Transcript Historical Geology

MOUNTAIN AND MOUNTAIN RANGES
A. Mountain
1. any part of the Earth’s surface sufficiently elevated above its
surroundings to have a distinct summit and to be considered worthy of
a name
2. peaks must rise at least 300 m above the surrounding terrain
B. Mountain Range
1. a series of mountains or mountain ridges that are closely related
in position, direction, age, and mode of formation
2. a group of mountain ranges found grouped together in an elongate
zone is a mountain chain
C. Continents
1. Shield-large areas of exposed ancient (Precambrian) rocks
2. Platform-buried ancient rocks underlying each continent
3. Craton-shield + platform together; nucleus of a continent
4. accretion- growth; deformed rocks along edges of cratons…
Teton Range in Rocky Mtnsyoung, note high sharp peaks
Smoky Mtns, Appalachians
older, smooth tops
DEFORMATION AND RUPTURE OF ROCKS
A. Tectonic Stress
1. tectonic force applied to rocks
2. three types of stress
a. compressive stress--squeezes rock and tend to shorten distances parallel to the
squeezing direction
b. tensional stress--pulls rock apart
--the opposite of compressive stress and tends to increase the distance parallel to the stress
c. shear stress--acts in parallel but opposite directions
--deforms rock by causing one part of a rock mass to slide past another
B. Strain
1. the distortion or deformation that results from stress
2. rocks can respond to tectonic stress in 3 different ways
a. elastic strain--directly proportional to the amount of stress: return to shape
b. non-elastic strain-- a rock undergoes plastic deformation
c. rupture-- rock breakage
C. Factors Controlling Rock Response to Stress
1. the nature of the material
-- all rocks first react to stress with small amounts of elastic deformation
-- with continuing stress, different types of rocks may behave in different ways
2. temperature-- the higher the temperature, greater the tendency of rocks to behave plas
3. pressure-- high pressure favors plastic behavior
4. time-- stress applied over a long time, rather than suddenly, also favors plastic
behavior of rocks
Stress and possible deformation: A. compression causes shortening of
rock layers by folding and faulting. B. tension causes lengthening and
faulting. C. shear stress causes deformation along planes
Behavior of rocks
under stress:
a. If rock is brittle,
it follows B, and
fractures at certain
point.
b. If rock is ductile,
it follows A, and is
deformed. Or it can
fracture if enough
stress applied.
Flat lying beds= infer little deformation
GEOLOGIC STRUCTURES
A. Definitions
1. geologic structure -- any feature produced by deformation of rocks
2. fold -- a bend in rock
3. fault --a fracture along which rock on one side has moved relative
to rocks on the other side
4. joint -- a fracture without movement of rock on either side of the
break
B. Folds
1. anticline--a fold arching upward
2. syncline--a fold arching downward
(GO to slides….)
Folded rocks in Calico Mtns, CA. Three folds are visible from left to right
a syncline, an anticline, and a syncline
S
A
S
Rocks deformed by folding and fracturing. Notice the faultoffset of folded white layers within the brown rocks
These sedimentary rocks were deposited flat, lithified, then tilted into this position
B. Folds
1. anticline--a fold arching upward
2. syncline--a fold arching downward
3. limbs--sides of a fold
4. fold axis—a line dividing the two limbs of a fold and running along the crest
of an anticline or the trough of a syncline
a. plunge: when the axis is inclined or tipped at an angle
b. plunging fold: a fold with a plunging axis
5. axial plane: an imaginary plane that runs through the axis and divides a fold
as symmetrically as possible into two halves
6. types of folds
a. monocline: has only one limb
b. anticline: an arch-like structure
c. syncline: a bowl shaped structure
d. dome: a circular or elliptical anticlinal structure
e. basin: a circular or elliptical synclinal structure
Monoclines: simple bend or
flexure in otherwise flat lying beds.
Notice the strike and dip symbols
and the circled cross-indicating flat
lying beds.
Syncline and Anticlinewith axial planes, axis
and limbs
Folds and topography-anticlines and synclines do not correspond to
topographic highs and lows….
Fold relationship to topography-syncline at peak of mountain
Complexity associated with folding-identifying eroded anticlines and
synclines by strike and dip of folded rock layers and relative ages
Plunging folds:
a. Folds that tilt in
one direction.
B. Cross section viewStandard geologic
symbol used to indicate
plunging fold=long arrow
in center of map. Arrow at
end of line shows direction
of plunge.
c. Sheep Mountain anticline
In Wyoming. Eroded..
