STUDY GUIDE DEFINITIONS

System

entity comprised of diverse but interrelated parts that function as a complex whole

Component

the smallest set of chemical formulae needed to describe all the phases in a system

Phase

a physically separable portion of a system

State

attributes that characterize a system

Uniformitarianism

Assumes that the same natural processes that operate in the universe now, have always operated in the universe in the past, and at the same rates; and that the same laws of physics apply everywhere in the universe.

Hypothesis

an explanation that accounts for a set of observations and can be tested by further investigation

Law of Superposition

in a sequence of rock, the oldest layer will be below the youngest.

Law of Original Horizontality

Layers of sediment will be deposited flat at the bottom of a lake or ocean basin.

Law of Cross-Cutting Relationships

If a rock layer is cut by a fault or igneous intrusion it must be older than the layer that cuts it.

Law of Lateral Continuity

Rock layers will extend outwards until the environment that produced them changes

Theory

a coherent group of general propositions used as principles of explanation for a class of phenomena

Parsimony

principle that states that the best hypothesis is usually the simplest hypothesis, i.e., requires the fewest assumptions

Igneous rocks

rocks formed from cooling of magma

Lithosphere

the rigid, brittle outermost layer of a planet.  Its average thickness is 100 km (oceanic 80 km; continental 250 km).

Asthenosphere

mechanically weak region of the upper mantle of the earth that exhibits ductile (plastic) behavior. Its average thickness is 350 km.

Crust

the thin, rocky, outer layer of our planet.  The oceanic crust is basaltic, while the continental crust is granitic or granodioritic. Its thickness is approximately 10 km at the oceans and about 35 km at the continents.

Mantle

a solid rocky layer beneath the crust composed of mainly peridotite.

Paleomagnetism

remanent magnetism in ancient rock recording the direction of the magnetic poles at some time in the past.

Divergent plate boundary/divergent margin

a fracture in the lithosphere where two plates move apart. Also called a spreading center.

Subduction

the sinking of old, cold oceanic lithosphere into the asthenosphere.

Radius of the earth

approximately 6400 km

Curie temperature

temperature at which magnetic minerals acquire its magnetism, approximately 600 ºC

MORB

mid-ocean ridge basalt

Seamount

submerged ocean island.

Ocean island

volcanic islands associated with hot spot volcanism

Hot spot or mantle plume

areas of intense volcanic activity that remain in more or less the same location over long periods of time and are unrelated to plate boundaries.

Basalt

a mafic rock containing 50% silica and composed of the minerals pyroxene and plagioclase.  Found mainly at divergent boundaries and in some instances of hot spot volcanism.

Andesite

an intermediate rock (60% silica) found primarily at subduction zone volcanoes

Shield volcanoes

basaltic volcanoes commonly found in mid-ocean ridges and in ocean islands such as Hawaii or Iceland.

Trench

deepest parts of the ocean floor, resembling a crease or trough, where plates meet during subduction.

Bathymetry

topography of the ocean floor

Hydrothermal vent

hot springs associated with rift valleys

Polymerization

degree of linking of SiO₄ tetrahedra

Mineral

a naturally occurring inorganic solid possessing long range internal order; a naturally occurring inorganic crystal

Plagioclase feldspar

an aluminosilicate containing calcium and sodium that is the most common mineral in earth’s crust.

Olivine

an ultramafic mineral that is the main component of peridotite