EARTH AND PLANETARY SCIENCES 101 PHYSICAL GEOLOGY FALL, 2002

THE WAY THE EARTH WORKS

STRUCTURAL GEOLOGY, HOW EARTH MATERIALS CHANGE THEIR SHAPE, ORIENTATION, AND POSITION.......

I. Introduction

A. Study of the shapes, arrangements, and interrelationships among geologic materials

B. Why important?

1. importance for understanding how geologic materials deform, particularly in the brittle realm

2. faults (brittle fractures in rocks along which displacement has occurred) are wonderful passageways for fluids, magmas, etc.)

3. geologic structures are, simply, fascinating!

C. Tectonic forces--movement of lithosphere plates resulting in permanent deformation of earth materials.

1. Stress (force/area) produces changes in shape and/or volume; permanent changes in rocks.

2. stresses are thus compressive or tensile in character. for any plane, stresses can be resolved into a normal component (acting perpendicular to the plane) and a shear component (acting parallel to the plane).

3. types of strain (the result of forces applied to geologic materials)

a. elastic

b. plastic (as in folds, continuously deformed materials)

c. fracture (as in joints and faults).

4. Importance of “scale” in structural geology--varied, and really little dependence on scale

5. Strain Rate--rate of deformation of earth materials, especially permanent,continuous deformation, is very, very slow.

D. Importance of layered materials(reference to the paleohorizontal datum); sedimentary and volcanic rocks are of great importance in making interpretations of deformed areas

E.. Field geology--if you will, an important component of research in the geological sciences--making observations in the field, constructing geologic maps, INTERPRETING THE GEOLOGIC PAST

1. construction of geologic maps and cross sections (distribution and nature of rock units, using symbols, colors, etc.

2. orientation of strata--strike and dip of planes in space--unique orientation of a plane in space requires the strike direction (direction of a horizontal line intersecting the plane), the angle of dip, and the direction of dip.

3. tools--Brunton compass, maps, air photos, rock identification materials

3. importance of geologic mapping. Assessment of mineral potential of the crust, everywhere. Interpreting the geologic past.

II. Types of Geologic Structures

A. Fractures in rocks--joints (essentially no displacement) and faults

1. joint sets--pathways for fluids, etc.

2. faults (imply motion)--dip slip, strike slip, oblique slip

3. dip slip types of faults--normal and reverse, with special cases

a. example of the Rio Grande river valley

4. strike slip varieties--dextral or sinistral (right-lateral or left lateral)

a. example of the San Andreas fault

5. relations to orientation of stresses.

B. folds--bends in geologic materials--represent essentially continuous, “ductile” deformation

1. geometry--fold axis, fold limbs, fold axial plane, fold hinge, fold plunge angle, etc.

2. types of folds--anticlines, synclines

3. geometries of folds--isoclinal, recumbent, domes, basins, overturned, etc.