Plate Tectonics

Chapter 1:

  • 1915 - alfred wagner proposed continental drift and pangaea

  • 1960's - technology was developed to back up this theory

  • 1968 - plate tectonics theory was developed

  • The theory states that the Earth's outer shell is made up of about 20 plates

  • Most of these plates are made up of both a continent and an ocean

  • They are moving over a weak layer of hot rock, several 100 km below Earth's surface, which flows like slow moving plastic

  • One theory is that convection currents move the plates


  • The mountain chains in Canada are from plate bumping together

  • Plates play a role in forming our fossil fuels, oil, gas and coal

  • Formed when we were in a warmer, more tropical climate


Chapter 2:

Types of plate boundaries:

  • Divergent

    • Boundary between two lithospheric plates

    • Moving apart

    • Mostly along the ocean floor

    • Rift valleys: deep valley at mid-ocean ridge. The sea floor spreading process is when molten rock is forced up through rifts/cracks in the valley, cools and hardens into new crust. Old crust moves away from the mid-ocean ridge.

    • Rift valleys are broken into segments separated by fracture zones. Movement in fracture zones is the source of earthquakes along mid-ocean ridges

    • Mid-ocean ridges: mid-atlantic ridge, east pacific rise

    • Hot springs rise up from hydrothermal vents along rift valleys

  • Convergent

    • Boundary between two plates that are moving towards each other/converging

    • Subduction

      • An oceanic plate goes below another plate

      • Can be 2 oceanic plates, or 1 oceanic and 1 continental

      • Subduction boundary

      • Ocean-ocean subduction forms: a deep sea trench, and volcanic island arc

      • Deep-sea trench: long deep trench formed along the boundary. The deepest part of the ocean

      • Volcanic island arc: a chain of volcanic islands on the overriding plate

      • Ocean-continent subduction forms: a deep sea trench, a mountain chain, and inland volcanoes

    • Collision: 

      • Two continental plates, continent-continent collision

      • Continents may weld together to form one continent

      • Collision boundary

      • Crust at boundary forms mountain range

      • Himalayas formed from Indian subcontinent and eurasian continent

  • Transform

    • Transform boundary

    • Two plates that are sliding past each other

    • The fracture zones that offset the segments of a mid-ocean ridge are transform boundaries

    • Movement along transform boundaries is not uniform


Chapter 3:

  • Earth's plates are moving (evident in earthquakes and volcanic activity)

  • Three hypotheses to plate movement causes

  • Mantle Convection - Heat from Earth's inner and outer cores is transferred through the mantle by mantle convection

  • Magma is is hooter that its less dense surroundings and rises upwards at a mid ocean ridge

  • This upwelling occurs on one side of  convection cells the current moves away from the mid ocean ridge, it drags the lithospheric plate with it

  • The cooler denser rocks of the lithospheric plate sink down at a subduction boundary, along a downwelling zone in the mantle that compensates for the upwelling on the other side of the convection cell

  • Ridge push- 

  • The molten magma that rises at a mid-ocean ridge is very hot and heats the rocks around it. 

  • As the asthenosphere and lithosphere at the ridge are heated, they expand and become elevated above the surrounding sea floor. 

  • This elevation produces a slope down and away from the ridge.

  • Because the rock that forms from the magma is very hot at first, it is less dense and more buoyant than the rocks farther away from the mid-ocean ridge

  • As the newly formed rock ages and cools, it becomes more dense

  • Gravity then causes this older, denser lithosphere to slide away from the ridge, down the sloping asthenosphere

  • As the older, denser lithosphere slides away, new molten magma wells up at the mid-ocean ridge, eventually becoming a new lithosphere.

  • The cooling, subsiding rock exerts a force on spreading lithospheric plates that could help drive their movements

  • This force is called ridge push

  • Slab Pull- 

  • Found at a subduction boundary 

  • A denser plate subducts below less dense plate

  • The edge of the subducting plate is cold and heavy so it sinks in the mantle

  • The rest of the plate is pulled down along with it. The force that pulls the plate down is called slab pull.

Chapter 4:

Reconstructing the past

  • Different kinds of evidence

  • Rock in mountains show evidence of past subduction, suggesting that they formed at a convergent boundary

  • Geologist can use data about the ages of rock to form the ocean basins to reconstruct earth's past

  • The magnetic record of igneous rocks on the continents can revel latitude at which the rock was formed even if the rock has moved from its original position

  • Fossils also provide clues

  • Fossils that once lived in shallow seas have been found on mountaintops

  • Rock that show evidence of having been covered by glaciers in the past

  • Tropical regions that show evidence of having had moving ice sheets in the past


  • Pangea

    • 250 million years ago all continents were welded together

    • Geologists cannot use data from the ocean floor to make models of pangea, because the oldest oceanic crust is not old enough. Subduction has destroyed all older oceanic crust.

