(idea) by Gorgonzola Sun Jul 11 2004 at 5:35:54

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/

I like it! 2 C!s