Transcript Ch. 11 Lecture1

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
Crystalline
Metamorphic
core
limestone
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