Transcript Photo of the Grand Canyon

Mariner 10
Mercury
Discovery Rupes (Discovery scarp)
Thrust faults formed as the planet’s interior
cooled and shrank, causing compression in the
crust
Offset in a
stream along the
San Andreas
Fault, CA. This
photo is part of
the classic
collection of
John Shelton’s
book “Geology
Illustrated”,
considered to be
one of the most
influential
popular
scientific works
of the 20th
Century.
•Tectonism When forces build up on rocks in the earth, the rocks eventually slip or
break releasing the stored up energy in a sudden earthquake. Faults (planar offsets in
rocks and landscape) are the features that form when the earth slips and moves.
•Tectonism Graben are tectonic valleys (not erosional) formed in areas that are
being pulled apart (extensional). A normal fault forms on each side of the valley
and the middle drops down. These Graben valleys are in Canyonlands, Utah.
•Tectonism Normal faulting on a grand scale: The Teton
Mountains in Wyoming are on the up side of the fault, Jackson
Hole and other areas east of the mountains are on the down side.
•Tectonism A reverse fault forms under compressional forces. Notice the overlap in
the layers indicating shortening that results from compression. (Montana)
•Tectonism If rocks are very hot and under high pressure at the time of
compression, they will fold (deform plastically) rather than break. These metamorphic
rocks in Eastern Connecticut must have been deformed deep within the Earth and
brought to the surface by erosion much later.
•Tectonism
Syncline in a
Washington
County
Maryland
roadcut.
Notice that
this picture
also dispels
any notion
that a syncline
must be in a
valley!
Plunging
syncline
•Tectonism Plunging syncline just north of Half Moon Bay, CA. The word plunge
indicates that the entire fold has been tilted, in this case downward in the direction away
from the airplane.
Sequence of events:
1) deposition of
sediments that
become rocks, 2)
rocks are buried, 3)
rocks are folded
deep in the roots of
a mt range, 4) mts
erode away and the
folded rocks are
exposed with a river
flowing over them,
5) new tectonism
begins to uplift new
mountains and the
river erodes across
them.
•Tectonism This Space Shuttle picture reveals not only 300mya folding in the
Macdonnel mountain range in Australia (with compression in a north-south direction),
but an amazing sequence of events: Because it’s not likely that the river ever flowed up
over the mountains, the river must have been there before the mountains, and eroded
through the mountains at the same rate the mountains were rising up.
Stone Mountain, GA above
Weathering and Erosion.
Mechanical Weathering results from
unloading (causing exfoliation
parallel to the surface), freezing and
thawing of water (causing cracking in
rocks and shattering along joints),
heating and cooling (which causes
rocks to expand and contract,
breaking them apart), and other
factors.
Weathering and Erosion. Oxidation of iron combined with leaching of soluble
elements produces the red soils characteristic of humid, warm climates. Less-extensively
weathered bedrock is seen in the lower part of the image.
Weathering and
Erosion.
Gravity is one of the
primary erosional
forces. Mass Wasting
occurs when rocks
move downward under
the force of gravity.
Despite the warning
sign and flag, not all
tourists in Kefalonia
Greece understand the
risk!
Weathering and Erosion.
Soil moves downward under the force
of gravity in a process called creep.
The trees on the Wasatch Plateau in
Utah chart the movement of the soil
by always turning to grow upward as
the rotation of the soil tries to tilt them
over.
Weathering and Erosion.
Gravity and rain conspire to
create many unusual features.
Here resistant boulders provide a
protective cap for cone-shaped
Hoodoos.
Weathering and
Erosion.
Spectacular arches in
Arches NP, Utah are
produced by a
combination of
mechanical weathering
by wind blown sand,
chemical and
mechanical weathering
by water, presence of a
resistant cap rock,
vertical fractures in the
original rock layer
(now long gone), and
gravity.
Weathering and Erosion. Rainwater runoff has carved the castles and spires of Bryce
Canyon NP, Utah. Exposed roots of the small pine tree reveal the rate at which sediment
is eroding from the upper rim of the erosional amphitheater.
Weathering and Erosion. Dendritic (tree-like) drainage patterns form from surface
rivers flowing over areas of high rainfall and uniform underlying rock. Eastern flank of
Andes in Argentina. Space Shuttle image.
Weathering and Erosion. Classic work of river erosion, Grand Canyon as seen from
the south rim. More resistant layers form cliffs, less resistant layers form slopes.
Weathering and Erosion.
Sometimes after a river has reached its base level and a meandering channel is
established, tectonic events cause the region to be uplifted, and downward erosion is
renewed. The result is entrenched meanders, such as these along the San Juan River
in Utah.
Weathering and Erosion.
As a region erodes down to base level, a broad plain develops, often containing a few
remnant buttes or hills. Imagine the now-eroded layer of sandstone that must have once
stretched between the buttes. Mitten Buttes, Arizona, Cutler Formation.
Weathering and
Erosion. Energy from
wind-driven waves (not
just tides) can cause
significant coastal
erosion.
Clockwise from Upper left:
Ocean Beach San Francisco,
Great lakes, Oregon Coast.
Weathering and
Erosion.
One of the clues that
convinced early
geologists in the
1800s that the Earth
had to be “very old”
was the presence of
erosion surfaces
underneath other
rock. The feature
shown here (from
Portugal), tells the
story first of
sediments that were
deposited and turned
to rock (the lower in-place rocks). Then, these rocks were folded deep in the Earth’s crust
by powerful compressional forces that must have built mountains. Then the overlying
rocks and mountains were eroded away (producing the erosional surface). New
sediments were deposited on top of the erosional surface and turned to rock, and finally,
erosion exposed the whole sequence. This feature is called an Angular Unconformity.