These symbols indicate an
anticline as opposed to a syncline…
Other complexities:
a. Inclined fold-axial
plane not vertical, limbs
dip at different angles
b. Overturned folds-both
fold limbs dip in same
direction, but one limb is
inverted.
c. Recumbent folds-folds
overturned so far that
they are lying on their
sides.
d. Recumbent fold in
Switzerland
Reading a geologic map: how
can tell if circular area is an eroded
anticline or syncline?:
a. In anticline, oldest exposed
rocks are in center
b. In syncline, youngest exposed
rocks are in center…
D. Joints
1. a fracture similar to a fault except that rocks on either side of the fracture have not
moved
2. examples of jointsa. columnar joints in basalt
-- occurs when basalt cools and shrinks causing polygonal cracking
b. exfoliation of granite
-- sometimes called sheet jointing
-- caused by distortion of the rock at shallow depth
3. joints become less abundant with increasing depth because both pressure and
plastic behavior of rock increase with depth and both inhibit jointing.
E. Strike and Dip
1. strike: the direction in which tilted layers of rock are oriented
-- always given as a compass direction
2. dip: the angle of inclination of the bedding plane
-- measured from a horizontal plane
3. strike and dip are used to measure any type of layering or other flat surfaces
in rocks
Joints: fractures along which no movement has
taken place.
a. Erosion along parallel joints in Arches Nat Park
b. Joints intersecting at right angles yield a rectangular pattern in Wales.
Concepts of Strike and Dip:
a. Intersection of horizontal plane
(sea level) and inclined plane (surface
of rock layer) forms a line known as
the strike. The dip is the angle of
deviation of the inclined layer from
the horizontal. Notice the strike and
dip symbol with 50 adjacent to it
indicating the angle of dip.
b. Natural example of strike and dip.
The strike of the dipping rock layer
is marked by the intersection of
the layer with the water.
7. folding always results in shortening of distances in rocks, and in turn, shortening is produced by
compression, reflecting compressional tectonic stress
-- folding is less commonly formed during crustal extension
C. Faults
1. slip is the distance that rocks on opposite sides of a fault have moved
2. numerous closely spaced fractures collectively is referred to as a fault zone
3. repeated movement of faults and fault zones
a. tectonic stress commonly continues to be active in the same place over
long period of time
b. once a fault forms, it is easier for movement to occur again along the same
fracture than for a new fracture to develop nearby
4. ore deposits often concentrated in faults
--miners referred to the side of a fault that hung over their heads as the hanging wall
and the side they walked on as the footwall.
a. Fault terminology
b. Polished fault
plane and scarp,
Klamath Falls, Ore.
c. Fault breccia-zone
of rubble along a fault
in Bighorn Mtns, Wyo
5. normal fault -- a fault in which the hanging wall moves down
relative to the footwall; normal faults result form extensional
tectonic force
6. graben – wedge-shaped block of rock dropped downward
between two normal faults
7. reverse fault – a fault in which the hanging wall moves up
relative to the footwall
8. thrust fault – a special type of reverse fault that is nearly
horizontal
9. strike-slip fault – the fault is vertical, or nearly so, and the
motion along the fault is
horizontal
-- transform faults that offset the oceanic ridge are special
types of strike-slip faults
10. horst- wedge shaped block of rocks moved upward between
two normal faults
Types of Faults:
I. Dip-slip faults
a. Normal fault- hanging wall
block moves down relative to
footwall block.
b. Reverse and c. Thrust faulthanging wall block moves up
relative to footwall block.
II. Lateral faults
d. Strike-slip fault: all movement
is parallel to strike of the fault.
e. Oblique-slip fault: combination
of strike slip and dip-slip movements
Two small normal faults cutting through layers of volcanic ash, ORE
Reverse fault in sandstone layers-to the right of the hammer
Reverse Fault
Normal fault
Thrust fault in Glacier Nat’l Park: Precambrian on top of Cretaceous
Right lateral offset of gully along San Andreas Fault in CA
Deformation and origin of Mountains
Pluton intrudes into sedimentary rocks
Erosion of softer overlying sedimentary rocks reveals plutons
and forms small mountains
a. Block faulting-origin of horsts and grabens
Humboldt Range, NEV….Horst bounded by normal faults
PLATE TECTONICS AND MOUNTAIN BUILDING
A. Orogenic Activity
1. orogeny refers to the process of mountain building
2. an orogen or orogenic belt is a long and relatively narrow region near a tectonically
active continental margin where any or all of those processes have formed mountains
B. Orogenic Activity and Plate Boundaries
1. rift boundary
a. mid-oceanic ridge
b. continental rifting is responsible for the uplift
2. transform fault boundary
a. high cliffs and escarpments form along transform faults offsetting the mid-oceanic
ridge
3. convergent plate boundaries’
a. ocean-ocean convergence
volcanic mountain ranges
b. ocean-continental convergence
volcanic mountain ranges
c. continental-continental convergence
folded mountain belts
Orogenic activity concentrated in circum-Pacific and Alpine-Himalayan
orogenic belts-Plate Tectonics and Mountain Building
F. Geologic Structures and Plate Boundaries
1. each of the 3 different types of plate boundaries is characterized by different
large-scale tectonic stresses
a. spreading centers are extensional
b. transform boundaries are regions of crustal shearing
c. convergent boundaries are dominated by compression.
