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Plate Tectonics

By Geridil and Simran

simi (:

on 22 January 2017

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Transcript of Plate Tectonics

Tidal Islands
Tidal islands are a type of continental island where land connecting the island to the mainland has not completely eroded, but is underwater at high tide
The famous island of Mont Saint-Michel, France is an example of a tidal island
Barrier Islands
Barrier islands are narrow and lie parallel to coastlines. Some are a part of the continental shelf (continental islands)
Made of sediment—sand, silt, and grave
They act as barriers between the ocean and the mainland.
They protect the coast from being directly battered by storm waves and winds
Other barrier islands formed during the most recent ice age. As glaciers melted, the sea level rose around coastal sand dunes, creating low-lying, sandy islands
By: Simran and Geridil
The Earth's crust is broken into plates that move around relative to each other
As a result of these movements, 3 main plate boundaries are formed:
- divergent boundary
- convergent boundary
- transform boundary
The biggest faults mark the boundary of two plates
Plate Boundaries
Divergent Boundary
A boundary where 2 plates are moving away from each other
A sign of this boundary is common volcanic activity
Occurs above rising convection currents that flow laterally underneath the lithosphere and drags the plates along the direction of the flow

Convergent Boundary
A boundary where 2 separate plates are pushing/moving towards each other
Earth's unchanging size implies that the crust must be destroyed at the same rate it is created
The location where one plate sinks (subducts) into another is called a subduction zone
Type of convergence taking place is determined on the kind of lithosphere involved
Convergence can occur between: an oceanic & large continental plate, 2 large oceanic plates or 2 large continental plates
Transform Boundary
Locations where 2 plates grind past each other horizontally
The fracture zone that forms a transform plate boundary is a transform fault
Most transform faults are found in the ocean basin and connect offsets in mid-ocean ridges and subduction zones
• Can be distinguished from the typical strike-slip fault because the movement is in the opposite direction
• Transform fault is formed between 2 different plates, each moving away from the spreading center of a divergent boundary
Jagged edges of plates strike and "lock" each other into place for an amount of time where stress begins to build up
Produces many earthquakes due to the stress from the fault line being released in quick bursts when the plates slip into new positions
Effects: recurring earthquake activity and faulting
The oceanic plate is overridden by the continental plate
Oceanic plate is forced down into mantle by subduction
Oceanic plate is descended into higher temperature environment where partial melting begins
Partial melting produces magma chambers above subducting oceanic plate
Magma chambers melt and fracture their way upwards as they have less density than the surrounding material
Buoyant magma chambers that rise to surface without solidifying will break through in a volcanic eruption
Effects: earthquake activity, ocean trench, a line of volcanic eruptions, and destruction of oceanic lithosphere
Oceanic Convergence
One of the plates subduct into the other (normally the older plate)
Subducting plate begins to melt as it's forced deeper into the mantle
Magma chambers with lower density than surrounding rock material ascends through overlying rock material
Magma chambers that reach the surface break through to form a volcanic eruption cone
First the cone will be beneath ocean surface but then grow to be higher than sea level producing an island chain
Effects: deeper earthquakes, oceanic trench, chain of volcanic islands, and destruction of oceanic lithosphere
Continental Convergence

