DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

types across (now separated) continental coastlines,
by similarities of geology in such situations,
and by direct measurement via laser satellite
geodesy, very long baseline interferometry,
and the Global Positioning System. The theory
states that the Earth’s lithosphere is broken
up into segments, or plates, which move
atop the more fluid asthenosphere. Currently
the Earth’s surface is composed of six major
plates and nine smaller plates. The interaction
of the plates along their boundaries produces
most of the volcanic and tectonic activity
seen on Earth. New crust is created and plates
move away from each other at divergent boundaries
(mid-ocean ridges). Plates collide and are
destroyed or severely deformed at convergent
boundaries, where one plate is subducted under
the other (deep sea trenches) or the two plates are
uplifted to form a mountain range. Thus, the Atlantic
Ocean is growing, and the Pacific Ocean is
shrinking. Plates slide past one another at transform
boundaries. The driving mechanism for
plate tectonics is believed to be convection cells
operating in the Earth’s asthenosphere. It is understood
that a giant hot plume ascends from the
core-mantle boundary, whereas cold slabs drop
through the mantle as a cold plume, producing
descending flow. The surface expression of hot
plumes are the volcanism of Hawaii, Iceland,
and other hot spots not directly associated with
plate tectonics.
The plates move with velocities of between
1 and 8 cm/yr. Earth appears to be the only
body in the solar system where plate tectonics
currently operates; remnant magnetism suggests
plate tectonics may have been effective in the
past on Mars.
platonic year See year.
Pleiades Young star cluster, Messier number
M45, which is a bright star forming region, and
has numerous young hot (B-type) stars, of which
seven are prominent; the Pleiades are the seven
sisters, daughters of Atlas.
Pleione Variable type B8 star at RA 03 h 49 m ,
dec +24 ◦ 07’; “Mother” of the “seven sisters” of
the Pleiades.
© 2001 by CRC Press LLC
pocket beach
Plimsoll’s mark After Samuel Plimsoll; the
name given to the load marks painted on the
sides of merchant ships indicating the legal limit
of submergence. The British began the system
around 1899, and the U.S. adopted a similar system
in 1930. Load lines include FW (fresh water),
S (summer), W (winter) WNA (winter in
the North Atlantic), and IS (Indian Summer) for
the relatively calm period October to April in
the Indian Ocean.
Pluto The ninth planet from the sun. Named
after the Roman god of the underworld, Pluto
has a mass of M = 1.29 × 10 25 g, and a radius
of R = 1150 km, giving it a mean density of
2.03 g cm −3 and a surface gravity of 0.05 that
of Earth. Its rotational period of 6.3867 days,
and its rotation axis has an obliquity of 122.5 ◦ .
The slow rotation means that the planet’s oblateness
should be very nearly 0, but its small size
may allow significant deviations from sphericity.
Pluto’s orbit around the sun is characterized
by a mean distance of 39.44 AU, an eccentricity
of e = 0.250, and an orbital inclination of
i = 17.2 ◦ . Its large eccentricity means that
for part of its orbit it is closer to the sun than
Neptune, most recently during the 1980s and
1990s. Its sidereal period is 247.69 years, and its
synodic period is 367 days. An average albedo
of 0.54 gives it an average surface temperature
of about 50 K. Its atmosphere is very thin, and
mostly CH4. Pluto is probably composed of a
mixture of ices, organic material, and silicates.
Because of its small size and icy composition,
Pluto is more properly an inner member of the
Kuiper belt, rather than a full fledged planet.
Pluto has one satellite, Charon, which is nearly
as massive as it is.
pluton A magma body (intrusion) that has
solidified at depth in the Earth’s crust.
plutonic Indicative of igneous rock consisting
of large crystals. This suggests slow crystalization,
as would happen at depths beneath the
Earth.
pocket beach A beach comprised of sediments
that are essentially trapped in the longshore
direction by headlands which block longshore
sediment transport.
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