A deviation from the average or expected value.  In paleomagnetism, a “positive” anomaly is  a stronger-than-average magnetic field at the earth’s surface.

The plastic layer of the earth, at whose upper boundary the brittle overlying lithosphere  is detached from the underlying mantle.

The term applied by American geologists to Wegener’s hypothesis of long-distance horizontal movement of continents he called “continental displacement.”

Tectonic boundary where two plates collide.  Where a tectonic plate sinks beneath another plate and is destroyed, this boundary is known as a subduction zone.  Subduction zones are marked by deep-focus seismic activity, strong earthquakes, and violent volcanic eruptions.

A part of the earth’s continental crust that has attained stability that has not been deformed for a long period of time.  The term is restricted to continents and includes their most stable areas, the continental shields.

In solid earth geophysics this means a chemical of phase change in materials with depth.  At discontinuities the earth materials become denser and earthquake waves change velocity.

Found at mid-ocean ridge or spreading center where new crust is created as plates move apart.  For this reason they are sometimes referred to as constructive boundaries and are the sites of weak shallow-focus earthquakes and volcanic action.

Point within the earth where an earthquake originates.

An elongate flat-floored valley bounded by faults on each side.

A point (or area) on the lithosphere over a plume of lava rising from the mantle.

The condition of equilibrium, comparable to floating, of large units of lithosphere above the asthenosphere.  Isostatic compensation demands that as mountain ranges are eroded, they rise-just as a   block of wood floating in water would rise if a part of it above the waterline were removed.

The northern part of the Permo-Triassic landmass of Pangaea.  (See Gondwana)

The rigid outer layer from the earth’s surface down to the asthenosphere.  It includes the  

      crust and part of the upper mantle and is of the order of 100 km (60 mi) thick.  The “plates” in plate  

      tectonic theory.

Study of the remanant magnetism in rocks, which indicates the strength and direction of the earth’s magnetic field in ancient landmasses.

super continent that existed from about 300-200 Ma, which included all the continents we know today.

The magnetic positive (north) or negative (south) character of a magnetic pole.

A hypothesis that oceanic crust forms by convective upwelling of lava a mid-ocean ridges and moves laterally to trenches where it is destroyed.

The sinking of one lithospheric plate beneath another at a convergent plate margin.  (See convergent boundary.)

Fault-bounded bodies of rock that have been transported some distance from their place or origin and that are unrelated to adjacent rock bodies or terranes.

Plate boundaries with mostly horizontal (lateral) movement that connect spreading centers to each other or to subduction zones.