Glossary - The Solid Earth - page 661

moment magnitude magnitude of an earthquake as estimated by using the seismic moment;
usual symbol M O .
momentum linear momentum of a body, the product of its mass and velocity; angular
momentum of a body, the product of its moment of inertia and angular velocity.
neutrino particle with no mass and no charge, but which can possess both energy and
momentum; emitted in the decay of some elementary particles.
nuclear fission splitting of a nucleus into fragments, with release of energy.
obduction process that involves breaking off a piece of a subducting plate and
thrusting it upwards onto the overriding plate; ophiolites were obducted into their present
positions.
offset horizontal distance.
olivine mineral (Mg,Fe) 2 Si 2 O 4 .
ophiolite fragment of crust and upper-mantle rocks, now exposed on land, analogous to
oceanic crust and mantle.
orogen region or belt of rocks that have been deformed together at a particular time; literally,
mountain creation.
orthopyroxene mineral (Mg,Fe) 2 Si 2 O 6 .
outer core outer liquid shell of the Earth's core, probably iron with some oxygen; inner
radius, 1221 km; outer radius, 3480 km.
Pangaea the single supercontinent of the Permian and Triassic, which broke up during the
Jurassic.
partial melting melting of part of a rock; because a rock is composed of different minerals,
each with its own melting behaviour, melting does not take place at one temperature (as for
ice at 0 ◦ C) but takes place over a range of temperatures; melting starts at the solidus
temperature and continues, non-linearly, as the temperature increases to the liquidus
temperature, at which the rock is totally molten.
passive margin continental margin formed during initial rifting apart of continents to form
an ocean; frequently has thick sedimentary deposits.
peridotite rock with over 90% mafic minerals, usually dominated by olivine.
period the time taken for one complete wave to pass any point; the inverse of the frequency.
phase change change of minerals from one crystallographic structure to another more
compact form at the greater temperatures and pressures within the Earth, or the reverse.
pillow lava lava with pillow-like features, erupted under water.
plate tectonics the 'carpentry' or 'architecture' of the Earth's surface; the system of large
lithospheric plates that move across the Earth's surface as spherical caps; most igneous and
tectonic activity occurs along the boundaries between, rather than within, the plates.
pluton body of magma that has solidified within the crust.