Plate Boundaries PDF

# Types of Plate Boundaries ## Divergent Boundaries Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. Imagine two giant conveyor belts, facing each other but slowly moving in opposite directions as they tr...

# Types of Plate Boundaries ## Divergent Boundaries Divergent boundaries occur along spreading centers where plates are moving apart and new crust is created by magma pushing up from the mantle. Imagine two giant conveyor belts, facing each other but slowly moving in opposite directions as they transport newly formed crust away from the ridge crest. ### Mid-Atlantic Ridge - The rate of spreading along the Mid-Atlantic Ridge averages about 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 between the continents of Europe, Africa, and the Americas into the vast ocean that exists today. - An example of divergent boundaries is Iceland where the island of Iceland is actually being pulled apart, and the result are lava fountains, like Krafla Volcano. ### African Rift Valley - A 6,000-mile crack in the Earth's crust, stretching from Lebanon to Mozambique is known as the African Rift Valley. - The force of the plates pulling apart caused huge chunks of the crust to sink and force up molten rock in volcanic eruptions. The process, called rifting, is still in progress with many active and semi-active volcanoes located along the African Rift Valley. Further evidence of volcanic activity along the rift is provided by the presence of numerous boiling hot springs. ### The Red Sea - The Red Sea (just off the east coast of Africa) is spreading. - The land to either side is actually pulling away. - The Red Sea continues to get bigger each year. ## Convergent Boundaries Convergent boundaries exist where one plate is subducted underneath another plate. **Subduction - one plate sinks underneath another plate** Where convergent plates exist, the Earth's crust is being destroyed as it slowly moves underneath another section of crust and melts into the mantle. ### Ocean-Continent Collision - Underneath the ocean water there are a number of long, narrow, curving trenches thousands of kilometers long and 8 to 10 km deep cutting into the ocean floor. - Trenches are the deepest parts of the ocean floor and are created by subduction. - Off the coast of South America, the oceanic Nazca Plate is pushing into and being subducted under the continental part of the South American Plate. - At the same time, the South American Plate is being lifted up, creating the Andes Mountains. - Strong, destructive earthquakes and the rapid uplift of mountain ranges are common in this region. Such earthquakes have been known to jolt the land up several meters. ### Ocean-Ocean Collision - When two oceanic plates collide, it can result in the formation of volcanoes. One oceanic plate sinks beneath the other, and over millions of years, the erupted lava and volcanic debris pile up on the ocean floor. - Finally, a volcano rises above sea level to form an island volcano. - Such volcanoes are typically strung out in chains called **island arcs**. - This is how the Aleutian Islands have formed, and why they experience numerous strong earthquakes. ### Continent-Continent Collision - The Himalayan Mountains were created when two continental plates met head-on, and neither was subducted. - Continental rocks are relatively light and, like two colliding icebergs, resist downward motion. - Instead, the crust buckled and was pushed upwards and sideways. - The result: Mountains! - India collided into Asia 50 million years ago, causing the Eurasian Plate to crumple up and override the Indian Plate. - After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights. - The Himalayas, towering as high as 8,854 m above sea level, are the highest continental mountains in the world. ## Transform Boundaries - Most transform boundaries are found on the ocean floor. - A few occur on land, an example is the San Andreas fault of California. - Transform boundaries are the result of two plates sliding past each other. ### The San Andreas Fault - Stretches about 1,300 km long and in some places tens of kilometers wide. - Slices through two thirds of the length 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 relative to the land on the east side of the fault zone (on the North American Plate). # Earthquakes! Remember all boundaries can cause earthquakes! Why? - **Divergent:** underwater sea-floor spreading leading to volcanic mountains like the Mid-Atlantic Ridge. On land volcanoes like Krafla (Iceland) and large separations of land usually with mountains, like in Africa. - **Convergent:** Ocean-Ocean, Ocean-Continent, Continent-Continent collision. ## Plate Boundaries - Milky Way Activity! 1. **Transform:** Carefully unwrap your Milky Way candy bars. With someone at your table, attach the two pieces of candy so that one side touches. Slowly slide the two “plates” (candy) past each other and watch what happens. What kind of plate boundary are you demonstrating? Draw it next to the term on your web! 2. **Convergent:** Now split the next two plate boundaries so that you only demonstrate one more. Your candy can only withstand one more demonstration before this gets too messy! Watch as two people at your table... Start with the candy pieces touching. Slowly apply compression force (push them together). What forms? What kind of plate boundary is this? Draw it next to the term on your web! 3. **Divergent:** So you probably know what is next... Have another person carefully bite off one sliver from a side of the chocolate on each piece of candy (don't do anything to the Starburst). Attach the two "exposed" sides. Carefully pull them apart. What is exposed from the candy? What kind of plate boundary is this? Draw it! ## Think.... - What kind of boundary is at the uplift zone? **Convergent** - What kind of boundary is at the spreading center? **Divergent** - What kind of boundary is at the subduction zone? **Convergent** The document included 35 slides with images of global maps showing the movement of tectonic plates throughout history. The images show various stages of plate movement starting with the supercontinent Pangaea and culminating in future projections of continental drift over the next 250 million years.