A refinement of Alfred Wegener's continental drift theory.

The Earth's outermost layers are the crust (where we live) and the upper mantle. The crust is 35-100km thick on land, but only 5-10km thick in the oceans. Together the crust and upper mantle form the lithosphere.

Below lies the molten asthenosphere. Slow convection currents in the asthenosphere create volcanic action in the deep fissures of the sea. New material is forced into the lithosphere by this action, and the lithosphere is forced outward.

The lithosphere is broken into around a dozen large and small sections called plates. Differences in the composition and density of the rock in the crust makes the continental plates more buoyant than the sea plates. (Sea plates are composed of moon-rock-like basalts.) Pressure on the plates at the boundaries results in them moving about. They may drift apart (extenional), where new material is created at mid-ocean ridges by an upwelling of lava from below; collide (compressional), where subduction forces crust back into the magma layer ocean trenches, or grind alongside each other (transform). The San Andreas fault is a transform type of fault where the Pacific plate grinds against the North American plate.

Plates move constantly but travel only a few centimeters every year.

Major plates: The African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific, and South American plates.

Smaller plates: the Anatolian, Arabian, Carribean, Cocos, Juan de Fuca, Philippine, and Somali plates.

Plate names from http://www.solarviews.com/eng/earthint.htm#plate

Evidence for plate tectonics (which replaces the old theory that mountains and suchlike were due to shrinkage as the Earth cooled):


  1. Jigsaw fit - The obvious fit between Africa and South America and other landmasses that can be fitted together.
  2. Matching fossils - Identical plant fossils of the same age found in rocks in South Africa, Australia, Antartica, India and South America, suggesting they were joined at one point. Identical crocodile fossils have been found in Brazil and South Africa.
  3. Living creatures - The exact same species of earthworm is found on the tip of South America and the tip of South Africa. They travelled over on the big raft we now call America.
  4. Identical Rock Sequences - Rock strata of similar ages found in various countries show remarkable similarity.
  5. Magnetic Stripes - Magnetic stripes on the ocean floor have a symmetrical pattern of stripes, evidence that the two sides are speeding away from each other.

History of plate tectonic theory

In 1912, Alfred Wegener proposed that the continents as they are now used to form a single landmass or supercontinent, which he called Pangaea, to explain the close match of shapes on continental boundaries. He postulated that they were surrounded by a single global ocean, but that Pangea broke apart and the remnants "drifted" to their current locations.

Whilst evidence seemed to support this, Wegener couldn’t produce an acceptable theory to explain the movement of continents. In 1930 Arthur Holmes suggested a mechanism to explain continental drift. Currents of heat and thermal expansion within the Earth's mantle, he propounded, could force the continents toward or away from one another, creating new ocean floor and building mountain ranges carrying the continental fragments on larger sections of the earth’s crust.

Data to back up this theory came with the discovery of the Great Global Rift. A German expedition in 1925, using deep sounding techniques, had determined that a continuous mountain range ran along the middle of the Atlantic Ocean, where it surfaced at Iceland, continued around Africa, through the Indian Ocean, between Australia and Antartica, and north through the Pacific Ocean. This range was called the Mid Ocean Ridge.

In 1953, Americans Maurice Ewing and Bruce Heezen discovered that a deep canyon ran through the Mid Ocean Ridge occasionally coming very close to land. The rift appeared to be breaks in the earth's crust, but perfectly fitted breaks, as if they were joints made by a carpenter. This Great Global Rift outlined sections of the earth's crust, which Ewing and Heezen called tectonic plates (tectonic being derived from the Greek for carpenter.) They found six major plates ( African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific, and South American) and seven smaller ones ( Anatolian, Arabian, Carribean, Cocos, Juan de Fuca, Philippine, and Somali). They discovered also that the majority of the world’s volcanic and earthquake activity occurred at the points that plates met – the Pacific plate alone accounting for 80 percent of the earth’s earthquake energy.

The work of Harry Hess and others in the 1960s led to the refining of plate tectonics theory to its current state. As it stands, there are still gaps, but it’s the most complete explanation of global dynamics available.

The Theory

Background

The Earth divides into three chemical layers: core, mantle and crust. The core consists primarily of iron and nickel and although it has been cooling for 4.5 billion years, it’s still extremely hot. It is subdivided into a solid inner core and a liquid outer core. The middle layer of the planet, the mantle, comprises minerals rich in iron, magnesium, silicon, and oxygen. The crust (subdivided into Oceanic and Continental) is made up of rock, rich in the oxygen and silicon with smaller amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. Oceanic crust is made of basalt (the most common rock on earth), while Continental crust consists of lower density materials like granite.

Plate tectonics is concerned with the mantle and the crust.

The outermost part of the mantle and the crust form the rigid outer layer of the earth - the lithosphere - which is the ‘plate’ of tectonic theory. Beneath this is the lower mantle - the asthenosphere – which, although solid, flows (solids can flow – look at toothpaste) . It’s suggested that this flow may be caused by mantle convection which pushes the plate in the same way that hot air rises to be deflected by ceilings, alternatively gravity may exert a stronger force on the older, colder ocean floor pulling it towards the core, with more force than newer lighter material.

Whatever causes the flow, there are four types of boundaries where plate tectonic activity occurs: divergent boundaries, where new crust is formed; convergent boundaries, where crust is consumed; collisional boundaries, where land masses collide; and transform boundaries, where plates slide against each other.

Divergent Boundaries

New ocean floor is continuously created along the Great Global Rift as heated magma rises from the Earth’s core, cooling on contact with the sea.

