Crustal Deformation

Review of Chapter 11

Isostasy• Balance in possible vertical movement of

the plates– Gravity bears down– Heated aesthenosphere is buoyant

(heat=expand=less dense=rise & cool=shrink=more dense=fall)

– Usually equal or nearly so

• Imbalance upsets isostasy and results in a rise or fall of plates

• Fall somewhere results in rise elsewhere and vice versa

• Isostasy explains the vertical distribution of Earth's crust. George Bedell Airy proposed that the density of the crust is everywhere the same and the thickness of crustal material varies. Higher mountains are compensated by deeper roots. This explains the high elevations of most major mountain chains, such as the Himalayas. G H Pratt hypothesized that the density of the crust varies, allowing the base of the crust to be the same everywhere. Sections of crust with high mountains, therefore, would be less dense than sections of crust where there are lowlands. This applies to instances where density varies, such as the difference between continental and oceanic crust.

Deformation

• All changes in volume or shape of rock

• Reaction to stress

• From confining pressure– Greater with depth– Shallow = brittle– Deeper = ductile

Stress

• A force that acts on rocks to change shape and/or volume

• Compression – squeeze

• Tension – pull apart

• Shear – an an angle

Strain

• Change in shape or volume of rock as a result of stress

• Stress must exceed resistance of rock which is variable– Rock composition– Temperature– Pressure– Amount/type/speed of stress

Types of Deformation

Brittle

Types of Deformation

Elastic (reversible)

Types of Deformation

Plastic (permanent)

Rocks deform elastically when exposed to stresses. Experiments have proven that most rocks at depth deform plastically once their elastic limit is surpassed. Surface rocks also deform elastically, but turn brittle and fracture when they exceed their elastic limit.

Strike and Dip

• Strike is the direction in which strata lie

• Dip is the angle of inclination from the surface at which strata lie

The structure of a slope is called its strike and dip. Geologists draw these symbols on maps to define the way beds of rock are at angles, or the way they are dipping. Strike and dip are at right angles to each other. The arrow points in the direction of dip.The angle of dip is the angle (in degrees) off of horizontal that the bed is dipping. It is usually measured with a Brunton compass and is given by a number like 28 degrees or 45 degrees.

Folds

• Flat rock bent into undulations, usually by compression

• Sides are called limbs and come off the axis

• May have 1 or 2 limbs

• Axis may not be horizontal = plunging

Anatomy of a Fold

Types of Folds-Anticline

Types of Folds-Syncline

Types of Folds-Monocline

Types of Folds-Recumbent

Types of Folds

Fold sizes• Small in hand-held sizes

• Large; seen only from the air– Ridge– Valley

Domes and Basins

• Gentle upwarping or downwarping of crustal rock produce domes and basins

• Erosion of these structures results in an outcrop pattern that is roughly circular or elongated

Domes and Basins

sandstone

limestone

Crystalline

Metamorphic

core

Faults

• Rock breaks in response to severe stress (fracture or joint)

• Movement along fracture (horizontal and/or vertical

• Surface of movement is fault plane

Dip-slip faults

• Movement is vertical (on the dip)

• Footwall below and hanging wall above

• Normal, reverse, thrust

Normal Fault

Fault block valley

Fault block ridge

Fault scarp

Strike-slip Fault

Oblique-slip fault

Joint to Fault