Understanding plate motions [This Dynamic Earth, USGS] (2024)

Scientists now have a fairly good understanding of how the plates moveand how such movements relate to earthquake activity. Most movement occursalong narrow zones between plates where the results of plate-tectonic forcesare most evident.

There are four types of plate boundaries:

• Divergent boundaries -- where new crust is generated as the platespull away from each other.
• Convergent boundaries -- where crust is destroyed as one plate divesunder another.
• Transform boundaries -- where crust is neither produced nor destroyedas the plates slide horizontally past each other.
• Plate boundary zones -- broad belts in which boundaries are not welldefined and the effects of plate interaction are unclear.

Illustration of the Main Types of PlateBoundaries [55 k]

Divergent boundaries

Divergent boundaries occur along spreading centers where plates are movingapart and new crust is created by magma pushing up from the mantle. Picturetwo giant conveyor belts, facing each other but slowly moving in oppositedirections as they transport newly formed oceanic crust away from the ridgecrest.

Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge.This submerged mountain range, which extends from the Arctic Ocean to beyondthe southern tip of Africa, is but one segment of the global mid-ocean ridgesystem that encircles the Earth. The rate of spreading along the Mid-AtlanticRidge averages about 2.5 centimeters per year (cm/yr), or 25 km in a millionyears. This rate may seem slow by human standards, but because this processhas been going on for millions of years, it has resulted in plate movementof thousands of kilometers. Seafloor spreading over the past 100 to 200million years has caused the Atlantic Ocean to grow from a tiny inlet ofwater between the continents of Europe, Africa, and the Americas into thevast ocean that exists today.

Mid-AtlanticRidge [26 k]

The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge,offers scientists a natural laboratory for studying on land the processesalso occurring along the submerged parts of a spreading ridge. Iceland issplitting along the spreading center between the North American and EurasianPlates, as North America moves westward relative to Eurasia.

Map showing the Mid-Atlantic Ridge splitting Iceland and separating theNorth American and Eurasian Plates. The map also shows Reykjavik, the capitalof Iceland, the Thingvellir area, and the locations of some of Iceland'sactive volcanoes (red triangles), including Krafla.

The consequences of plate movement are easy to see around Krafla Volcano,in the northeastern part of Iceland. Here, existing ground cracks have widenedand new ones appear every few months. From 1975 to 1984, numerous episodesof rifting (surface cracking) took place along the Krafla fissurezone. Some of these rifting events were accompanied by volcanic activity;the ground would gradually rise 1-2 m before abruptly dropping, signallingan impending eruption. Between 1975 and 1984, the displacements caused byrifting totalled about 7 m.

LavaFountains, Krafla Volcano [35 k]

ThingvellirFissure Zone, Iceland [80 k]

In East Africa, spreading processes have already torn Saudi Arabia awayfrom the rest of the African continent, forming the Red Sea. The activelysplitting African Plate and the Arabian Plate meet in what geologists calla triple junction, where the Red Sea meets the Gulf of Aden. A newspreading center may be developing under Africa along the East African RiftZone. When the continental crust stretches beyond its limits, tension cracksbegin to appear on the Earth's surface. Magma rises and squeezes throughthe widening cracks, sometimes to erupt and form volcanoes. The rising magma,whether or not it erupts, puts more pressure on the crust to produce additionalfractures and, ultimately, the rift zone.

HistoricallyActive Volcanoes, East Africa [38 k]

East Africa may be the site of the Earth's next major ocean. Plate interactionsin the region provide scientists an opportunity to study first hand howthe Atlantic may have begun to form about 200 million years ago. Geologistsbelieve that, if spreading continues, the three plates that meet at theedge of the present-day African continent will separate completely, allowingthe Indian Ocean to flood the area and making the easternmost corner ofAfrica (the Horn of Africa) a large island.

Summit Crater of 'Erta'Ale [55 k]

OldoinyoLengai, East African Rift Zone [38 k]

Convergent boundaries

The size of the Earth has not changed significantly during the past 600million years, and very likely not since shortly after its formation 4.6billion years ago. The Earth's unchanging size implies that the crust mustbe destroyed at about the same rate as it is being created, as Harry Hesssurmised. Such destruction (recycling) of crust takes place along convergentboundaries where plates are moving toward each other, and sometimes oneplate sinks (is subducted) under another. The location where sinkingof 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 continentalplates.

Oceanic-continental convergence

If by magic we could pull a plug and drain the Pacific Ocean, we would seea most amazing sight -- a number of long narrow, curving trenchesthousands of kilometers long and 8 to 10 km deep cutting into the oceanfloor. Trenches are the deepest parts of the ocean floor and are createdby subduction.