Powerful collision occurs
2 thick continental plates collide with both having densities lower than the mantle preventing subduction
- may be little subduction or the heavier lithosphere below the continental crust might break free from the crust and subduct
Crust/continent margin sediments might be caught in the collision zone between continents forming a highly deformed melange of rock
Extensive folding & faulting of rocks may be caused by the intense compression
Deformation can extend hundreds of miles into the plate interior
Effects: intense folding and faulting, broad folded mountain range, shallow earthquake activity, and shortening/thickening of plates within the collision zone
Plate Tectonics
Past Supercontinents
Formed about 300 million years ago
Future Supercontinents
There's a strange pattern of "magnetic striping" on the ocean floors where magnetic fields in the rock are facing opposite directions
Based on the behaviour of the current plate tectonics, there will be a new supercontinent, called Pangaea Ultima or Pangaea Proxima, about 250 million years from now
Most likely North America and South America will be pushed east by the Pacific Plate
- also swallowing up the Atlantic Ocean as they move towards Europe and Asia
Africa will move north towards Europe
- possibly creating mountains that dwarf Mount Everest
Australia is expected to move northwest into Asia (Pangaea Proxima) or southwest towards Antarctica (Pangaea Ultima)
Two other possibilities:
- Amasia: where Antarctica stays at its current position
- Novopangaea: Antarctica moves towards the other continents (between Asia, Australia and South America
Theory developed from 1950's through 1970's plate tectonics is the modern version of continental drift
Earths outer shell is divided into several plates that slide over the mantle
Plate Tectonics Theory Development
In 1912, Wegener hypothesized that all of the continents were at one time connected as one large landmass which he called
The continental drift theory was first proposed by Alfred Wegener in 1915
- on the basis of fossil evidence, how the coastlines seemed to fit together and other features
- his theory was not widely accepted at the time
His hypothesis stated that about 200 million years ago it broke apart and began slowly moving to their current positions. His hypothesis is referred to as "
continental drift.
Wegener also speculated on sea-floor spreading and the role of the mid-ocean ridges
Seven largest minor plates
Secondary Plates
Arabian plate
Caribbean plate
Juan de Fuca plate
Cocas plate
Nazca plate
Philippine plate
Sea plate
Scotia plate
There are seven major or eight major plates(depending on how they are defined) and many minor plates
Number of Plates
Primary Plates
North American plate
South American plate
Indo-Australian plate
Eurasia plate
African plate
Pacific plate
Antarctic plate
Oceanic Divergent Boundary
Occurs beneath oceanic lithosphere
Lithosphere is lifted by the rising convection current below, creating a mid-ocean ridge
Lithosphere is stretched and produces a deep fissure
Magma flows into the fissure and then solidifies
Magma spreads and forms new ocean floor & oceanic crust
Effects: submarine mountain range, volcanic activity, shallow earthquake activity, new seafloor and widening ocean basin
Continental Divergent Boundary
Thick continental plate is arched upwards from convection current's lift
Pulled thin and fractured into rift-shaped structure
Normal faults develop on both sides of the rift and central blocks slide downwards
Streams may flow into sinking rift valley to form a long linear lake
Ocean water might flow in if rift sinks below sea level and produce a shallow sea
New ocean basin could be produced if the rift continues to sink
Effects: rift valleys, numerous normal faults bounding a central rift valley, shallow earthquake activity, and volcanic activity
Tertiary Plates
Scientists have identified about 58 of them
Lwandle plate
Panama plate
Rivera plate
North Andes
Alfred Wegener, German meteorologist
In the 1960's, scientists developed a new theory that expanded Wegener's idea of continental drift.
According to the theory of plate tectonics, Earth's crust and part of the upper mantle are broken into sections.
These sections, called plates, move on the mantle. Plates are made of the crust and a part of the upper mantle. These two parts combined are the lithosphere.
The mantle below the lithosphere is called the asthenosphere.The rigid plates of the lithosphere float and move around on the asthenosphere.
The Theory
Layers Of Earth
Earth is made up of three main layers: core, mantle and crust.
The innermost layer is the inner core, which is solid and dense, made of mostly iron.
Next layer is the outer core. The outer core is mainly liquid iron and is extremely hot.
The mantle, which is a dense, hot layer of semi-solid rock approximately 2,900 km thick.
A portion of the upper mantle, called the asthenosphere
The outermost layer is the crust
The lithosphere is part of the upper mantle and the crust.
The Lithosphere
About 100 kilometers thick
Broken into huge sections called plate tectonics that are always moving
Tectonic plate are composed of oceanic lithosphere and thicker continental lithosphere each topped by own kind of crust
Extreme heat from the mantle part of the lithosphere makes it easier for the plates to move
Continental Lithosphere
Oceanic Lithosphere
Layer of igneous, sedimentary rock that forms the continents and continental shelves
Layer mostly consists of granite rock
Thickness-40km to 200km (40km is crust)
Scientists believe that there was no continental crust originally
process primarily result of volcanism and subduction
Less dense than oceanic crust but is thicker
Consists mainly of volcanic lava rock called basalt
Thickens as it ages and moves away from mid-ocean ridge
New oceanic crust produced at mid-ocean ridges and recycled back to the mantle at subduction zones
Oceanic crust is younger
-oldest crust is 170 millions years old
Supercontinents are giant landmasses made up of more than one continental core
Common working definition: it involves about 75% of the existing continental crust
There is a cycle of supercontinents that form and break apart up to every 400-500 million years, driven by plate tectonics
Constant changing affects the planet's climate and biology
Speeds range from 10-100mm per year
