Plate Tectonics

Parts of Mount Everest

Everest was the seafloor 470 million years ago.

Qomolonga Formation:

• made of limestone

• contained sea fossils (this is how scientists suspected it was underwater)

• at the summit

Yellow Band:

• heated limestone

• had some microfossils

• 26,000 - 29,000 feet

Everest Series:

• Sedimentary rock

• No fossils

• 23,000 - 26,900 feet

Theory of Plate Tectonics

• Scientific revolution that unfolded between 1915 and 1970

• states that the earth's surface is broken into rigid tectonic plates that are in constant motion driven by interactions with the mantle

Alfred Wegner:

• he proposed that the continents were once all part of land mass called Pangaea (meant all land in greek)

• since the continents are not currently connected, hypothesis called *Continental Drift*

• well supported with 4 pieces of evidence

• movement of continents is unsupported

• pangea was the thing proven, not plates or plate motion.

4 Pieces of Evidence for Continental Drift

Evidence \#1: *Continents fit together like puzzle pieces* (South America and Africa fit together best)

Evidence \#2: *Fossils match* (many fossils, animals, and plants are found on different continents. How would they cross the ocean?)

Evidence \#3: *The Rocks and Mountain Ranges Match* (The Caledonian Mountains in northern Europe match in age and mineralogy with the Appalachian Mountains in eastern North America)

Evidence \#4: *Paleoclimate* (Evidence of Tropics or polar climates in the past in areas that aren't hot or cold climates today. They were located in Pangea at a different latitude)

What is a tectonic plate?

A tectonic place is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. It is a thin outer layer of earth that is broken into pieces.

Tectonic plates move about 5-10 cm per year.

Tectonic plates are lithosphere moving on top of the mantle.

• Lithosphere (crust + uppermost mantle) is cooler and under less confining pressure so it breaks if stressed.

• Asthenosphere (upper mantle) is hot and under pressure so it flows.

*There are 7 major plates:*

North American

South American

Pacific

Eurasian

African

Australian

Antarctic

*And 5 minor ones (that we need to know):*

Nazca

Caribbean

Indian

Arabian

Cocos

Earth

2 methods to divide the earth:

*Composition (what the layers are made of):*

Crust

Mantle

Core (inner and outer)

*Mechanical (how the layers respond to stress):*

Lithosphere (solid, strong)

Asthenosphere (plastic, weak and deformable)

Mesosphere (solid)

Outer core (molten, flowing)

Inner core (solid)

Ocean Exploration

• There was more ocean exploration driven by the search for submarines in ww2 (sonar).

• Marie Tharp and Bruce Heezen compiled all the soundings into the first map of the North Atlantic seafloor in 1957

• They discovered that the ocean floor was not flat, it was full of ridges, rifts, and mountains.

Seafloor spreading - Harry Hess

• Harry Hess was a former Navy Admiral and Princeton geologist who studied the seafloor.

• used the research and maps for Tharp and Heezen, publishing the "sea floor spreading hypothesis".

• proposed that the mid-atlantic ridge was a spreading center, a place where plates are moving apart driven by convection (divergence)

• people then looked back to Wegner's Pangaea

A 1968 - 1983 Deep Sea Drilling Expedition collected data about the age of the seafloor finding evidence that the oceanic seafloor was much younger than the continental crust. These discoveries went along with the ideas of seafloor spreading.

How seafloor spreading works

When the plates diverge, the magma in the asthenosphere rises, melting, creating new oceanic crust. Near deep sea trenches, the denser ocean floor is subducted into the asthenosphere (powered by convection). The continents will "grow apart" as the new crust is created. The youngest seafloor is in the center of the ocean while the older ocean floor is by the coast.

Hot Spots

Not all volcanos are on plate boundaries.

• Hot spots are rising plumes of magma causing volcanism

• Not all hotspots are on plate boundaries, they are pretty random in location

• Could be driven by displacement of mantle by subducting lithosphere.

• They can form hot spot island chains like Hawaii.

• They show the movement of plates over a nonmoving mantle plume. Made by one plume in place with the movement of plates.

Location of Volcanos and Earthquakes

Earthquakes and Volcanos are mainly on plate boundaries, but some of them are not. For volcanos, this is because the plates are moving apart (divergent) and it creates a space for magma to rise from a plume to the surface. The tectonic plates are always moving, and the edges get stuck as they are rigid, building pressure and stress. When the pressure gets too high, energy is released causing the earthquake.

Plate Tectonic Theory

Tectonic plates with continents and ocean crust move as one slab across asthenosphere. A science-based driving mechanism: convection including mantle plumes, slab pull & ridge push. It is backed up with evidence, age and shape of ocean floor, hotspot chains and locations of earthquakes + volcanos.

Divergent Plate Boundary

• plates are moving apart

• mild volcano eruptions and shallow earthquakes

Examples: East African Rift Valley, East Pacific Rise, Indian Ridge, Mid-atlantic ridge.

Process of rifting:

As the plates diverge, a rift (crack) forms in the middle of them. In the middle of that is a ridge formed by the upflow of magma due to the break in the lithosphere. There will be two convection cells, one per plate made of the rising magma. This rift gets bigger and bigger it can be filled in by water forming lakes, seas or oceans.

Transform Plate Boundary

• tectonic plates move horizontally past each other

• no up or down motion

• earthquakes are shallow

• the plates will slide by each other but they get stuck together, building tension and pressure.

• strike-slip fault: the area between the two plates that is split apart

Convergent Plate Boundary

• plates are moving towards each other

• the density of the crust determines what happens at the boundary

• there are three types of boundaries depending on whether the plates are oceanic or continental (CC, OC, OO).

• Continental crust is warmer and lighter while oceanic is colder and denser (it will sink)

• Oceanic lithosphere is typically about 50-100 km thick. The continental lithosphere is thicker, about 150 km.

Convergent Continental-Continental (CC)

• plates will collide and crumple

• folded/collision mountain ranges may form

• suture zone forms due to deformation and mountain building