Results of Stress

Chapter 5.2

Rock Deformation1.Rock Deformation

a.Caused by the high temperatures and pressures of geologic stress.

b.It is possible for the rock to return to its original shape if the force (stress) is applied and removed slowly.

c.When the stress passes the limits of the rock, the rock can be deformed permanently and may break.

Folding1.Folding

a.When rock responds to stress by becoming permanently deformed without breaking.

b.Folding can be easily observed where the rocks are compressed inward and the layers of the rock move into new positions without breaking. (Cracks may appear, but the rock layers remain intact.)

Folding

a.Size of folds can vary greatly.i.Some rock folds can be found within a small hand-size rock while others span of thousands of miles and must be seen from air.

Foldinga.There are three types of rock folding:

Anticline, Syncline, & Monocline. i.An anticline fold is an up-curved fold in

which the oldest layer is in the center of the fold.

1.Mountain ridges are generally found along anticlines.

Folding

ii. A syncline fold is a down-curved fold in which the youngest layer is in the center.

1.Valleys are generally found along synclines.

iii. A monocline fold is a fold in which both sections remain horizontal.

Faulting & Fracturing1.Faulting & Fracturing

a.Rocks do not always respond to stress by folding. Under certain conditions rocks will actually break when stress is applied.

b.Faulting & Fracturing of rocks generally occurs nearer to the Earth’s surface where the rocks are cooler and under less pressure.

c.A fracture is a break in rock where there is no rock movement on either side of the break.

Faulting & Fracturinga.A fault is a break in rock along

which rocks on either side of the break move. i.A fault plane is the surface of a fault along which any motion occurs.

ii.The hanging wall is the rock above the fault plane.

iii.The footwall is the rock below the fault plane.

Faultsa.There are four types of faults: Normal

Faults, Reverse Faults, Thrust Faults, and Strike-Slip Faults. i.A normal fault is a fault in which the

hanging wall moves down relative to the footwall.

1.These faults generally form along divergent boundaries where the crust is being pulled apart.

Faultsi.A reverse fault forms when

compression causes the hanging wall to move up relative to the footwall.

ii.A thrust fault is a reverse fault where the fault plane is nearly horizontal.

iii.A strike-slip fault is a fault where the rock on either side of the fault plane slide horizontally past each other.