Plate Tectonics [EARTH SCIENCE]

• A plate boundary is a point where two or more plates interact with one another. They can relatively move towards each other, move away from one another, or even slide past one another. These movements were explained by the plate tectonic theory, continental drift theory, and seafloor spreading.
• The continental drift theory was proposed by Alfred Wegener that stated that there was one giant landmass called Pangaea that drifted away from one another.
• Seafloor spreading was proposed by Harry Hammond Hess. This theory supported the continental drift theory by providing an explanation of why plates drifted from one another.
• 3 Types of plate boundaries: convergent, divergent, and transform
• Types of convergent boundary

  • Oceanic-Continental Convergence
  • Oceanic-Oceanic Convergence
  • Continental-Continental Convergence
• A convergent boundary is characterized as two plates colliding with one another. This happens due to a process called subduction. The convergence of plates may happen in three ways – oceanic plate to continental, oceanic to oceanic, and continental to continental.

  Convergent boundaries are also known as destructive boundaries because as one plate interacts with another plate, colliding parts are destroyed.
• Oceanic-Continental Convergence

  Oceanic plates subduct beneath the continental plate because of density differences. As the denser plate goes beneath the less dense plate, the subducting part melts, causing the magma underneath to rise forming volcanic arcs. On the point of subduction, a trench is also formed. A good example of this is the Nazca Plate off the coast of South America where the Nazca Plate subducts under the South American Plate.
• Oceanic-Oceanic Convergence

  In this convergence, the older, denser slab will sink back into the mantle forming a subduction zone. As the subducting plate goes beneath the less dense plate, it forms a deep depression called a trench. The Aleutian

  Islands off the tip of Alaska were formed by magma generated when the Pacific Plate descended below some oceanic lithosphere on the margin of the North American Plate.

  A trench is a valley formed in the seafloor as two plates converge.
• Continental-Continental Convergence

  Continental plates cannot dive into the asthenosphere because of the nature of their densities. Because of this, they will bump into one another and push each other up in a process called orogenesis. As they collide, mountain ranges are formed and they may continue growing as the collision continues as well. The Indian plate carries the landmass of India toward China as it compresses the Eurasian continental plate. Today the Himalayan Mountains are the result of this collision.
• A divergent boundary happens when two plates drift away or diverge from one another. This type of boundary is found above the rising plume of a convection cell.

  Convection happens when materials have a temperature dif erence. Hotter materials rise and cooler materials sink.

  Divergent boundaries are also known as constructive boundaries because these lead to the formation of new crust.
• Divergent boundary

  As the plates move away from one another, the magma from the asthenosphere rises and fills the gap. The melted rocks harden and become a new ocean floor. This process is how the new Earth surface is created. (Notes: takes thousands of years) (Possible to form volcanic eruption)

  A rift valley is also an example of a divergent boundary. This may cause a single landmass to break apart just like the African continent. The East African Rift Valley is marked by a series of long lakes that start near the southern end of the Red Sea and move southward toward Mozambique.
• Iceland has a divergent boundary running through its middle – The North

  American plate diverges from the Eurasian plate as the Mid-Atlantic Ridge runs through it.
• A transform plate boundary happens when two plates slide past one another. Unlike the convergent and divergent boundaries that create landforms, this plate boundary creates earthquakes as they move. Parts of plates that interact are destroyed, creating a fault valley. The San Andreas Fault is a transform boundary that separates the North American and Pacific Plates.
• Most transform plate boundaries occur in ocean basins where they offset oceanic ridges.
• Seafloor Spreading

  Studies show that volcanic activity under the sea causes magma from beneath the earth’s crust to rise to the surface, forming a very long ridge along the middle of the oceans that separate the large continents. When continental plates collide, one of the plates splits up into two layers: a lower layer of dense mantle rock and an upper layer of lighter crystal rock. As the mantle layer subducts, the upper layer is peeled off and crumples up against the other plate, thus forming mountain ranges, like the Alps. These are called crumpled mountains.
• The Continental Drift hypothesis paved the way for the emergence and acceptance of the plate tectonics theory. It was proposed by a German meteorologist and geophysicist named Alfred Wegener.

