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Plate Tectonics & Continental Drift

Continental Drift

How can a whole continent move? The underlying geology is known as plate tectonics. This theory holds that large slabs of the earth's crust shift around on the planet's mantle, a layer of rock so hot that it flows. Moving at speeds of up to four inches per year, the shifting plates result in continental drift, the slow rearrangement of the Earth's land and sea floor. The plate tectonic theory states that the Earth's outer shell consists of a series of rigid plates known as the lithosphere /asthenosphere . These plates move in response to conduction/ convection currents within the mantle. The interactions of the plates at their boundaries yield earthquakes, volcanic activity, ocean trenches, island chains, mountain ranges and other features.

Evidence supporting continental drifthttp://www.exploratorium.edu/origins/antarctica/ideas/gondwana2.html

Evidence Description

Layers of the Earth

The crust is found on the surface of the Earth to 50 km below the Earth’s surface where it reaches temperatures of 200 °C.

The mantle is thought to be composed mainly of olivine-rich rock. It has different temperatures at different depths. The temperature is lowest immediately beneath the crust and decreases/ increases with depth. Geothermal gradients are responsible for different rock behaviors and the different rock behaviors are used to divide the mantle into two different zones known as the upper and lower mantle. Rocks in the upper mantle are cool /hot and soft/ brittle , while rocks in the lower mantle are cool/ hot and soft /brittle .

The core is thought to be composed mainly of an iron /calcium and nickel alloy. This composition is assumed based upon calculations of its density and upon the fact that many plants/ meteorites , which are thought to be portions of the interior of a planetary body, are iron-nickel alloys. The core is the earth's source of internal heat because it contains materials that are not radioactive/ are radioactive, which release heat as they break down into more stable substances. The inner core is solid, 1250 km thick and can reach temperatures of 6000 °C, hotter than the surface of the sun! The outer core is liquid, 2200 km thick and reaches temperatures of 5000 °C.

Crust* 0-50km* 200 Celcius °C

Upper Mantle* Made of magma* Cool & Brittle

Lower Mantle* Made of magma* Hot & soft

Outer Core* Made of iron & nickel* liquid

Inner Core* Made of iron & nickel* very hot b/c of radioactivity* solid

Types of Crust

There are two different types of crust: thin /thick oceanic crust that underlies the ocean basins and thin/ thick continental crust that underlies the continents. These two different types of crust are made up of different types of rock. The 10km thin oceanic crust is composed of primarily of granite/ basalt and the thicker 32-50 km continental crust is composed primarily of granite /basalt . The low /high density of the thick continental crust allows it to "float" in high relief on the low/ high density mantle below.

Oceanic Crust* 5-10km* Made of basalt

Continental Crust* 32-50km* Made of granite* low density

Magma* High density

1. Divergent: Oceanic & Oceanic“Seafloor Spreading”

Divergent plate boundaries are locations where plates are moving towards/ away from one another. This occurs because rising/ sinking convection currents push up on the bottom of the lithosphere, lifting it and flowing laterally beneath it. This lateral flow causes the plate material above to be dragged along in the direction of flow. At the crest of the uplift, the overlying plate is made thick/ stretched thin , breaks and pulls apart.

When a divergent boundary occurs beneath oceanic lithosphere, the rising convection current below lifts the lithosphere producing a mid-ocean /mid-continental ridge. Extensional forces stretch the lithosphere and produce a deep fissure. When the fissure opens, pressurereduces /increases on the super-heated mantle material below. It responds by melting and the new magma flows into the fissure. The magma then liquefies/ solidifies and the process repeats itself.

Example: The Mid-Atlantic Ridge

Effects:

A submarine mountain range such as the Mid-Atlantic Ridge Volcanic activity in the form of fissure eruptions Shallow earthquake activity Creation of new seafloor and a widening ocean basin.

