REVIEW OF PHYSICAL GEOLOGY

Plate Tectonics and Structural Geology

Plate Tectonics

Internal Structure of the Earth

The earth has a layered structure due to differentiation that occurred early in the earth's history, whereby denser material in the molten earth tended to descend toward the center and lighter material tended to rise toward the surface. The major layers of the earth, from the top down are

• crust - made up of rocks of relatively low density and very thin compared to the earth's radius

  • oceanic crust - composed largely of mafic igneous rocks; denser than continental crust and less thick

  • continental crust - composed largely of felsic igneous and metamorphic rocks with a relatively thin veneer of sedimentary rocks; less dense than oceanic crust and thicker

• mantle - made up of iron-rich rocks; most of the earth is mantle

• outer core - a liquid region consisting mostly of molten iron

• inner core - consisting mostly of solid iron at the center of the earth

For purposes of plate tectonics theory, it is convenient to identify the following two layers.

• lithosphere - consists of the crust and the solid part of the uppermost mantle

• asthenosphere - a plastic zone of partial melting just beneath the lithosphere

Tectonic Plates

The lithosphere is broken up into a series of tectonic plates. They are called "plates" because they are typically thousands of kilometers wide and only scores of kilometers deep and so have a plate-like shape. They are in motion, riding on the structurally weak athensophere, which is plastic due to partial melting. They typically move a few centimeters per year. Since the continents are carried along on the plates they are part of,

the continents also move. This motion is called continental drift.

Plate Boundaries

As tectonic plates move, they interact with each other at plate boundaries, which is where most of the tectonic action occurs on the planet. There are three types of plate boundaries.

• divergent - where two plates are moving apart

  • when mature, are marked by spreading centers where new oceanic crust is being created

  • incipient divergence is marked by rift valleys, such as that in eastern Africa

  • characterized by shallow earthquakes and non-explosive, basaltic volcanism

• convergent - where two plates are colliding

  • ocean-ocean convergence - one oceanic plate subducted under the other; marked by crustal compression, island-arc systems, deep and shallow earthquakes, and explosive volcanism

  • ocean-continent convergence - oceanic plate subducted under the continental plate; marked by crustal compression, deep and shallow earthquakes, and explosive volcanism

  • contient-continent convergence - no subduction; marked by extreme crustal compression and relatively shallow, sometimes powerful, earthquakes but little or no volcanism

• transform fault - where two plates slide past each other

  • shallow earthquakes but little or no volcanism

  • occasional powerful earthquakes if the fault cuts across continental crust (San Andreas fault zone)

Plate Tectonic Processes

• seafloor spreading - the creation of new oceanic crust at spreading centers as two oceanic plates move apart; oceanic crust grows older as you move away from spreading centers

• subduction - the process whereby oceanic crust descends into the mantle at convergent boundaries due to the crust's density'

• volcanic chains - linear ranges of volcanoes along convergent plate boundaries, forming arc-shaped volcanic islands where both plates are oceanic, called island arcs

• orogeny - mountain building at plate boundaries; occurs at all plate boundaries but is especially intense at convergent boundaries (orogenies can also occur away from plate boundaries, but that is not common)

• hot spot - a volcanic province typically distant from a plate boundary; thought to be due to magma rising from deep in the mantle with no direct connection to plate tectonics

• mantle convection - the rising of hot material in the mantle and the sinking of cooler material, such has what occurs in a boiling pot (but much more slowly); thought to be the driving force behind plate tectonics

Structural Geology

Faults

A fault is a fracture in rock along which there has been a lateral displacement (slip). The plane defined by this fracture is called the fault plane. Geometrically, faults are characterized by strike and dip.

• strike - the compass direction of the line where the fault plane intersects the (horizontal) surface of the earth.

• dip - the largest angle between the fault plane and the horizontal. In other words, the angle measured from the horizontal to the steepest direction of the plane. (Any plane in the earth, such as the bedding planes of sedimentary rocks can have a strike and dip)

• The strike and dip at a given location are always at a right angle to each other.

In addition to strike and dip, the two faces of a non-vertical fault plane are designated as the foot wall and the hanging wall.

• foot wall - the face that lies underneath the fault plane, supporting the hanging wall

• hanging wall - the face that lies above and is supported by the foot wall

There are two basic types of fault: dip-slip and strike-slip. However, many faults are partly dip-slip and partly strike-slip.

• dip-slip fault: This is a fault where one side of the fault slips upward and the opposite side downward. There are two types of dip-slip fault, normal and reverse.

  • normal fault - a fault where the hanging wall moves downward with respect to the foot wall; a normal fault indicates crustal extension (crust being pulled apart by tensional forces)

  • reverse fault - a fault where the hanging wall moves upward with respect to the foot wall; a reverse fault indicates crustal compression; a reverse fault with a small dip is called a thrust fault

• strike-slip fault: This is a fault where one side of the fault slips horizontally with respect to the other side.

Folds

There are three major kinds of folds in layered rocks. Two of these indicate crustal compression: anticlines and synclines. One can indicate crustal extension: monocline

• anticline - a fold where the sedimentary beds arch upward; when exposed by erosion the beds dip away from the axis of the fold (the "center" of the fold, where the folding is greatest); moving away from the axis of the fold, the beds get progressively younger

• syncline - a fold where the sedimentary beds warp downwards; when exposed by erosion the beds dip toward the axis of the fold; moving away from the axis of the fold, the beds get progressively older

• anticlines and synclines have axial planes, which, roughly speaking, divide the fold in half (technically the axial plane is the plane of maximum folding); each "half" is called a limb

• monocline - a fold where the beds are horizontal on both sides of the fold but have been folded to form a step-like structure, with the beds on one side lower than those on the other

Some other important terms to know about anticlinal and synclinal folds:

• overturned fold - where the beds on one side of the fold have been folded to the point they are now upside down

• recumbant fold - where the axial plane has been bent to the horizontal or beyond