    • Gondwana: before the formation of Pangea, a large continent from south pole to equator. Eventually joined Pangea. Turns into south america, southern europe, africa, near east, india, australia, new zealand, antarctica. 

    • Formation of Pangea: Gondwana and other small landmasses merged 

    • Breakup of Pangea: broke into Gondwana and Laurasia, than they broke  up into smaller pieces to form today's continents

    • Landmasses may have converged and broken apart many times before Pangea

    • Tracking Plate Movements: technique called satellite laser ranging shoots a laser to a satellite and uses time to determine distance to satellite. Comparing measurements over many years shows how ground station location has changed.

    • One pattern predicts that in 100-150 million years australia and antarctica will converge, africa will collide with europe and form a new mountain range


  • Plate Tectonics and Continental Growth

    • Plate tectonics has affected the shapes of the continents as well as their positions.  

    • Processes associated with plate movements have added rock materials to the margins of the ancient continent cores. 

    • The shapes that are familiar today have formed gradually.

    • At the core of each continent is an expanse of ancient rock called the craton

    • Around 2.5 billion years ago, this core continental material stabilized. 

    • Before then, Earth's surface was probably too hot and unstable for continents to form. 

    • One example of a craton is the North American craton, which is exposed at the surface in most of eastern Canada. Geologists call this part of the craton the Canadian Shield. 

    • The remainder of the North American craton lies buried under a platform of sediments. 

    • The North American craton shows the approximate shape of the continent one billion years ago. The remainder of North America has been added to the craton as the continent developed into its present dimensions.

  • Sources of Growth Material for Continents

    • Deep sea sediments: can be added to edge of continent when ocean-continent subduction occurs

    • Igneous rock: 

      • Plutons (large igneous rocks) form from magma that cools

      • Volcanoes at subduction boundaries eject materials to add to the edge of the continent

      • Volcanic rock build up from volcanic island chains can merge with continent due to plate movements

    • River sediments: consist of eroded rock and soil, builds up at edge of continent

    • Terranes: a large block of lithospheric plate that has moved VERY far and attached to the edge of a continent. Found on all continents. May have been a primary process in the growth of western North america. 

    • Characteristics of Terranes: surrounded by major faults, rocks and fossils do not match nearby terranes

Questions

Describe what scientists hypothesize Earth’s surface looked like 200 million years ago and 180 million years ago.

Gondwana stretched from the south pole to the equator, and other small land masses existed. Then pangea formed and all continents merged together.


What is a craton?

An expanse of ancient rock at the core of a continent. Canadian shield. Cratons help geologists determine past shapes of continents, as the craton formed a billion years ago.

Describe at least two processes that contribute to the growth of continents over time.

Deep sea sediments can be added to the edge of the continent when ocean-continent subduction occurs 

River sediments that consist of eroded rock and soil can build up at the edge of a continent.

Chapter 5

Shield Volcanoes

  • Because of its low viscosity, basaltic lava tends to flow long distances before hardening.

  • In some cases, the lava builds up in layers, forming shield volcanoes with broad bases and gently sloping sides. 

  • The broad base of a shield volcano can support a mountain of enormous height. For example, Mauna Loa, a volcano on the island of Hawaii, rises 4170 meters above sea level and its base is 5000 meters below sea level; thus, its total height is 9170 meters.

  • Because shield volcanoes discharge basaltic lavas, they tend to be less explosive than other types of volcanoes. 

  • Basaltic lava flows, however,may be frequent and copious, causing damage to homes, highways, and other property.


Cinder Cones

  • Simplest type of volcano

  • Smaller than other volcanoes

  • Form in groups on the sides of other volcanoes

  • Forms when molten lava is thrown into the air from a vent

  • Fragments break apart and then harden before hitting the ground

  • Fragments accumulate into a cone shaped mound with an oval base




Composite Volcanoes

  • Composite volcanoes develop when layers of materials from successive explosive eruptions accumulate around a vent. 

  • The materials include hardened lava flows and other pyroclastic material.

  • Magma, water, and gases exploded in a massive cloud of superheated ash and stones. Some of the hot ash mixed with the snow and ice on the mountain to form a fast-moving mudflow called a lahar 

  • In some areas, the debris from the lahar and landslides reached a depth of about 46 meters.

  • After a violent eruption, a composite volcano may remain relatively quiet for a long period of time. 

  • Beneath the surface, however, gas-rich magma may again be building up pressure, eventually leading to another explosive eruption.


Calderas

  • Formed from magma the top of a volcano collapses into a crater shaped basin

  • Sometimes found at the top of active volcanoes

  • Can be filled with water to form a lake

  • Heat from volcano can heat hot springs and geysers

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Lava Plateaus

  • Sometimes plate tectonics results in the formation of a long, narrow crack or fissure in Earth’s surface. 

  • Basaltic lava pouring from the fissure spreads across the land, forming a lava plateau.

  • The basaltic lava that formed the Columbia Plateau in the northwestern United States is over one kilometer thick in some places and covers an area of about 164,000 square kilometers.