Structure of the Earth Layers of the earth: Crust Ocean Crust 5 – 10 km thick (0 km at mid-ocean ridge) Continental Crust30 – 50 km thick (70km under Himalayas) Lithosphere 50 – 100 km thick Mantle Upper Mantle or Asthenosphere400 – 650 km thick Lower mantle2100 km thick Core Outer Liquid Core:2300 km thick Inner Solid Core:1200 km thick

Structure The outer liquid core is made up of an electroconductive layer of iron and nickel and about 10% sulfur. This movement creates the dynamo that is responsible for the Earth’s magnetic field and magnetosphere (which protects the Earth from harmful solar radiation). The inner core is also iron-nickel, but immense pressure keeps this layer solid.

Structure of the Earth Density of Crust Continental Crust: Granitic rocks 2.7 g/cm 3 Oceanic Crust: Basalt 3.0 g/cm 3 Upper Mantle (Asthenosphere): Ultramafic 3.2 g/cm 3 Lower Mantle: Ultramafic 4.5 g/cm 3 Core: Iron The crust “floats” on the heavier mantle. Since the continental crust is lighter, it will float higher in the mantle than the oceanic crust. This is similar to blocks of wood floating in water.

The Crust “floats on the Mantle Note that pine floats higher in water than the Oak because it is less dense. This is similar to Continental Crust (Basalt) floating higher in the mantle than Oceanic Crust (Granite) Note also that the higher the Oak floats above water, the greater the amount of Oak below water. This is similar to mountain ranges – the crust extends high above sea level but also has very deep roots.

Properties of the mantle Important points: 1) Asthenosphere is plastic. It flows when under stress similar to a liquid. 2) Lithosphere is less dense than asthenosphere, and thus floats on asthenosphere. 3)Similar to ice floating on water.

Mantle Convection Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere (the two components of the upper mantle), is divided into a number of plates that are continuously being created and consumed at their opposite plate boundaries

Earth’s Interior Video 1) 2)

Plates The Earth’s crust is composed of at least 15 large plates. 7 of these plates are very large – Pacific, North American, South American, Eurasian, African, Australian, Antarctic.

Types of Plate Boundaries 1)Divergent Plate Boundary Constructive Margin New crust is being created 2)Convergent Plate Boundary Destructive Margin Crust is being destroyed (subduction/mountain building) 3)Transform Fault Boundary Conservative margin No crust created or destroyed

Comparison of Plate Margins

Comparison of Plate Boundaries

Location of Volcanoes with respect to Plate Boundaries.

Convergent Plate Boundaries Three types: 1)Ocean-Continent 2)Ocean-Ocean 3)Continent- Continent

Ocean-Continent Boundary When an oceanic plate pushes into and subducts under a continental plate, the overriding continental plate is lifted up and a mountain range is created. As the oceanic plate sinks into the subduction trench, the part of the subducting plate in contact with the continental plate creates a zone in which earthquakes occur. The two plates can get locked and when enough stress builds, the plates suddenly move past each other in an earthquake. Seismometers can pinpoint the location and depth of these earthquakes. These earthquakes plot a perfect path of the subducting oceanic crust slab. Such earthquakes are often accompanied by uplift of the land by as much as a few meters.

Subduction Zone Earthquakes This is a chart of earthquake epicentres where the Nazca Plate is subducting under the South American Plate. Note that the green and blue dots trace a perfect outline of the subducting slab. v=Wt_jJUnTFhg Subduction Zone Video

Ocean-Continent Boundary Ocean subducts under Continent and is slowly melted Trench forms on ocean-continent margin Zone of dynamic metamorphism Ocean crust melts due to dewatering. Magma rises and mixes with continental crust to form Andesitic volcanoes. High Seismic activity and potential for Tsunamis Examples – Andes Mountains, South America, Rocky Mountains.

Ocean-Continent Boundary As oceanic crust melts (it has a basaltic composition with some sediments included). This magma rises through granitic continental crust and the magma mixes. Rock Type: andesite Volcanism: stratovolcano (intermediate chemistry, potentially high gas content leading to explosive volcanism with pyroclastic flows)

Ocean-Ocean Boundary When two oceanic plates converge one is usually subducted under the other and in the process a deep oceanic trench is formed. The Marianas Trench, for example, is a deep trench created as the result of the Phillipine Plate subducting under the Pacific Plate. Oceanic-oceanic plate convergence also results in the formation of undersea volcanoes. Over millions of years, however, 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.