Off the coast of South America along the Peru-Chile trench, the oceanicNazca Plate is pushing into and being subducted under the continental partof the South American Plate. In turn, the overriding South American Plateis being lifted up, creating the towering Andes mountains, the backboneof the continent. Strong, destructive earthquakes and the rapid uplift ofmountain ranges are common in this region. Even though the Nazca Plate asa whole is sinking smoothly and continuously into the trench, the deepestpart of the subducting plate breaks into smaller pieces that become lockedin place for long periods of time before suddenly moving to generate largeearthquakes. Such earthquakes are often accompanied by uplift of the landby as much as a few meters.

Convergence ofthe Nazca and South American Plates [65 k]

On 9 June 1994, a magnitude-8.3 earthquake struck about 320 km northeastof La Paz, Bolivia, at a depth of 636 km. This earthquake, within the subductionzone between the Nazca Plate and the South American Plate, was one of deepestand largest subduction earthquakes recorded in South America. Fortunately,even though this powerful earthquake was felt as far away as Minnesota andToronto, Canada, it caused no major damage because of its great depth.

Ring of Fire [76 k]

Oceanic-continental convergence also sustains many of the Earth's activevolcanoes, such as those in the Andes and the Cascade Range in the PacificNorthwest. The eruptive activity is clearly associated with subduction,but scientists vigorously debate the possible sources of magma: Is magmagenerated by the partial melting of the subducted oceanic slab, or the overlyingcontinental lithosphere, or both?

Oceanic-oceanic convergence

As with oceanic-continental convergence, when two oceanic plates converge,one is usually subducted under the other, and in the process a trench isformed. The Marianas Trench (paralleling the Mariana Islands), for example,marks where the fast-moving Pacific Plate converges against the slower movingPhilippine Plate. The Challenger Deep, at the southern end of the MarianasTrench, plunges deeper into the Earth's interior (nearly 11,000 m) thanMount Everest, the world's tallest mountain, rises above sea level (about8,854 m).

Subduction processes in oceanic-oceanic plate convergence also resultin the formation of volcanoes. Over millions of years, the erupted lavaand volcanic debris pile up on the ocean floor until a submarine volcanorises above sea level to form an island volcano. Such volcanoes are typicallystrung out in chains called island arcs. As the name implies, volcanicisland arcs, which closely parallel the trenches, are generally curved.The trenches are the key to understanding how island arcs such as the Marianasand the Aleutian Islands have formed and why they experience numerous strongearthquakes. Magmas that form island arcs are produced by the partial meltingof the descending plate and/or the overlying oceanic lithosphere. The descendingplate also provides a source of stress as the two plates interact, leadingto frequent moderate to strong earthquakes.

Continental-continental convergence

The Himalayan mountain range dramatically demonstrates one of the most visibleand spectacular consequences of plate tectonics. When two continents meethead-on, neither is subducted because the continental rocks are relativelylight and, like two colliding icebergs, resist downward motion. Instead,the crust tends to buckle and be pushed upward or sideways. The collisionof India into Asia 50 million years ago caused the Indian and Eurasian Plates to crumple up along the collision zone. After the collision, the slow continuousconvergence of these two plates over millions of years pushed up the Himalayasand the Tibetan Plateau to their present heights. Most of this growth occurredduring the past 10 million years. The Himalayas, towering as high as 8,854m above sea level, form the highest continental mountains in the world.Moreover, the neighboring Tibetan Plateau, at an average elevation of about4,600 m, is higher than all the peaks in the Alps except for Mont Blancand Monte Rosa, and is well above the summits of most mountains in the UnitedStates.

Above: The collision between the Indian and Eurasian plateshas pushed up the Himalayas and the Tibetan Plateau. Below: Cartoon crosssections showing the meeting of these two plates before and after theircollision. The reference points (small squares) show the amount of upliftof an imaginary point in the Earth's crust during this mountain-buildingprocess.

| The Himalayas: Two Continents Collide |

Transform boundaries

The zone between two plates sliding horizontally past one another is calleda transform-fault boundary, or simply a transform boundary. Theconcept of transform faults originated with Canadian geophysicist J. TuzoWilson, who proposed that these large faults or fracture zones connecttwo spreading centers (divergent plate boundaries) or, less commonly, trenches(convergent plate boundaries). Most transform faults are found on the oceanfloor. They commonly offset the active spreading ridges, producing zig-zagplate margins, and are generally defined by shallow earthquakes. However,a few occur on land, for example the San Andreas fault zone in California.This transform fault connects the East Pacific Rise, a divergent boundaryto the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, anotherdivergent boundary to the north.

