Explanation of concepts: Movement happens at plate boundaries

Types of plate boundaries

1. Divergent Plate Boundaries:
Almost all the Earth’s new crust forms at divergent boundaries, but most are not well
known because they lie deep beneath the oceans. These are zones where two plates move away from each other, allowing magma from the mantle to rise up and solidify as new crust.


[ ocean floor divergent boundary interactive graphic ]

Example of a land-based divergent boundary:


[ East Africa Rift Zone ]

2. Convergent Plate Boundaries:

The size of the Earth has not changed significantly during the past 600 million years, and very likely not since shortly after its formation 4.6 billion years ago. The Earth's
unchanging size implies that the crust must be destroyed at about the same rate as it is being created, as Harry Hess surmised. Such destruction (recycling) of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) under another. The location where sinking of a plate occurs is called a subduction zone.

The type of convergence -- called by some a very slow "collision" -- that takes place between plates depends on the kind of lithosphere involved. Convergence can occur
between an oceanic and a largely continental plate, or between two largely oceanic plates, or between two largely continental plates.

Subduction


[ interactive subduction animation ]

a. Continental vs. Oceanic Plate Convergence
Oceanic-Continental place convergence occurs when an oceanic plate moves under (subducts) a continental plate. Oceanic-continental convergence creates many of the Earth's active volcanoes, such as those in the Andes and the Cascade Range in the Pacific Northwest.


[ Example of subduction from the Cascades & Juan de Fuca plate ]

[ Example of subduction from New Zealand ]

b. Oceanic vs. Oceanic Plate Convergence


[ ocean crust convergence animation ]

As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed. Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. Magmas that form island arcs are produced by the partial melting of the descending plate and/or theoverlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.


[ aerial photograph of Aleutian Islands ]

[ Aleutian Islands map ]

[ More good images from the Alaska Volcano Observatory ]


c. Continental vs. Continental Plate Convergence

When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused 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. Most of this growth occurred during the past 10 million years. The Himalayas, towering as high as 8,854 m above sea level, form the highest continental mountains in the world. Moreover, the neighboring Tibetan Plateau, at an average elevation of about 4,600 m, is higher than all the peaks in the Alps except for Mont Blanc and Monte Rosa, and is well above the summits of most mountains in the United States.


[ India subduction interactive animation ]

[ Example of Himalayas from space ]

50 years ago, Hillary and Tenzing reached the top of Mount Everest, the first people to accomplish that feat. Since then, the peak has risen 7 feet due to plate convergence.

3. Transform Plate Boundaries:
The zone between two plates sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary. The concept of transform faults originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults or fracture zones connect two spreading centers (divergent plate boundaries) or, less commonly, trenches (convergent plate boundaries). Most transform faults are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes. However, a few occur on land, for example the San Andreas fault zone in California. The San Andreas is one of the few transform faults exposed on land. The San Andreas fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. Along it, 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 zone (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).


[ Transform Plates of the San Andreas Fault ]

4. Types of faults:


[ The four different fault lines interactive animation ]

5. Which direction are the plates moving?

The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimeters per year (cm/yr), or 25 km in a million years. 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. The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about
3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).

[ Predicted Future Location of Plates ]

Plate tectonic maps and Continental drift animations by C. R. Scotese, PALEOMAP Project (www.scotese.com)

[ Plate Directional Movement ]

[ Plate Boundary Interactions ]