2. different geologic structures are commonly associated with each type of boundary
a. divergent boundaries produce normal faults, grabens, little or no folding of rocks
b. transform plate boundaries are strike-slip faults
c. convergent continent plate boundaries commonly associated with folding have
reverse and thrust faulting
--at convergent continent-ocean or ocean-ocean boundaries subduction is
accompanied by crustal extension and normal faulting
ISLAND ARCS
A. Subduction Complex
1. The area in front of the island arc
2. subduction of plate highly deforms sediment and part of the oceanic crust
3. growth of the subduction complex occurs by addition of the newest slices at the
plate boundary
4. bottom of the complex-underthrusting forces the subduction complex upward,
forming a sedimentary basin called a forearc basin between the complex and island ar
the forearc basin fills with sediment derived form erosion of the volcanic islands and
becomes a part of the island arc
Origin of volcanic Island Arc:
a. Subduction of oceanic plate
b. Continued subduction, accretion begins,
continued plutonism; beginning of deformation
by thrusting and folding.
c. Thrusting of back-arc basin sediments
onto the adjacent continent and suturing
of the island arc to continent.
THE BUILDING OF TWO MOUNTIAN CHAINS: THE ANDES AND THE HIMALAYAS
A. Similarities and Differences
1.immense and great height of the peaks are similar
2. both rise abruptly from adjacent low-lying regions
3. igneous rocks are the most abundant rock type in the Andes
4. highly folded and thrust-faulted sedimentary and metamorphosed sedimentary
rocks dominate Himalayan geology
5. both chains formed at convergent plate boundaries
a. Andes at ocean-continental collision’
b. Himalayas at a continental-continental collision
B. The Andes: Subduction at a Continental Margin
Today the Andes are a relatively narrow mountain chain consisting predominantly of
volcanic and plutonic rocks produced by subduction at a continental margin
1. the chain also contains extensive sedimentary rocks deposited in basin that
formed by sinking of the crust on both sides of the mountains
2. some of these rocks were deformed and metamorphosed by tectonic forces and
elevated temperature and pressure related to the emplacement of huge volumes
of magma into the crust
3. this type of a plate margin is called an Andean margin.
3 Stages to development of
Andes Mtns, So America:
a. Prior to 200 MYA, west coast
of S America was a passive margin
b. An orogeny began when W coast
became an active margin- breakup
of Pangaea.
c. Continued deformation, volcanism
and plutonism…formed Andes Mtns
C. The Himalayan Mountain Chain: A Collision Between Continents.
1.formation of an Andean-type margin
2. continent-continent collision -- India and Asia
3. Himalayas today
a. The combination of underthrusting of India beneath Tibet and the horizontal
squashing of Tibet has produced unusually thick continental crust beneath the
Himalayas and the Tibetan Plateau
b. Frequent moderate earthquakes and occasional large and destructive ones
characterize the region.
4. the junction between two welded continents is called a suture zone or a continental
suture
a. recognized by a lack of continuity between rock types, ages, and structures on
either side of the suture
b. rocks of the suture zone are commonly deformed and sheared by the collision
c. bits and scraps of oceanic crust and mantle occur in suture zones
Continent-Continent collision: formation of Himalayas as India meets
Asia. Through time, subduction has been long-lived; ultimately had
thrusting of sediments and Indian continent onto Asia….
Continental Evolution:
Continental accretion
through time due to
multiple orogenic
events suturing onto
edge of continent…
Multiple collisionsEither continent-continent
Or continent-oceanic
Geologic Maps-their construction and Use
Construction of geologic map from surface exposures
a. Valley with outcrops, much of it covered by soil
b. Data from outcrops used to infer what is present
in covered areas between outcrops. Lines represent
boundaries between different types of rocks.
c. A geologic map (top) showing area as if soil was
removed. Strike and dip measurements indicated.
Orientation of rocks recorded by strike and dip used
to construct the cross section across the bottom.