The Blanco, Mendocino, Murray, and Molokai fracture zonesare some of the many fracture zones (transform faults) that scar the oceanfloor and offset ridges (see text). The San Andreas is one of the few transformfaults exposed on land.

The San Andreas fault zone, which is about 1,300 km long and in places tensof kilometers wide, slices through two thirds of the length of California.Along it, the Pacific Plate has been grinding horizontally past the NorthAmerican 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 movingin a northwesterly direction relative to the land on the east side of thefault zone (on the North American Plate).

SanAndreas fault [52 k]

Oceanic fracture zones are ocean-floor valleys that horizontally offsetspreading ridges; some of these zones are hundreds to thousands of kilometerslong and as much as 8 km deep. Examples of these large scars include theClarion, Molokai, and Pioneer fracture zones in the Northeast Pacific offthe coast of California and Mexico. These zones are presently inactive,but the offsets of the patterns of magnetic striping provide evidence oftheir previous transform-fault activity.

Plate-boundary zones

Not all plate boundaries are as simple as the main types discussed above.In some regions, the boundaries are not well defined because the plate-movementdeformation occurring there extends over a broad belt (called a plate-boundaryzone). One of these zones marks the Mediterranean-Alpine region betweenthe Eurasian and African Plates, within which several smaller fragmentsof plates (microplates) have been recognized. Because plate-boundaryzones involve at least two large plates and one or more microplates caughtup between them, they tend to have complicated geological structures andearthquake patterns.

Rates of motion

We can measure how fast tectonic plates are moving today, but how do scientistsknow what the rates of plate movement have been over geologic time? Theoceans hold one of the key pieces to the puzzle. Because the ocean-floormagnetic striping records the flip-flops in the Earth's magnetic field,scientists, knowing the approximate duration of the reversal, can calculatethe average rate of plate movement during a given time span. These averagerates of plate separations can range widely. The Arctic Ridge has the slowestrate (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).

EasterIsland monolith [80 k]

Evidence of past rates of plate movement also can be obtained from geologicmapping studies. If a rock formation of known age -- with distinctive composition,structure, or fossils -- mapped on one side of a plate boundary can be matchedwith the same formation on the other side of the boundary, then measuringthe distance that the formation has been offset can give an estimate ofthe average rate of plate motion. This simple but effective technique hasbeen used to determine the rates of plate motion at divergent boundaries,for example the Mid-Atlantic Ridge, and transform boundaries, such as theSan Andreas Fault.

GPSSatellite and Ground Receiver [63 k]

Current plate movement can be tracked directly by means of ground-basedor space-based geodetic measurements; geodesy is the scienceof the size and shape of the Earth. Ground-based measurements are takenwith conventional but very precise ground-surveying techniques, using laser-electronicinstruments. However, because plate motions are global in scale, they arebest measured by satellite-based methods. The late 1970s witnessed the rapidgrowth of space geodesy, a term applied to space-based techniquesfor taking precise, repeated measurements of carefully chosen points onthe Earth's surface separated by hundreds to thousands of kilometers. Thethree most commonly used space-geodetic techniques -- very long baselineinterferometry (VLBI), satellite laser ranging (SLR), and the Global PositioningSystem (GPS) -- are based on technologies developed for military and aerospaceresearch, notably radio astronomy and satellite tracking.

Among the three techniques, to date the GPS has been the most useful forstudying the Earth's crustal movements. Twenty-one satellites are currentlyin orbit 20,000 km above the Earth as part of the NavStar system of theU.S. Department of Defense. These satellites continuously transmit radiosignals back to Earth. To determine its precise position on Earth (longitude,latitude, elevation), each GPS ground site must simultaneously receive signalsfrom at least four satellites, recording the exact time and location ofeach satellite when its signal was received. By repeatedly measuring distancesbetween specific points, geologists can determine if there has been activemovement along faults or between plates. The separations between GPS sitesare already being measured regularly around the Pacific basin. By monitoringthe interaction between the Pacific Plate and the surrounding, largely continentalplates, scientists hope to learn more about the events building up to earthquakesand volcanic eruptions in the circum-Pacific Ring of Fire. Space-geodeticdata have already confirmed that the rates and direction of plate movement,averaged over several years, compare well with rates and direction of platemovement averaged over millions of years.

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Last updated: 09.15.14
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