The speed of this floor creation varies along the ridge. Between North America and Europe, the rate is about 3.6 cm per year. At the East Pacific rise, it’s 12.6 inches 32.2 cm annually.

The newly created floor pushes the plates outward as it forms – a process called sea floor spreading.

Convergent Boundaries

Despite the seafloor spreading described above, Earth retains a constant size – one of the reasons is subduction at convergent boundaries

At these boundaries parts of the crust slide beneath other parts. This invariably occurs where an ocean floor meets a land mass at a boundary, since the land mass is more buoyant, but also happens where two ocean floors meet. Deep trenches are formed at these boundaries, as oceanic plates bend downward toward the core.

At a depth between 300 and 700 kilometers, the rock of the subducting plate becomes molten and some of this molten material rises back to the surface as volcanic activity, though most becomes part of the asthenosphere.

Collisional Boundaries

When two land masses meet neither is able to slide under the other. Instead, they crush together, thrusting material upwards along collisional boundaries, forming mountain ranges. As boundaries move over time new ranges arise, and older ones begin slowly to erode – The Himalayas are still slowly getting higher and some ranges in New Zealand are growing by double-figure centimeters every year.

Transform Boundaries

Transform boundaries don’t create or absorb crust. Instead, the two plates rub against each other, generating tension which is released suddenly and often violently in a forward jerk as they slide past each other – this release leads to earthquakes – transform boundaries are often called fault lines.

The San Andreas Fault is the best known – here the Pacific plate to the west of the fault is moving northwest, while the North American Plate on the east is moving southeast.

This movement is very slow, but undeniable – eventually Los Angeles, on the Pacific plate will be north of San Francisco, on the North American. But don’t hang around waiting – it’s going to take 16 million years.

I thought a list of all the currently active ("neotectonic") plates might be useful; this turned out to be less of a wild goose chase than my previous geographical list-hunting exercises.

Ever since geologists found a mechanism to make Alfred Wegener's theory of continental drift work, they have been searching to identify and define the plates themselves.

We should start by defining the term "tectonic plate": A unitary section of the Earth's lithosphere which is in motion independently from the rest of the lithosphere. Considering a plate as a rigid body is something of a simplification, since plates can undergo continuous deformation as they slide around and collide, but it's a good one for identifying the things.

Even with a definition in hand, the task can be daunting. Geology is as competitive an academic field as any other, and controversies are likely to arise in plate identifications. In addition, the Earth's surface is constantly moving. Although plate motions are slow in relation to a human lifespan, problems arise because there are countless crustal blocks which may have been independent plates in the past, and are now fused to another plate (lowering them to the status of a "terrane"), as well as blocks which are likely to separate from a plate in the future. It is tempting to label these block as plates now. There are other esoteric problems, such as telling an overthrust zone from a subduction zone. At some point, one simply has to stop analyzing.

Plates are usually identified by determining their boundaries (or "margin"s), but often, plates can be distinguished by spotting their independent motions. Boundaries and motions are collected from field and remote sensing observations:

  • Prominent geomorphic features such as surface faults, and the Mid-Ocean Ridge.
  • The locations of events such as earthquakes and volcanic eruptions.
  • Satellite tracking of ground stations to determine their movements relative to one another.
  • Magnetic anomalies in rocks, which reveal the positions of the continents in the past, allowing us to determine their relative movement.

An important characteristic of each plate is its Euler pole, which is the fixed point on the Earth's surface (usually relative to the Pacific Plate) that the plate's motion can be characterized as a rotation around. The Euler Pole is determined by combining localized movement observations using moment tensor inversions.

As of this writing, the most comprehensive model of the Earth's lithosphere appears to be the one constructed by Peter Bird of UCLA2. In this model, called "PB2002", Bird lists 52 plates, from the familiar large ones to some small and very bizarre ones. Bird describes the limitations of his model, going so far as to stop attempting the identifcation of new plates in complex areas he labels as "orogens". Some of his plate identifications are controversial. Most of the new plates appear in a complex stretching from Eastern Indonesia through Melanesia.

Classifications and size divisions are mine.

  • (O) = Oceanic plate, composed of oceanic crust (sima)
  • (C) = Continental plate, composed mostly of continental crust (sial)
  • (A) = A volcanic arc which has become separated from a larger plate by the process of back-arc rifting. The remnant is usually classified as an oceanic plate.
  • (SF) = Subduction fragment, cut from an oceanic plate betwen a spreading center, a subduction zone, and a transform fault
  • (SCM) = Spreading-center microplate, created when a block caught between discontinuous segments of a spreading rift disconnects and rotates between the plates around it.

Other notes:

  • NIB = did not appear in Peter Bird's paper, identity is debatable.
  • Plates with two stars appear in the traditional 14-plate model of the Earth's lithosphere.
  • Plates with one star also appeared on NASA's Digital Tectonic Activiy Map2, either labeled or obviously bounded.
  • Bird measured the size of the plates in terms of solid angle relative to the center of the Earth, because his paper demonstrates a loglinear relationship between a given solid angular size and the number of plates that are at least that size. It should be noted that one steradian is 1/(4π) of the total surround; on the the Earth's surface, this equates to about 40,000,000 square kilometers.

Big plates:

Medium-sized plates:

Small Plates:

Very small plates (180,000 to 570,000 km2):

Microplates (less than 180,000 km2):


1Peter Bird, An updated digital model of plate boundaries, Geochemistry Geophyisics Geosystems (aka G3), 4(3),1027
Available online at http://element.ess.ucla.edu/publications/2003_PB2002/2003_PB2002.htm

2http://geodynamics.gsfc.nasa.gov/dtam/

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