Evidence for all past supercontinents are limited because the seafloor is always regenerating itself
Plate Movement
How we first knew plates move?
2 lines of evidence unlocked mystery
seemingly related rock types great distances apart and the magnetic polarity recorded by ocean floor rocks
Evidence of past rates of plate movement on a smaller scale can be obtained from geologic mapping
This simple technique has been used to determine rates of plate motion at divergent boundaries-Mid-Atlantic ridge and transform boundaries-San Andres fault
Ocean floors were the key to finding out about plate movement
Average rate of plate movement can range widely
Arctic ridge has the slowest rate(less than 2.5cm/year)
East Pacific Rise has the fastest rate(more than 15 cm/year)
Challenging question because we are not able to get into the Earth's interior to observe it
Primary forces behind plate movement is thermal convection
Causes of Tectonic Plate Movement
Thermal Convection
Thermal convection is when heat from the core of the Earth is transferred to the surface of the Earth by the mantle
Liquid rock near the mantle is heated and rises toward the crust. The rock near the surface is cooler and sinks back down toward the core.
At the top of the mantle, the rock encounters the thin crust, and, as it pushes it aside, lava flows out from the mantle to form new oceanic crust. As this happens, the plates smash into each other, slide past each other or are pushed under another plate.
This movement of the plate along with the upwelling of the mantle by the convection currents may also cause secondary actions that assist in plate movement.
Detection Of Plate Movement
Current plate movement can be tracked directly by means of space-based geodetic measurements; geodesy is the science of the size and shape of the Earth
Since plate motions are global in scale, they are best measured by satellite-based methods
Space geodesy, a term applied to space-based techniques for taking precise, repeated measurements of carefully chosen points on the Earth’s surface separated by hundreds to thousands of kilometers
The Global Positioning System (GPS) has been the most useful for studying the Earth’s crustal movements
By repeatedly measuring distances between specific points, geologists can determine the movement along faults or between plates
By monitoring the interaction between the Pacific Plate and the surrounding mostly continental plates, scientists are learning more about events that build up to earthquakes and volcanic eruptions in the Pacific “Ring of Fire”.
Global Positioning System (GPS)
GPS is a relatively new technology that uses signals sent from satellites orbiting the Earth to very accurately determine the location on receivers on the surface
GPS is based on technologies developed for military and aerospace research, notably radio astronomy and satellite tracking
Satellites continuously transmit radio signals back to Earth.
To determine its precise position on Earth (longitude, latitude, elevation), each GPS receiver must simultaneously obtain signals from
at least four satellites
Technology allows scientists to measure plate movements as small as a few millimeters per year
Satellites and GPS recievers
The Appearance and Disappearance of Islands
Appearance Of Islands
Oceanic Islands
Oceanic islands can form from different types of volcanoes
As volcanoes erupt, they build up layers of lava that may eventually break the water’s surface. When the tops of the volcanoes appear above the water, an island is formed
While the volcano is still beneath the ocean surface, it is called a sea mount
One type forms in subduction zones, where one tectonic plate is shifting under another
Japan sits at the site of four tectonic plates.The heavy oceanic plates (the Pacific and the Philippine) are sub ducting beneath the lighter Eurasian and North American plates
One type forms when tectonic plates rift, or split apart from one another
In 1963, the island of Surtsey was born when a volcanic eruption spewed hot lava in the Atlantic Ocean near Iceland. The volcano was the result of the Eurasian tectonic plate splitting away from the North American plate
One type forms as continent shifts over a hot spot
Hot Spot
Another type of oceanic island forms as a continent shifts over a “hot spot.”
A hot spot is a break in the Earth’s crust where material from the mantle bubbles or rushes up. The crust shifts, but the hot spot beneath stays relatively stable
A single hot spot formed the islands of the U.S. state of Hawaii.
Hawaii’s “Big Island” is still being formed by Mauna Loa and Kilauea, two volcanoes currently sitting over the hot spot.
The newest Hawaiian island, Loihi, also sits over the hot spot, but is still a seamount about 914 meters (3,000 feet) beneath the Pacific.
Continental Islands
Continental islands were once connected to a continent. They still sit on the continental shelf. Some formed as Earth’s shifting continents broke apart
Slow movements of the Earth’s crust broke apart Pangaea into several pieces that began to drift apart. Breakup occurred, some large chunks of land split. These fragments of land became islands
Formed because of changes in sea level. Ice covered large parts of the Earth, Water was locked in glaciers, and the sea level was much lower as glaciers began to melt, the sea level rose.
The ocean flooded many low-lying areas, creating islands such as the British Isles, which were once part of mainland Europe
Large continental islands are broken off the main continental shelf, but still associated with the continent, called microcontinent
May form through the weathering and erosion of a link of land that once connected an island to the mainland
Tidal Islands
Tidal islands are a type of continental island where land connecting the island to the mainland has not completely eroded, but is underwater at high tide
The famous island of Mont Saint-Michel, France is an example of a tidal island
Barrier Islands
Barrier islands are narrow and lie parallel to coastlines. Some are a part of the continental shelf (continental islands)
Made of sediment—sand, silt, and grave
They act as barriers between the ocean and the mainland.
They protect the coast from being directly battered by storm waves and winds
Other barrier islands formed during the most recent ice age. As glaciers melted, the sea level rose around coastal sand dunes, creating low-lying, sandy islands

