The Origin of Ocean Basins

Continental Drift Based upon the fit of continental outlines, fossils, and geologic evidence. Alfred Wegener proposed his hypothesis of continental drift. The continents are sections of a past super continent called Pangea, which broke apart and the fragments plowed through the oceanic crust to their present locations.

Sea-Floor Spreading Sea floor spreading demonstrates that the sea floor moves apart at the oceanic ridges and new oceanic crust is added to the edges.

Sea-Floor Spreading Starts with magma welling up (rising) at the crest (top) of the mid-ocean ridge and then cooling to form new oceanic crust. As this happens, older oceanic crust is pushed away from the mid-ocean ridge (think conveyor belt). Once the oceanic crust meets the less dense continental crust, it is subducted (pushed under) back into the mantle.

Sea-Floor Spreading

Sea-Floor Spreading Convection currents in the mantle drive this process, pushing magma up at the mid-ocean ridge and pulling it back down where ocean crust meets continental crust.

Magma Convection

Alfred Wegener and Continental Drift Wegener was an astronomer and meteorologist who, in the early 20th century, noticed that the continents looked a little like puzzle pieces and could be pieced together. Scientists had noticed that oceanic fossils were found on top mountains, and warm climate plant fossils had been found in very cold places. They also noticed that some continents that were not connected had similar fossils, but wondered how similar creatures could have traveled such a great ocean. Using your provided “puzzle”, try to put together a super continent called Pangea based on the shape of the continents and the fossil record for each area. Include the key and answer questions.

Questions 1. What do you think caused the continents to move from being connected to being separated by oceans? 2. What factors did you use to place the pieces where you did? 3. Why do you think someone who was not a geologist (Wegener) was able to come up with this idea, while it eluded experts? 4. Between which continents does the Mid-Atlantic Ridge run? Do you think this is a factor in the creation of the Atlantic Ocean? Why or why not?

Earth’s Magnetic Field Earth’s magnetic field changes over many, many years and has in the past swapped ends!

Earth’s Magnetic Field The geomagnetic field is the magnetic field of the Earth. 1. Magnetometers detect and measure Earth’s magnetic field. 2. Moving across the ocean floor away from the oceanic ridges, magnetometers alternately record stronger (positive) and weaker (negative) magnetic fields (called magnetic anomalies) in the sea floor rocks. 3. Magnetic anomalies and the rocks causing them form parallel bands arranged symmetrically about the axis of the oceanic ridge.

Earth’s Magnetic Field 4. As basaltic rocks crystallize, some minerals align themselves with Earth’s magnetic field, as it exists at that time, imparting a permanent magnetic field, called paleomagnetism, to the rock. 5. Periodically Earth’s magnetic field polarity (direction) reverses poles.

Earth’s Magnetic Field Rocks forming at the ridge crest record the magnetism existing at the time they solidify. Sea floor increases in age away from the ridge and is more deeply buried by sediment because sediments have had a longer time to collect. Rates of sea-floor spreading vary from 1 to 10 cm per year for each side of the ridge and can be determined by dating the sea floor and measuring its distance from the ridge crest. Continents are moved by the expanding sea floor.

Global Plate Tectonics Because Earth’s size is constant, expansion of the crust in one area requires destruction of the crust elsewhere. 1. Currently, the Pacific Ocean basin is shrinking as other ocean basins expand. 2. Destruction of sea floor occurs in subduction zones.

Global Plate Tectonics 3. Seismicity is the frequency, magnitude, and distribution of earthquakes. Earthquakes are concentrated along oceanic ridges, transform faults, trenches and island arcs. 4. Tectonism refers to the deformation of Earth’s crust.

Plate Tectonics

Global Plate Tectonics 5. Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench downward in the direction of the island arc.

Global Plate Tectonics 6. Plate boundaries associated with plate tectonics: Convergent Plate Boundary – Where 2 plates collide. Associated with Benioff Zones.  Divergent Plate Boundary – Where 2 plates separate and spread apart.   Transform Plate Boundaries – Where 2 plates slide across one another, like in the San Andreas Fault.

Global Plate Tectonics F. Earth’s surface is composed of a series of lithospheric plates. 1. Plate edges are trenches, oceanic ridges, and transform faults. 2. Seismicity (earthquakes) and volcanism (volcanoes) are concentrated along plate boundaries.

Global Plate Tectonics 3. Movement of plates is caused by thermal convection of the "plastic" rocks of the asthenosphere which drag along the overlying lithospheric plates. 4. Mantle plumes originate deep within the asthenosphere as molten rock which rises and melts through the lithospheric plate forming a large volcanic mass at a "hot spot."

Global Plate Tectonics G. Wilson Cycle refers to the sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins. Stages in basin history are: 1. Embryonic—rift valley forms as continent begins to split. 2. Juvenile—sea floor basalts begin forming as continental sections diverge. 3. Mature—broad ocean basin widens, trenches develop, and subduction begins. 4. Declining—subduction starts to eliminates much of sea floor and oceanic ridge. 5. Terminal— Ocean basin disappears, very little water left, much like a river or lake. 6. Suturing - last of the sea floor is eliminated and continents collide forming a continental mountain chain.

Global Plate Tectonics