Plate Tectonics II Geological Oceanography

We have now learned about the Earth’s plates The Earth’s surface is a mosaic of rigid plates all moving with respect to each other These movements result in many of the structural features we see on Earth, like mountains, trenches and ocean basins Plate movement also shapes continents, for example leading to formation of mountains like the Himalayas and huge valleys like the African Rift Valley Teacher’s Note: Many islands like the Hawaiian islands are also formed by plate motion; The difference here is that these islands were formed by plate motion over a hot spot in the mantle far from a plate boundary where as this lesson focuses on interactions at plate boundaries.

Now we’ll learn more about plate movement or plate tectonics As plates moves, they always interact with other plates The ways that plates interact along their boundaries shape geological features of Earth’s surface and the terrain The terrain of the seafloor includes seafloor (abyssal) hills, mountain chains (ridges, islands and seamounts), trenches and great platforms, all formed by tectonic processes

Map of the ocean floor This map shows that the ocean floor has underwater mountain chains (shown by the tan lines) Photo: http://oceanexplorer.noaa.gov/explorations/05galapagos/background/mid_ocean_ridge/media/globalpb_600.html; Accessed: November 2010 Photo: NOAA-OE/WHOI

There are three types of plate boundaries Divergent Boundary: Two plates move apart (called Spreading Centers) Convergent Boundary: Two plates collide (called Subduction Zones) Conservative Boundary: Two plates slide past one another (called Transform Faults)

Divergent boundary Two plates move away from each other, pulling crust apart As crust pulls apart, magma (molten rock) rises up to form new crust and seafloor, a process called seafloor spreading The spreading center is raised because of the heat of the magma and sometimes has a central rift valley, as in the mid-Atlantic Ridge Most spreading centers are found within the new seafloor they are forming Spreading center Plate 1 Plate 2 Magma

Divergent boundaries and mid-ocean ridges Mid-ocean ridges are continuous mountain chains found in all the world’s ocean basins Mid-ocean ridges occur along divergent boundaries (spreading centers) The entire mid-ocean ridge system is the longest continuous mountain chain on earth! (Over 40,000 miles long and found primarily below 2000 m) Examples of mid-ocean ridges: Mid-Atlantic Ridge: runs down center of Atlantic Ocean basin East Pacific Rise: runs down Pacific Ocean basin, parallel to South and Central America Teacher’s Notes: The Mid-Atlantic Ridge has a deep rift valley at crest that approximates the size of the Grand Canyon! It has a relatively slow spreading rate of 2-5 cm/yr. The East Pacific Rise by contrast does not have a significant rift valley and a relatively fast spreading rate of 6 – 16 cm/yr. Source (http://oceanexplorer.noaa.gov/explorations/05galapagos/background/mid_ocean_ridge/mid_ocean_ridge.html, Accessed April 2011).

Close-up of ocean ridge This image shows a mid-ocean ridge mountain chain called the East Pacific Rise Photo: NOAA-OE/WHOI Photo: http://oceanexplorer.noaa.gov/explorations/05galapagos/background/mid_ocean_ridge/media/epr_relief_image_600.html; Accessed: November 2010

Convergent boundary Two plates collide with each other Oceanic-Oceanic plates: Can form trenches or island arcs like the Aleutian Islands Oceanic-continental plates: Oceanic plate gets subducted, pushed beneath, the continental plate Forms oceanic trenches and volcanic mountains on land, like the Cascade Mountains and the Andes Mountains Continental-continental plates: Form mountains as crust folds together, like the Himalayas Teacher’s Note: Note on Convergent boundary between two oceanic plates. The denser of the two oceanic plates will be subducted and material from the melting oceanic plate will come to the surface and form a chain of volcanic islands.

Aleutian Islands: A result of plate tectonics A recent NOAA voyage to the Aleutian Island area uncovered rarely seen Subarctic oceanic life, like this snailfish This map shows the location of the Aleutian Islands, which were created as a resulted of oceanic-oceanic plate convergence Photo: NOAA Photo: http://www.pmel.noaa.gov/home/accomp/fy99/foci.shtml; Accessed: November 2010

Conservative boundary Two plates slide past one another in opposite directions, parallel to, and along a transform boundary, also known as a transform fault This results in earthquakes, like those in California along the San Andreas Fault (the boundary between the Pacific and North American plates) Teacher’s Note: The diagram is a top-down view of the plates. Plate 1 Plate 2

Viewing the global terrain The plate movement you learned about results in the geological terrain that makes up our planet Examples include mountains, trenches, valleys and volcanoes