Ocean-Ocean Boundary Ocean subducts under Ocean and is slowly melted Trench forms on ocean-ocean margin Zone of dynamic metamorphism Ocean crust melts due to dewatering. Magma rises and mixes with more oceanic crust to form Basaltic-Andesitic volcanoes. High Seismic activity and potential for Tsunamis Examples – Japan, Phillipines, Aleutian Islands (Alaska), Indonesia, South Pacific.

Ocean-Ocean Boundary As oceanic crust melts (it has a basaltic composition with some sediments included). This magma rises through more basaltic crust, so the composition tends to be more mafic. Rock Type: basalt, andesite Volcanism: stratovolcano (less explosive volcanism and fewer pyroclastic flows)

Island Arc Subduction under Japan

Continent-Continent Boundary When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights.

Continent-Continent Boundary Continental slabs do not subduct but form mountain ranges and high plateaus. Zone of regional metamorphism Volcanism is not associated with this boundary. High Seismic activity. Examples – Himalayas, Ural Mountains.

Continent-Continent Boundary An ocean existed between India and China. As India eventually collided with Asia (China), the ocean between them was consumed and subducted below what is now the Himalayan mountains.

Continent-Continent Boundary Since the two continents do not subduct, no volcanism is involved. Since high mountain ranges are created, the crust is thickest at these locations and extreme metamorphism occurs. Often melting leads to granitic intrusions. Rock Type: granite Volcanism: none

Divergent Plate Boundaries Divergent plate boundaries generally occur at Seafloor Spreading centers (Mid-Ocean Ridges). These are found in all oceans.

Sea-Floor Spreading Subduction Zone Video:

Mid-Continent Rift

Passive Margin Passive continental margins are those passively moving away from sites of seafloor spreading (e.g., Eastern North America, see image above). Technically, the term continental margin includes the regions know as the continental shelf, slope and rise; however, it often is used more generally to describe the entire continental- oceanic transitional zone.

Passive Margin

Transform Plate Boundary Transform-Fault Boundaries are where two plates are sliding horizontally past one another. These are also known as transform boundaries or more commonly as faults. Most transform faults are found on the ocean floor. They commonly offset active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes.

Transform Plate Boundary Transform Boundaries ch?v=tIuk2blBzHs ch?v=tIuk2blBzHs ch?v=iPqWE6hoxfU ch?v=7aGUSkDHb9w

San Andreas Fault (California) The San Andreas is one of the few transform faults exposed on land. 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).

Mantle Hot Spots

Mantle Hot Spots – under Oceanic Crust Some volcanism occurs in the middle of plates. Rising mantle plumes are hot enough to “punch” through the mantle and produce volcanoes that rise from the ocean floor and in the case of Hawaii’s big island, rise well above the surface. These mantle “plumes” or hot spots remain stationary while the oceanic plate moves over the hot spot. As a result, the volcano loses its source of magma and becomes extinct and is carried (westward in the case of the Hawaiian Islands) and a new volcano forms.

Oceanic Hot Spot Ultramafic basaltic magma is generated by the hot spot which mixes with basaltic oceanic crust Rock Type: basalt Volcanism: shield volcanoes. The mafic magma has low gas content and low viscosity creating runny lava that crates volcanoes with very gentle slopes. Shield volcanoes produce spectacular lava fountains but are the least dangerous form of volcanism

Mantle Hot Spots – under Continental Crust Yellowstone happens to be hot spot located beneath a continent. The North American continental plate is travelling westward over this mantle hotspot

Continental Hot Spot Mafic basaltic magma is generated by the hot spot which mixes with the granitic continental crust Rock Type: rhyolite Volcanism: vast caldera volcanoes. The magma gains high silica and gas content from the melted continental material creating volcanoes with Felsic chemistry. No volcanic cone is created, however this type of volcanism is extremely explosive producing caldera that may be 100 km in diameter. The dormant Yellowstone caldera current heats the geyser basins in the park.