Divergent Boundaries and Their Impact

Divergent Boundaries

• Divergent boundaries occur along spreading centers
• Plates move apart
• New crust is created by magma pushing up from the mantle
• Like two conveyor belts moving in opposite directions
• Newly formed crust moves away from the ridge crest

Mid-Atlantic Ridge

• The rate of spreading along the Mid-Atlantic Ridge averages 2.5 centimeters per year or 25 km in a million years
• Seafloor spreading over the past 100-200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water to the vast ocean that exists today

Iceland

• An example of a divergent boundary is Iceland
• The island is being pulled apart
• This results in lava fountains, like the Krafla Volcano

African Rift Valley

• A 6,000-mile crack in the earth's crust, stretching from Lebanon to Mozambique
• Known as the African Rift Valley
• The force of the plates pulling apart causes huge chunks of crust to sink and molten rock to erupt in volcanic eruptions
• Volcanic activity is ongoing along the rift
• Numerous hot springs are present

The Red Sea

• The Red Sea (east coast of Africa) is spreading apart
• The land on either side is pulling away, and the Red Sea gets bigger each year

Convergent Boundaries

• Convergent boundaries exist where one plate is subducted underneath another plate
• Subduction occurs where one plate sinks underneath another plate

Ocean-Continent Collision

• Underneath the ocean water, there are a number of long, narrow, curving trenches
• Trenches are thousands of kilometers long and 8–10 km deep
• They cut into the ocean floor and are a result of subduction
• Trenches are the deepest parts of the ocean floor

Ocean-Continent Collision (South America)

• The Nazca plate is pushing into the South American plate and being subducted
• This causes the uplift of the South American plate, creating the Andes Mountains
• Strong, destructive earthquakes are common in this region.

Ocean-Ocean Collision

• When two oceanic plates collide, one plate sinks beneath the other
• This results in the formation of volcanoes
• Erupted lava and volcanic debris pile up on the ocean floor
• A volcano rises above sea level to form an island volcano
• Island volcanoes are typically in chains called island arcs

Continent-Continent Collision

• When two continental plates collide, neither is subducted
• This happens because the continental rocks are relatively light
• As the plates impact, the crust buckles and is pushed upward and sideways
• This forms mountains, such as the Himalayas, that are exceptionally high mountains

Himalayan Mountains

• The Himalayas were formed when the Indian and Eurasian plates collided 50 million years ago
• This collision caused the Eurasian plate to crumple up and override the Indian Plate
• The continuous convergence of the two plates over millions of years pushed up the Himalayas, and the Tibetan Plateau to their present heights
• The Himalayas are the highest continental mountains, reaching 8,854m above sea level

Transform Boundaries

• Most transform boundaries are found on the ocean floor
• A few occur on land, like the San Andreas fault in California
• Transform boundaries exist as a result of two plates sliding past each other

The San Andreas Fault

• Stretches about 1,300 km long and tens of kilometers wide
• Slices through two-thirds of California
• The Pacific Plate has been grinding horizontally past the North American Plate for 10 million years, at an average rate of about 5 cm/yr
• Land on the west side of the fault (on the Pacific Plate) is moving in a northwesterly direction compared to the land on the east side of the fault zone

Plate Boundaries and Earthquakes

• All types of plate boundaries can result in earthquakes