Scientists now have a fairly good understanding of how the plates move and how such
movements relate to earthquake activity. Most movement occurs along narrow zones
between plates where the results of plate-tectonic forces are most evident.
There are four types of plate boundaries:
Divergent boundaries -- where new crust is generated as the plates pull away
from each other.
Convergent boundaries -- where crust is destroyed as one plate dives under
another.
Transform boundaries -- where crust is neither produced nor destroyed as the
plates slide horizontally past each other.
Plate boundary zones -- broad belts in which boundaries are not well defined
and the effects of plate interaction are unclear.
Illustration of the Main Types of Plate Boundaries [55 k]
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. Picture two giant
conveyor belts, facing each other but slowly moving in opposite directions as they
transport newly formed oceanic crust away from the ridge crest.
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge. This
submerged mountain range, which extends from the Arctic Ocean to beyond the
southern tip of Africa, is but one segment of the global mid-ocean ridge system that
encircles the Earth. 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. This rate may
seem slow by human standards, but because this process has been going on for
millions of years, it has resulted in plate movement of thousands of kilometers.
Seafloor spreading over the past 100 to 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.
Mid-Atlantic Ridge [26 k]
The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers
scientists a natural laboratory for studying on land the processes also occurring along
the submerged parts of a spreading ridge. Iceland is splitting along the spreading
center between the North American and Eurasian Plates, as North America moves
westward relative to Eurasia.
Map showing the Mid-Atlantic Ridge splitting Iceland and separating the North
American and Eurasian Plates. The map also shows Reykjavik, the capital of Iceland,
the Thingvellir area, and the locations of some of Iceland's active 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 widened and new ones
appear every few months. From 1975 to 1984, numerous episodes of rifting (surface
cracking) took place along the Krafla fissure zone. Some of these rifting events were
accompanied by volcanic activity; the ground would gradually rise 1-2 m before
abruptly dropping, signalling an impending eruption. Between 1975 and 1984, the
displacements caused by rifting totalled about 7 m.
Lava Fountains, Krafla Volcano [35 k]
Thingvellir Fissure Zone, Iceland [80 k]
In East Africa, spreading processes have already torn Saudi Arabia away from the rest
of the African continent, forming the Red Sea. The actively splitting African Plate and
the Arabian Plate meet in what geologists call a triple junction, where the Red Sea
meets the Gulf of Aden. A new spreading center may be developing under Africa
along the East African Rift Zone. When the continental crust stretches beyond its
limits, tension cracks begin to appear on the Earth's surface. Magma rises and
squeezes through the widening cracks, sometimes to erupt and form volcanoes. The
rising magma, whether or not it erupts, puts more pressure on the crust to produce
additional fractures and, ultimately, the rift zone.
Historically Active Volcanoes, East Africa [38 k]
East Africa may be the site of the Earth's next major ocean. Plate interactions in the
region provide scientists an opportunity to study first hand how the Atlantic may have
begun to form about 200 million years ago. Geologists believe that, if spreading
continues, the three plates that meet at the edge of the present-day African continent
will separate completely, allowing the Indian Ocean to flood the area and making the
easternmost corner of Africa (the Horn of Africa) a large island.
Summit Crater of 'Erta 'Ale [55 k]
Oldoinyo Lengai, East African Rift Zone [38 k]
Convergent 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.
Oceanic-continental convergence
If by magic we could pull a plug and drain the Pacific Ocean, we would see a most
amazing sight -- 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 along the Peru-Chile trench, the oceanic Nazca Plate
is pushing into and being subducted under the continental part of the South American
Plate. In turn, the overriding South American Plate is being lifted up, creating the
towering Andes mountains, the backbone of the continent. Strong, destructive
earthquakes and the rapid uplift of mountain ranges are common in this region. Even
though the Nazca Plate as a whole is sinking smoothly and continuously into the
trench, the deepest part of the subducting plate breaks into smaller pieces that become
locked in place for long periods of time before suddenly moving to generate large
earthquakes. Such earthquakes are often accompanied by uplift of the land by as much
as a few meters.
Convergence of the Nazca and South American Plates [65 k]
On 9 June 1994, a magnitude-8.3 earthquake struck about 320 km northeast of La Paz,
Bolivia, at a depth of 636 km. This earthquake, within the subduction zone between
the Nazca Plate and the South American Plate, was one of deepest and largest
subduction earthquakes recorded in South America. Fortunately, even though this
powerful earthquake was felt as far away as Minnesota and Toronto, 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 active volcanoes,
such as those in the Andes and the Cascade Range in the Pacific Northwest. The
eruptive activity is clearly associated with subduction, but scientists vigorously debate
the possible sources of magma: Is magma generated by the partial melting of the
subducted oceanic slab, or the overlying continental 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 is formed. The
Marianas Trench (paralleling the Mariana Islands), for example, marks where the fast-
moving Pacific Plate converges against the slower moving Philippine Plate. The
Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the
Earth's interior (nearly 11,000 m) than Mount Everest, the world's tallest mountain,
rises above sea level (about 8,854 m).
