Download Key to Investigation 2: Plate Tectonics

Key to Investigation 2: Plate Tectonics
1a. All three phenomena mostly appear concentrated on continental edges, but this effect is secondary.
They are actually on pate boundaries. The edges of the continental margins are either active plate
boundaries, such as the West Coast of the US, or they are passive margins such as the East Coast of the
US. When Pangaea was forming about 300 million years ago, the East Coast was an active margin,
with earthquakes, mountain building and volcanoes. The remnants of these are still evident throughout
the region.
1b. It is the moving of plates rubbing against each other that cause earthquakes. Subduction zones are
primarily the cause of volcanoes. Converging or colliding plates push up mountains. If plates are not
locally interacting with each other, there is little geologic activity, such as in Central Australia, the
Great Plains (US) and northern Europe.
1c. In most cases, plotted points will be on or very near plate boundaries.
1d. The Earth’s crust is divided into about 20 plates that can move in relation to one another. Because
Earth’s interior is so hot, partially melted rock carries that heat toward the lower temperatures at the
surface (via convection currents). These massive convection currents in the upper mantel move the
plates across Earth’s surface. Much of the heat is released in spreading centers at midocean ridges,
which is the longest mountain range on Earth (46,000 miles long). These spreading centers drive plate
movement, which is why there are many volcanoes and earthquakes there.
Each year, the Atlantic Ocean is widening by about two inches (b/c of the reasons above).
North and South America are being pushed westward, and Europe and Africa are moving eastward into
the Pacific Ocean, where colliding takes place, pushing up mountain ranges and, in subduction zones,
producing volcanic chains of mountains.
2. Ring of Fire describes the coastline of the Pacific Ocean because the whole region is one where
plates are converging and there are many volcanoes (due to subduction zones).
3a-c. These two volcanoes are in the east Africa rift valley, where there is a spreading center that is
moving in 3 directions, a triple junction. This same process broke apart Pangaea 200 million years ago.
Two of the directions are flooded and under water, the Red Sea and the Gulf of Aden, where Arabia
has been pulled from the African continent. In the third direction, there has been much spreading, and
the crust has not stretched enough to fall below sea level and flood. The volcanoes are a result of
magma reaching the surface through the thinned and stretched crust. The Red Sea may someday
become another ocean.
In the eastern US, the same thing happened 200 million years ago as Africa started to be pushed
away from the east Coast. The long linear basins of the East Coast and Palisades (Jersey!), a volcanic
sill, across the Hudson River from New York City, are remnants of that spreading.
4a-d. Hot spots are thought to be plumes of magma that come from deep in the mantle. The chemical
composition of magma is different from subduction magmas in that they have little silica. The hot spot
seems to remain in a fixed position in the mantle as the magma burns through to the surface, producing
volcanic activity. As the tectonic plates move over these hot spots, the magma burns through, leaving
a trail of volcanoes.
The Hawaiian Islands are an example of such a trail of volcanoes. In the continental US, the
hot spot that is now under Yellowstone National Park (the poor quality picture) has burned its way
across the Snake River Plain and will probably continue into southern Montana.
By dating the age of the lavas at hot spots, and measuring how far apart they are from each
other, scientists can determine the rate and direction of plate movement.
5a. These two ranges are the results of the subcontinent of India colliding with the southern coast of
Asia about 40 – 50 million years ago. This collision is continuing today, causing the mountains to rise
about one inch per year, as well as causing numerous earthquakes in China.
5b. The Urals formed the same way the Himalayas did when northern Europe collided with what is
now Siberia (about 300 million years ago). The Urals have eroded a lot since then but were once
probably many times higher than they are today. We see a similar idea with the Appalachian
Mountains (US), which formed when Africa and North America collided during the formation of
Pangaea.