Mantle convection within the Earth's surface causes new material to constantly rise at rift zones and for plates to move away from the rift and toward one another at other ends
Laurentia (core of North America), Baltica (northern Europe), smaller fragments and a smaller supercontinent Gondwana (China, India, South America, Africa, and Antarctica) were created once Pannotia broke up
Between 550-350 million years ago Laurasia (North America and Eurasia) was formed
Laurasia and Gondwana joined to form Pangaea
Once it was completely formed, Pangaea covered one-third of the planet and was surrounded by a single ocean, called Panthalassa
Pangaea existed during the Permian and Triassic periods
In many areas, the modern-day continents seemed to fit together
- for example: northwestern coast of Africa and eastern coast of South America
Fossil distribution, distinctive patterns in rock strata in now unconnected parts of the world, and distribution of the world's coal
Fossil distribution: matching fossil remains have been found although ancient species in continents are currently separated by oceans
- fossil plants and animals such as mesosaurs that were only in South America and Africa could be found on many continents
Patterns in rock strata: there are distinctive patterns in rocks in continents that are far apart
- Appalachian Mountains and Caledonian Mountains fit together
Coal distribution: coal normally forms in warm & wet climates but coal in Antarctica has been found
Islands Threatened By Climate Change
Global sea levels are rising and the world’s land ice is disappearing
Sea levels have risen 6 to 8 inches in the past 100 years
Antarctica has been losing more than 100 cubic kilometers of ice per year since 2002
By 2100, the Intergovernmental Panel on Climate Change estimates that sea levels will rise as much as 20 inches
Rising sea levels may leave low-lying islands underwater
Low-lying islands have greater risk of storm surges and rising seas
Populations are moving to higher areas, or are trying to buy land from other countries to migrate its citizens, and some have developed new ways of farming to protect their agriculture
Climate change will result in changes in weather patterns and more extreme weather events
At risk of disappearing because of sea level rise caused by melting sea ice and ice sheets in Greenland and Antarctica
Changes in climate are blamed on carbon emissions from power plants, cars, and other human activities
Islands Threatened by Climate Change
Torres Strait Islands
Solomon Islands
Carteret Islands
Cape Verde
Island Nation of Kiribati
Disappearance Of Islands
Kiribati located in the South Pacific, is 6 feet above sea level on average
Island is predicted to be will uninhabitable in 60 years due to climate change
At risk of disappearing because of sea level rise caused by melting sea ice and and ice sheets in Greenland and Antarctica
In 2005, Kiribati's emissions per capita were only 7% of the global average and less than 2% of U.S. per capita emissions
Kiribati is a chain of 33 atolls and islands in the South Pacific
Home to more than 100,000 people
Average annual temperatures have steadily increased since 1950, the ocean has become more acidic and average rainfall has also increased
Since 1993, sea level has risen across Kiribati by 1 to 4 millimeters per year
Residents have built walls to protect their villages from rising waters
Sea level rise affect all aspects of life on Kiribati, including agriculture, health, and water quality
Subduction Zones
A volcanic arc is a chain of volcanoes formed above a subducting plate
Offshore volcanoes form islands, resulting in a volcanic island arc
Result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench
The oceanic plate is saturated with water, and volatiles such as water drastically lower the melting point of the mantle.
As the oceanic plate is subducted, it is subjected to greater and greater pressures with increasing depth.
This pressure squeezes water out of the plate and introduces it to the mantle.The mantle melts and forms magma at depth under the overriding plate.
The magma ascends to form an arc of volcanoes parallel to the subduction zone
Oceanic arcs form when oceanic crust subducts beneath other oceanic crust on an adjacent plate, creating a volcanic island arc
Disappearance of Hawaiian Islands
Hawaiian Islands are experiencing significant erosion due to the groundwater beneath the islands working to dissolve the island from within as well as the forces of soil erosion
- mountains on Oahu will eventually become leveled
- the groundwater's effect on dissolving the rock of the Hawaiian Islands is more significant than the forces of soil erosion
The future of the islands is going to be strongly influenced by plate tectonics
- Oahu is gradually moving northwest while its mountains are slowly being pushed upwards
- Oahu is estimated to continue growing for as long as 1.5 million years
- the force of groundwater will eventually overcome the island as it descends to a low-lying topography
Volcanic Arc
Convergent boundaries occur where one plate subducts underneath another plate with density lower than the subducting plate or collides with another plate in the case that both plates are composed of continental material.
As an oceanic plate subducts underneath another plate at a convergent boundary, such area is called a subduction zone
Islands can form at subduction zones like Japanese Islands
Volcanically active belt on Earth is known as the Ring of Fire, a region of subduction zone volcanism surrounding the Pacific Ocean
- interior of continent may have been dry as it was locked in massive mountain chains that blocked all moisture and rainfall
- based on coal deposits found in USA and Europe, areas near the equator had tropical rainforests similar to the Amazonian jungle
- the Southern end of Pangaea covered the South Pole and was heavily glaciated at times
- several animals flourished such as the Traversodontidae
- insects such as beetles and dragonflies flourished in the Permian period
- the Permian-Triassic extinction even (a.k.a the Great Dying) occurred around 252 million years ago and caused most species to go extinct
- the early Triassic period had archosaurs that rose crocodiles, birds and a plethora of reptiles
- earliest of dinosaurs began to emerge on Pangaea about 230 million years ago
- the continents began to drift apart before the dinasours became extinct, but were still rather close together

