Earth Science (EARTH-1141) - Structure Vocab

Chapter 7

deformation

general term for the processes of **folding, fauting, shearing, compression, or extension** of rocks as the result of various natural forces such as stress

geologic structures

basic geologic features (**Anticline, Syncline, Horst, Graben, Normal fault**) that result from forces associated with tectonic plates

stress

amount of force acting on a rock unit to change its shape and or volume

Uniform Stress

stress that is applied uniformly in all directions, and reduces the volume of a rock

differential stress

forces that are unequal in different directions, the force stronger in one direction an weaker in another

compressional stress

**differential stress** that squeezes rock mass as if it were in a vice, decreasing the volume. Occurs at **convergent plate boundaries.**

tensile stress

**differential stress** that pulls apart a rock body. Occurs at **divergent plate boundaries**.

shear stress

**differential stress** that causes two adjacent parts of a body to slide past one another. Occurs at **transform plate boundaries.**

Strain

the change in shape and or volume of a rock as a result of **stress,** can be temporary or permanent

Elastic Deformation

temporary **deformation** in which the rock returns to nearly its original size and shape when the stress is removed

Plastic Deformation

permanent **deformation** that produces a change i the size and shape of a rock body without fracturing it. Occurs at depths where temperatures and confining pressures are high.

Brittle Failure

**deformation** that involves the fracturing of rock once it can no longer take stress, common near the surface

folds

bent layer or series of layers that were __originally horizontal__ and subsequently deformed, usually formed deep underground.

Results from:

* **Compressive stress**


* **Plastic strain.**

Comprised of:

* **Axis**
* **Axial plane**
* **limbs**

Axis

hinge that cuts **fold** in half, and that the fold is bent around

Axial Plane

surface that connects all hinge lines of a **fold** into one plane

Limb

roughly symmetrical sides of **folds** that extend from the **hinge**, can be shared between two **folds**

anticline

**fold** in sedimentary strata that resemble a frown, opposite of **syncline**

syncline

**fold** in sedimentary strata that resembles a smile, opposite of an **anticline**

Symmetrical fold

**folds** with **limbs** that are mirror images of each other, and a vertical **axial plane**

Asymmetrical Fold

**folds** with limbs that are different lengths and a non-vertical **axial plane**

overturned fold

Extremely **asymmetrical fold** and a tipped over **axial plane** where both **limbs** dip in the same direction

recumbent fold

**overturned fold** that lies on its side, resulting in a horizontal **axial plane**

plunging folds

**fold** that occurs when a **anticline** is hit from a third direction and plunges into the planet, resulting in a tilted **hinge**

dome

large, roughly circular, regional structure that __frowns__ on all sides like an **anticline**

basin

large, roughly circular, regional structure that __smiles__ on all sides like an **syncline**

monocline

**fold** with only one **limb**

fault

a break in a rock mass along which movement has occured

dip slip fault

a fault in which the vertical movement is parallel to the dip of the fault surface

hanging wall block

rock surface immediately __above__ **fault** whose angle is such that a lantern can be hung

footwall block

rock surface immediately __below__ a **fault** whose angle is such that one can stand upon it

fault scarp

cliff created by movement along a **fault**

normal fault

**fault** where **hanging wall** moves __down__ relative to **footwall**

horst

uplifted block of crust bounded by **normal** faults

graben

valley formed by the downward displacement of a fault-bounded block between **normal fault**

joint

fracture in rock along which there has been no movement

orogenesis

the processes that collectively result in the formation of mountains

collisional mountains

mountains in which **compressive** horizontal forces have shortened and thickened the crust