forming a sunken valley known as a graben. Widening, magma erupted. Eventually the rift valley sank below...

sea level, forming the Red Sea. If the crust continues to thin, magma will well up creating a new spreading boundary between Africa and the Arabian Peninsula. Further north in Israel, the rift dips below sea level, forming the Dead Sea. At -400m it's the lowest point on the continental surface then steepest towards the continent.

Oceanic-Oceanic Plate Margins - When two oceanic plates meet, the denser/faster one will be subducted. The collision point is marked by the bending of the denser plate, forming a deep ocean trench. Continued subduction of the plate brings sea water, locked in the ocean crust, deep into...

the mantle. As the plate heats up the water is liberated, lowering the mantle melting point and causing partial melting. While the descending plate melts in the Benioff zone, magma rises as its less dense than the surrounding asthenosphere and so rises in plutons. This magma rises through cracks in the plate and may reach the surface. Over millions of years the lava builds up and may emerge to form a string of island volcanos along the plate margin creating an island arc.