Oceanic CrustOceanic Crust

Magma

Fissure

Ridge PUSH

2. Divergent: Continental & Continental

When a divergent boundary occurs beneath a thick continental plate, the pull-apart is not vigorous enough to create a clean, single break through the thick plate material. Here, the thick continental plate is arched downwards/ upwards from the convection current's lift, pulled thin by extensional forces, and fractured into a rift-shaped structure.

Example: The East Africa Rift Valley

Effects:

A rift valley sometimes occupied by a long linear lakes Numerous normal faults bounding a central rift valley Shallow earthquake activity along the normal faults. Volcanic activity sometimes occurs within the rift.

Fault MountainsRift Valley

Linear Lake

3. Convergent: Continental & Oceanic“Subduction”

When continental and oceanic plates collide the thinner and more dense /less dense oceanic plate is overridden by the thicker and more dense/ less dense continental plate. The oceanic plate is forced down into the mantle in a process known as "subduction". As the oceanic plate descends /ascends , it is forced into lower/ higher temperature environments. At a depth of about 160 km materials in the subducting plate begin to approach their melting temperatures and a process of partial melting begins.

Example: The Cascade Mountain Range

Effects:

A zone of earthquake activity that is shallow along the continent margin but deepens beneath the continent

An ocean trench immediately off shore of the continent A line of volcanic eruptions a few hundred miles inland from the shoreline Destruction of oceanic lithosphere

Oceanic

Continental

Cool warm

HOT!!!rock melts and pressure increases

Volcanoes

trench

Slab PULL

Convergent: Oceanic & Oceanic“Subduction”

When a convergent boundary occurs between two oceanic plates one of those plates will subduct beneath the other. Normally the older plate will subduct because of its lower/ higher density. The subducting plate is heated as it is forced deeper into the mantle and at a depth of about 160 km the plate begins to melt. Magma /lava chambers are produced as a result of this melting and the magma is lower in density than the surrounding rock material. It begins ascending /descending by melting and fracturing its way through the overlying rock material. Magma chambers that reach the surface break through to form a volcanic eruption cone. In the early stages of this type of boundary the cones will be just/ deep beneath the ocean surface but later grow to be higher than sea level. This produces an island chain. With continued development the islands grow larger, merge and an elongate landmass is created.

Example: Japan, the Aleutian islands

Effects:

A zone of progressively deeper earthquakes An oceanic trench A chain of volcanic islands Destruction of oceanic lithosphere.

4.Convergent: Continental & Continental

Older oceanic crustYounger oceanic crust

Cool

WarmerHOT!!!Melts and pressure increases

Volcanoes

Trench

Volcanic Islands

Hot Spot - e.g. Hawaiian Islands

In this type of convergent boundary a powerful collision occurs. The two thick continental plates collide and both of them have a density that is much lower /higher than the mantle, which prevents subduction. There may be a small amount of subduction or the heavier lithosphere below the continental crust might break free from the crust andsubduct.

Fragments of crust might be caught in the collision zone between the continents forming a highly deformed combination of rock. The intense compression can also cause extensive folding and faulting /volcanoes within the two colliding plates. This deformation can extend hundreds of kilometers into the plate interior.

Example: The Himalaya Mountain Range

Effects:

Intense folding and faulting A broad folded mountain range Shallow earthquake activity Shortening and thickening of the plates within the collision zone

5.Transform: Continental & Continental“Shearing”

Transform Plate Boundaries are locations where two plates slide past /go under one another. The fracture zone that forms a transform plate boundary is known as a transform fault. Most transform

Lots of pressure

Fault mountains

Fold mountains

faults are found in the ocean basin and connect offsets in the mid-ocean ridges. A smaller number connect mid-ocean ridges and subduction zones.

Example: San Andreas Fault Zone

Effects: Recurring earthquake activity and faulting Earthquakes are usually shallow Volcanic activity is normally not present because the typical magma sources of an upwelling

convection current or a melting subducting plate are not present.

http://aspire.cosmic-ray.org/labs/seismic/index.htm

North American Plate

Pacific Plate

Fault Mountains

Earthquakes