As a result of the movement of the plate tectonics and mantle convection, Pangaea began to break apart approximately 200 million years ago
A rift of new material due to weakness in the Earth's crust caused the supercontinent to separate
- magma began to push through and create volcanic rift zone
- the rift grew so much that it created a basin and Pangaea began to separate
First broke into Laurasia (Northern Hemisphere) and Gondwana (Southern Hemisphere)
- Laurasia split into North American and Eurasia
- Gondwana split into Africa, Antarctica, Australia and South America
Panthalassa formed new oceans as the continent continued to separate
The continents gradually moved into their new positions over millions of years
Continents still continue to move
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New data from the seafloor (topography and the magnetism of rocks) led to the plate tectonic theory in the 1960s
The mechanism to drive the movement of continents was proposed as seafloor spreading (where new plate material is born) based on symmetrical patterns of reversed/normal magnetic rocks on the sea floor
Magnetic minerals aligned in ancient rocks on continents also showed that the continents once shifted relative to one another
The theory explains why similar animal & plant fossils, and similar rock fossils can be found on different continents
Continental Drift Theory Continuation
Four main past supercontinents before Pangaea:
Kenorland/Paleopangaea (formed about 2500 million years ago)
Columbia (formed about 2100 million years ago)
Rondina (formed about 1100 million years ago)
Pannotia (formed about 600 million years ago)

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