Lab 5

Structural Geology and Plate Tectonics

Chapters 1, 5, and 6

Structural Geology: the study of rock deformation

Deformation:

Caused by stress (directed pressure)

Deformation, such as a change in shape, is called strain.

Three tools used by structural geologists:

1. Geologic map = shows the distribution of rocks at the Earth's surface

2. Geologic cross section = a vertical slice through the Earth; a cutaway view

3. Block diagram = a combination of geologic map and cross section; looks like a solid block, with a map on top and cross section on each of its visible sides; See Figure 1.3

System to describe rock orientation / attitude: Figure A

Strike: The compass direction of a line formed by the intersection of a horizontal plane (such as the surface of a lake) and an inclined stratum, fault, fracture, or other surface. It is expressed relative to north or south; For example, strike is expressed as "north x degrees east."

Dip: The angle between a horizontal plane and the inclined stratum, fault, or fracture. A thin stream of water poured onto an inclined surface always runs down the surface parallel to dip. The inclination of the water line down from the horizontal plane is the dip angle. Dip is always measured perpendicular to strike. The direction that the water runs down an inclined geologic surface is the dip direction and must be expressed together with the dip angle. (Example: 45^o North)

Strike and dip are shown on maps by using a T-shaped symbol. The long line shows the strike direction and the short line shows the dip direction. The dip is always drawn perpendicular to strike line. The accompanying number indicates the dip angle in degrees.

FOLDS

TERMINOLOGY (See Figure B)

Fold axis (or hinge line): each stratum is bent around an imaginary axis.

Axial plane: for all strata in a fold, the fold axes lie within this plane of the fold

Limb: Folds commonly have two sides, or limbs

EXAMPLES (See Figure 1.3-a)

Anticline: An "upfold" or "convex fold"; The core of the anticline contains the older rocks; The limbs of the anticline dip away from each other.

Syncline: A "downfold" or "concave fold"; The core of the syncline contains the younger rocks; The limbs of the syncline dip toward each other.

Monocline: Strata dip from a horizontal attitude in one direction only; The structure will be a large feature of gentle dip.

Plunging fold: A fold with an inclined fold axis; Plunge is the angle between the fold axis and horizontal; The trend of the plunge is the compass direction of the fold axis.

FAULTS (See Figure C and Figure 1.3-b)

Normal fault: Hanging wall moves downward relative to the foot wall. Caused by (ex)tensional (pull-apart) forces.

Reverse fault: Hanging wall moves upward relative to the foot wall. Caused by compressional forces.

Thrust fault: A reverse fault with an extremely low fault plane angle.

Strike-slip fault: Block movements are parallel to the strike of the fault. Generally, no vertical displacement; Also called a lateral fault.

Outcrop Thickness Variations See Figure 5.3 a&b

Thickness in map view can vary because of varying dips

PLATE TECTONICS

Pangaea

In 1915, Alfred Wegener suggested that today’s continents were once part of a "supercontinent" called Pangaea. He hypothesized that at some point in geologic time, Pangaea fragmented into pieces (the continents) and they gradually drifted to their present positions.

Plate boundaries

Surface plates of continental and ocean-floor rock move as large units.

The rigid upper layer of the earth is the lithosphere. The lithosphere is made of eight major plates.

The lithosphere includes continental and oceanic crust and the upper part of the mantle.

Because of stresses caused by the movement of the mantle material, the lithosphere is broken by a system of fractures that occur in specific zones called plate boundaries.

3 kinds of plate boundaries:

1. Diverging boundary See Figure D

Also called spreading centers, mid-oceanic ridges (ocean), and rifts (land)
Example: Mid-Atlantic Ridge
Basaltic lava wells up from Earth’s mantle through cracks in the oceanic crust. As the lava rises and occasionally pours onto the seafloor, it solidifies to form new oceanic crust.
The rising lava forces aside older crust, moving it horizontally away from both sides of the ridge. The continuous supply of new crust forms a ridge along the plate boundary.

2. Converging boundary See Figure E

Also called subduction zones and suture zones.
Old ocean crust ends up here as it migrates away from the spreading centers. If all of the old crust piled up on the Earth’s surface, then the Earth would grow larger and larger. But this formation of new crust is balanced by subduction.

Subduction: descent of old crust back down into the mantle where it is remelted. Regions where subduction and plate collision occur are called subduction zones.

Dense oceanic crust vs. less dense continental crust: when they slowly collide, the denser oceanic crust is forced to descend beneath the continental crust. Heat and friction increase the temp of the subducting plate and creates magma. It then erupts at the surface as a chain of volcanic islands and volcanoes along a continental margin. Example: the Andes Mountains of South America

Continental vs. continental crust (similar densities): when they slowly collide, simple subduction cannot occur. Like sliding two rugs together, they both rumple. A zone of intense deformation and mountain building is formed. This is called a suture zone. Example: the Himalayas

3. Transform boundary

Form where plates slip horizontally past each other.
Also called a transform fault (similar to strike-slip fault motion).
Formed by a system of large lithospheric fractures that occur perpendicular to spreading centers.
Example: San Andreas fault system in California

Rate of plate movement

Hot spots: (See Figure 6.6)

A center of volcanic activity that persist in a stationary location for tens of millions of years. It is the result of plumes of magma rising from the Earth’s mantle.
As a plate migrates over a hot spot, a volcano develops directly over the hot spot. As the plate slides on, the volcano that was over the hot spot becomes dormant and over time migrates many kilometers from the hot spot.
The result is a string of volcanoes, with the active end over the hot spot and the inactive end far from the hot spot.
It creates a succession of volcanoes that are progressively older with distance from the hot spot.
Examples: Hawaiian Islands and Emperor Seamount chains represent such a line of volcanoes that formed over the Hawaiian hot spot.
This is a great way to measure plate motion.

Rate of plate motion = (Travel distance) ¸ (Travel time)

Measured in centimeters per year.

Return to Labs and Assignments