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 the overlying oceanic lithosphere. The
descending plate also provides a source of stress as the two plates interact, leading to
frequent moderate to strong earthquakes.
Continental-continental convergence
The Himalayan mountain range dramatically demonstrates one of the most visible and
spectacular consequences of plate tectonics. 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 Indian and Eurasian Plates to crumple up along the collision zone. After
the collision, the slow continuous convergence of these 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.
Above: The collision between the Indian and Eurasian plates has pushed up the
Himalayas and the Tibetan Plateau. Below: Cartoon cross sections showing the
meeting of these two plates before and after their collision. The reference points
(small squares) show the amount of uplift of an imaginary point in the Earth's crust
during this mountain-building process.
| The Himalayas: Two Continents Collide |
Transform 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. This transform fault connects the East Pacific Rise, a
divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer
Ridge, another divergent boundary to the north.
The Blanco, Mendocino, Murray, and Molokai fracture zones are some of the many
fracture zones (transform faults) that scar the ocean floor and offset ridges (see text).
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).
San Andreas fault [52 k]
Oceanic fracture zones are ocean-floor valleys that horizontally offset spreading
ridges; some of these zones are hundreds to thousands of kilometers long and as much
as 8 km deep. Examples of these large scars include the Clarion, Molokai, and Pioneer
fracture zones in the Northeast Pacific off the coast of California and Mexico. These
zones are presently inactive, but the offsets of the patterns of magnetic striping
provide evidence of their 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-movement deformation
occurring there extends over a broad belt (called a plate-boundary zone). One of these
zones marks the Mediterranean-Alpine region between the Eurasian and African
Plates, within which several smaller fragments of plates (microplates) have been
recognized. Because plate-boundary zones involve at least two large plates and one or
more microplates caught up between them, they tend to have complicated geological
structures and earthquake patterns.
Rates of motion
We can measure how fast tectonic plates are moving today, but how do scientists
know what the rates of plate movement have been over geologic time? The oceans
hold one of the key pieces to the puzzle. Because the ocean-floor magnetic striping
records the flip-flops in the Earth's magnetic field, scientists, knowing the
approximate duration of the reversal, can calculate the average rate of plate movement
during a given time span. These average rates of plate separations can range widely.
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).
Easter Island monolith [80 k]
Evidence of past rates of plate movement also can be obtained from geologic mapping
studies. If a rock formation of known age -- with distinctive composition, structure, or
fossils -- mapped on one side of a plate boundary can be matched with the same
formation on the other side of the boundary, then measuring the distance that the
formation has been offset can give an estimate of the average rate of plate motion.
This simple but effective technique has been used to determine the rates of plate
motion at divergent boundaries, for example the Mid-Atlantic Ridge, and transform
boundaries, such as the San Andreas Fault.
GPS Satellite and Ground Receiver [63 k]
Current plate movement can be tracked directly by means of ground-based or space-
based geodetic measurements; geodesy is the science of the size and shape of the
Earth. Ground-based measurements are taken with conventional but very precise
ground-surveying techniques, using laser-electronic instruments. However, because
plate motions are global in scale, they are best measured by satellite-based methods.
The late 1970s witnessed the rapid growth of space geodesy, a term applied to space-
based techniques for taking precise, repeated measurements of carefully chosen points
on the Earth's surface separated by hundreds to thousands of kilometers. The three
most commonly used space-geodetic techniques -- very long baseline interferometry
(VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS) -- are
based on technologies developed for military and aerospace research, notably radio
astronomy and satellite tracking.
Among the three techniques, to date the GPS has been the most useful for studying the
Earth's crustal movements. Twenty-one satellites are currently in orbit 20,000 km
above the Earth as part of the NavStar system of the U.S. Department of Defense.
These satellites continuously transmit radio signals back to Earth. To determine its
precise position on Earth (longitude, latitude, elevation), each GPS ground site must
simultaneously receive signals from at least four satellites, recording the exact time
and location of each satellite when its signal was received. By repeatedly measuring
distances between specific points, geologists can determine if there has been active
movement along faults or between plates. The separations between GPS sites are
already being measured regularly around the Pacific basin. By monitoring the
interaction between the Pacific Plate and the surrounding, largely continental plates,
scientists hope to learn more about the events building up to earthquakes and volcanic
eruptions in the circum-Pacific Ring of Fire. Space-geodetic data have already
confirmed that the rates and direction of plate movement, averaged over several years,
compare well with rates and direction of plate movement averaged over millions of
years.
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