Mountain Building- SUMMARY
Mountains and Mountain Ranges are results of tectonic Stresses applied to rocks:
a. compressive stress--squeezes rock and tend to shorten distances parallel to the squeezing
direction
b. tensional stress--pulls rock apart
--the opposite of compressive stress and tends to increase the distance parallel to the stress
c. shear stress--acts in parallel but opposite directions
--deforms rock by causing one part of a rock mass to slide past another
Strain- the distortion or deformation that results from stress
rocks can respond to tectonic stress in 3 different ways
a. elastic strain--directly proportional to the amount of stress: return to shape
b. nonelastic strain-- a rock undergoes plastic deformation: changes shape
c. rupture-- rock breakage
Factors Controlling Rock Response to Stress
1. the nature of the rock
2. temperature-- the higher the temperature, greater the tendency of rocks to behave
plastically
3. pressure-- high pressure favors plastic behavior
4. Time
Mountain Building- SUMMARY
GEOLOGIC STRUCTURES
A. Definitions
1. geologic structure -- any feature produced by deformation of rocks
2. folds -- a bend in rock
- limb--sides of a fold
-fold axis—a line dividing the two limbs of a fold and running along the crest
of an anticline or the trough of a syncline
- plunge: when the axis in inclined or tipped at an angle
- plunging fold: a fold with a plunging axis
- axial plane: an imaginary plane that runs through the axis and divides a fold
as symmetrically as possible into two halves
types of folds
a. monocline: has only one limb
b. anticline: an arch-like structure
c. Syncline: a bowl shaped structure
d. dome: a circular or elliptical anticlinal structure
e. basin: a circular or elliptical synclinal structure
3. fault --a fracture along which rock on one side has moved relative to rock on the other side
a. normal fault -- a fault in which the hanging wall moves down relative to the footwall normal
faults result form extensional tectonic force
b. graben – a wedge-shaped block of rock that dropped downward between two normal faults
c. reverse fault – a fault in which the hanging wall move up relative to the footwall
d. thrust fault – a special type of reverse fault that is nearly horizontal
e. strike-slip fault – fault is vertical, or nearly so, and the motion along the fault is horizontal
-- transform faults that offset the oceanic ridge are special types of strike-slip faults
Mountain Building- SUMMARY
4. joint -- a facture without movement of rock on either side of the break
1. a fracture similar to a fault except that rocks on either side of the facture have not moved
2. examples of jointsa.
columnar joints in basalt- occurs when basalt cools and shrinks causing polygonal cracking
b. exfoliation of granite
Concepts of Strike and Dip:
1. strike: the direction in which tilted layers of rock are oriented
-- always given as a compass direction
2. dip: the angle of inclination of the bedding plane
-- measured from a horizontal plane
3. strike and dip are used to measure any type of layering or other flat surfaces
in rocks
Types of Faults:
I. Dip-slip faults
a. Normal fault-hanging wall block moves down relative to footwall block.
b. Reverse and c. Thrust fault-hanging wall block moves up relative to footwall block.
II. Lateral faults
d. Strike-slip fault: all movement is parallel to strike of the fault.
e. Oblique-slip fault: combination of strike slip and dip-slip movements
Mountain Building- SUMMARY
Terms used to describe complex folds:
a. plunging fold: fold that dips in one direction
b. Inclined fold-axial plane not vertical, limbs dip at different angles
c. Overturned folds-both fold limbs dip in same direction, but one limb is inverted.
d. Recumbent folds-folds overturned so far that they are lying on their sides.
PLATE TECTONICS AND MOUNTAIN BUILDING
A. Orogenic Activity
1. orogeny refers to the process of mountain building
2. an orogen or orogenic belt is a long and relatively narrow region near a tectonically
active continental margin where any or all of those processes have formed mountains
B. Orogenic Activity and Plate Boundaries
1. rift boundary
a. mid-oceanic ridge
b. continental rifting is responsible for the uplift
2. transform fault boundary
a. high cliffs and escarpments form along transform faults offsetting the mid-oceanic ridge
3. convergent plate boundaries’
a. ocean-ocean convergence
volcanic mountain ranges
b. ocean-continental convergence
volcanic mountain ranges
c. continental-continental convergence
folded mountain belts
Mountain Building- SUMMARY
Plate-Plate interactions, resulting mountain ranges formed:
a. Island Arcs: primarily from oceanic-oceanic
b. Andes type mountains: oceanic-continental plate interactions
c. Himalayan mountains: continent-continent interactions
End result is continental accretion…’growth’ of continents
Geologic Maps- interpreting complex geology from ….
Reading a geologic map: how can tell if circular area is an anticline or syncline?:
a. In anticline, oldest exposed rocks are in center
b. In syncline, youngest exposed rocks are in center…
Horst and grabenshorst: an uplifted fault block bounded on at least two sides by faults
graben: a fault-caused depression bounded on at least two sides by faults…
Text: Historical Geology
Small scale folds
in sedimentary rocks.
Pen is 13.5cm long