  Wegener hypothesized that long ago, there was a supercontinent that consisted of all landmasses on Earth. He named this supercontinent Pangaea (from the Greek words pan meaning “all” and gaia meaning “land”). He and other supporters of the continental drift hypothesis collected evidence to substantiate their claims.
• Evidence of the Continental Drift Hypothesis

  • Continental Jigsaw Puzzle
  • Fossils
  • Similar Rock Types and Geologic Features
  • Ancient Climates
• Evidence #1. Continental Jigsaw Puzzle

  If you take the boundaries of each continent and try to fit them together, you’d get a landmass similar to the configuration of Pangaea. Wegener argued that the remarkable fit of the continents was more than a coincidence, citing the almost perfect fit of South America and Africa.
• Evidence #2. Fossils

  Similar fossil remains of plants and animals were found on continents that are currently separated by large bodies of water. Paleontologists agreed that these organisms wouldn’t have been able to cross these oceans due to inherent characteristics (e.g., the Mesosaurus lived only in freshwater, the Glossopteris seeds were too heavy to be carried by the wind across great distances, etc.).
• Evidence #3. Similar Rock Types and Geologic Features

  Large mountain belts of similar ages and rock types could be matched with each other across continents. This is the case with the Appalachian Mountains in the eastern margin of North America being similar to the Caledonian Mountains in the western margin of Scandinavia
• Evidence #4. Ancient Climates

  According to Wegener, evidence suggesting that there were glaciers before in present-day continents (such as Africa, South America, and Australia) located in the equator supported continental drift. However, the opposition to the hypothesis suggested that this may be due to a period of extreme global cooling.
• Evidence #4. Ancient Climates

  Wegener asserted that this was not the case because there was also evidence showing that large tropical swamps co-existed with the glaciers at the time. Despite all this evidence, the continental drift hypothesis was still not accepted by the scientific community mostly because of one problem: Wegener could not explain how the continents drifted. It wasn’t until after his death would the mystery be solved.
• The Development of the Plate Tectonics Theory

  After World War II, extensive ocean exploration led to the discovery of the global oceanic ridge system which spans around the globe, making it the longest mountain range in the world (around 80,000 km long).

  New oceanic crust forms in the axis of this ridge system. Because of this, rocks become progressively older and thicker with sediment away from the axis. This phenomenon was termed as seafloor spreading by Harry Hess and Robert Dietz
• The Development of the Plate Tectonics Theory

  However, the dredging of the ocean floor showed that the oldest oceanic crust was no more than 180 million years old. If the new oceanic crust was constantly being generated along the ridge, where did the old oceanic crust go?
• The plate tectonics model states that the lithosphere is broken up into rigid slabs called tectonic plates or simply plates. These plates overlie the ductile asthenosphere, allowing them to be in constant motion with respect to one another
• There are seven major plates which cover 94% of the Earth’s surface area:

  • African plate
  • Antarctic plate
  • Eurasian plate
  • Indo-Australian plate
  • North American plate
  • Pacific Plate
  • South American plate
• As mentioned earlier, plates are always in constant motion. Because of this, the margins of the plates are always interacting with one another. The sites where these margins interact are called plate boundaries.
• There are also minor plates such as the Philippine Sea plate, Juan de Fuca plate, Cocos plate, Nazca plate, Scotia plate, and Arabian plate.
• Transform Plate Boundaries (conservative margins)

  These plate boundaries are characterized by two plates sliding past each other, not destroying or producing new crustal material. They are also called transform faults and are usually found in fracture zones.

  Fracture zones are linear breaks on the ocean floor that run perpendicular to oceanic ridges. An active transform fault lies between the two offset oceanic ridges, while the areas beyond the ridge zones are inactive zones.