GlossaryOort cloud a collection of comets that surround the Sun about a light-year
away
ophiolite(OH-fi-ah-lite) oceanic crust thrust upon continents by plate
tectonics
ore body the accumulation of metal-bearing ores where the hot, hydrother-
mal water moving upward toward the surface mixes with cold seawater
penetrating downward
orogens(ORE-ah-gins) eroded roots of ancient mountain ranges
orogeny(oh-RAH-ja-nee) an episode of mountain building by tectonic
activity
outgassing the loss of gas from within a planet as opposed to degassing, the
loss of gas from meteorites
ozone a molecule consisting of three atoms of oxygen in the upper atmos-
phere and that filters out harmful ultraviolet radiation from the Sun
Paleogene the Cenozoic Paleocene, Eocene, and Oligocene epochs
paleomagnetism the study of Earth’s magnetic field, including the posi-
tion and polarity of the poles in the past
paleontology (pay-lee-ON-TAL-o-gee) the study of ancient life-forms
based on the fossil record of plants and animals
Paleozoic(PAY-lee-eh-ZOE-ic) the period of ancient life, between 570
and 250 million years ago
Pangaea (pan-GEE-a) a Paleozoic supercontinent that included all the
lands of Earth
Panthalassa(PAN-the-lass-ah) the global ocean that surrounded Pangaea
peridotite the most common ultramafic rock type in the mantle
period a division of geologic time longer than an epoch and included in
an era
photosynthesis the process by which plants form carbohydrates from car-
bon dioxide, water,and sunlight
phyla groups of organisms that share similar body forms
phytoplankton marine or freshwater microscopic, single-celled, freely
drifting plant life
pillo w lava lav a extruded on the ocean floor giving rise to tabular shapes
placer(PLAY-ser) a deposit of rocks left behind by a melting glacier; any
ore deposit that is enriched by stream action
placoderm an extinct class of chordates, fish with armorlike plates and
articulated jaws
planetesimals small celestial bodies that accreted during the early stages of
the solar system
plate tectonics the theory that accounts for the major features of Earth’s
surface in terms of the interaction of lithospheric plates