Contributed by Aileen Alonso-Perez
The plate tectonics theory tells us that the Earth’s rigid outer shell (lithosphere) can slide over the plastic asthenosphere, which is the uppermost layer of the mantle. Tectonic plates move horizontally at rates that vary from 1 to 16 cm per year. Important geological processes take place along plate boundaries, such as the formation of mountain belts, earthquakes, and volcanoes.
URL: http://pubs.usgs.gov/publications/text/Vigil.html
Divergent boundaries occur where plates move apart and new crust is
created by magma pushing up from the mantle. Example: Mid-Atlantic Ridge. The
rate of spreading averages about 2.5 cm per year.
Convergent boundaries occur where plates move toward each other, and
sometimes one plate sinks under another. Subduction zone is the location
where sinking of a plate occurs. Convergence can occur in three different
ways:
· Oceanic-continental
convergence: is when an oceanic plate
converges with a continental plate, the denser oceanic plate sinks into the
mantle beneath the edge of the continent. Example: Andes, South America.
URL: http://pubs.usgs.gov/publications/text/Vigil.html
· Oceanic-Oceanic Convergence: Oceanic lithosphere is hot,
thin, and of low density; however, when it spreads away from the mid-oceanic
ridge, it becomes older, cooler, and denser. The denser one sinks into the
mantle. Volcanic fragments pile up on the ocean floor and form island arcs.
These zones experience numerous strong earthquakes. Example: The Aleutian
Islands, Alaska.
URL:
http://pubs.usgs.gov/publications/text/Vigil.html
· Continental-continental
convergence: when two continents meet
head-on, neither is subducted because the continental rocks are relatively light
and resist downward motion. Example: Himalayas is the highest continental
mountains in the world (8,854 m above sea level).
URL: http://pubs.usgs.gov/publications/text/Vigil.html
Transform boundaries: occur where two plates slide horizontally or past
one another. Most transform faults are found on the ocean floor; however,
a few occur on land. For example:
San Andreas. It extends for about 1000 km (about 600
mi) northwestward from the Imperial Valley to Point Arena and out to sea. The
average rate of motion across the San Andreas Fault
Zone during the past 3 million years is 56 mm/yr (2 in/yr). This is about
the same rate at which your fingernails grow. Assuming this rate continues,
scientists project that Los Angeles and San Francisco will be adjacent to one
another in approximately 15 million years.