Deposition and Stratigraphy. Animation of angular unconformity generation.
Deposition and
Stratigraphy.
Alternating episodes of
deposition and erosion are
necessary to explain the
presence of unconformities
such as this angular
unconformity at Shepherd
Point, Utah.
In general, erosion occurs
where rock is uplifted and
gravity, wind, water, or ice
carries weathered sediment
away. Deposition occurs
either where water/air
movement slows, allowing
particulate matter to settle out,
or where conditions are right
for sediments to crystallize
from solution.
Classic John Shelton photo
Deposition
and
Stratigraphy
.
The “V”
shaped canyon
in Death
Valley CA
reveals
erosion in the
upland. The
alluvial fan
forms where
sediment is
deposited in
the lowerenergy
environment
that occurs
where slope
decreases.
Deposition and
Stratigraphy.
The currently-active
part of the Mississippi
Delta as seen with the
ASTER imager of the
Terra Satellite.
Sediment is deposited
by branching
distributary channels
where the river water
stops flowing upon
encountering the sea.
With less water
movement, the
sediment settles out.
Sufficient sediment is
carried by the river to
extend the delta by
about 300 feet each
year.
Former Location
of Harbor
Deposition and Stratigraphy.
In the days when Paul the Apostle
spent two years preaching in Ephesus
on the western coast of Asia Minor,
the region of the Great Theater looked
out on a beautiful bay much like that
at modern Izmir. The walkway led
from the theater to the harbor. Years
later, sediment carried by the river
silted up the bay and the site is now
landlocked. Agricultural practices in
the region probably speeded up the
erosion and deposition process.
Deposition and Stratigraphy.
A
high energy environment, such as a
beach subject to severe storms, will
wash away small particles and deposit
larger particles. At left is a modern high
energy beach on the North Shore of
Lake Superior. Above is a
conglomerate (Schunemuck Mountain
Conglomerate) deposited on an ancient
Devonian high energy beach (much
later exposed and polished by glacial
erosion).
Above: Cypress swamp near Albany
Georgia.
Right: Barrier island and marsh
lagoon, Plum Island Massachusetts.
Deposition and Stratigraphy.
Low-energy environments of
deposition include a swamp (left)
and a lagoon (below on left,
protected from open ocean waves by
a barrier island). Sediments in the
swamp and lagoon will be muddy
and organic-rich. Sediments along
the higher-energy foreshore and
dunes will be sandy.
Deposition and
Stratigraphy.
MISR image of the
Great Barrier Reef,
Australia.
Deposition of
CaCO3 sediment
from solution in sea
water to make
limestone usually
occurs where the
supply of particulate
sediment is low.
Deposition of
chemical sediments
doesn’t depend on
the energy in the
environment of
deposition, since
there are no particles
to “settle out”.
Deposition and Stratigraphy. Coral reefs often form in warm climates where water
temperatures and salinities are very stable. The stability results in great diversity of life.
Great Barrier Reef, Australia
Deposition and Stratigraphy. Coral reefs often form in areas with lots of sunlight
and food, resulting in a great abundance of life GBR
Deposition and Stratigraphy. A common, if somewhat simplified, sequence of sediments
on a continental shelf (going from the beach out to sea) is sandstone (where moderate energy waves
wash finer sediment away), mud (both silt and clay deposited in deeper water where there is less
water movement), and carbonate sediments (CaCO3-based sediments deposited where little
particulate sediment reaches). These sediments become the rock types sandstone, shale, and
limestone.
A concept called Walther’s Law suggests that sediments that occur adjacent to each other in modern
environments may have become stacked on top of each other in the past due to migration of
environments through time.
Deposition and Stratigraphy. Many layers are stacked on each other in the Grand Canyon
AZ. Notice three layers in the lower right: TS = Tapeats sandstone, BAS = Bright Angel Shale,
and ML = Mauve Limestone. Notice that they are in the same sequence typical of shorelines, but
stacked on top of each other instead of side by side.
Deposition and Stratigraphy. The sequence (upward) of sandstone, shale, limestone can
occur in a stacked sequence if an ocean advances over an area slowly, depositing sandstone first,
then as the water deepens, shale and limestone later (and thus on top of the sandstone). Therefore,
this sequence at the Grand Canyon is the record of an ocean advancing over this area during the
Cambrian time period.
Deposition and Stratigraphy. When the sea transgressed over the region of the
Grand Canyon, it advanced over a pre-existing erosional surface. Rocks exposed at that
ancient surface included metamorphic Schist, Igneous Granite, and tilted PreCambrian
sedimentary layers. The surface beneath the Tapeats Sandstone (labeled with a “2”) is
called the Great Unconformity. The surface labeled “1” is an even more ancient
unconformity.
Deposition and Stratigraphy. Tilted Precambrian rocks lie below the Great Unconformity
in the lower half of the image. Above the unconformity is, in sequence, the Tapeats Sandstone
(making a cliff), Bright Angel Shale (making a slope), and the Mauve Limestone (making a short
cliff below the more prominent cliff-former at the top of the butte). This tells the following story:
1) ancient deposition, 2) tilting of rocks, 3) erosion of rocks, 4) advance of an ocean that deposited
new rocks, and finally 5) recent erosion of the canyon that has exposed it all.
•Life Processes
Tree roots cause both physical and
chemical weathering. Physical
weathering is due to the outward
pressure provided by turgor pressure
in the cells that can force cracks
wider. Chemical weathering is due
to acids produced by the tree and ion
exchanges of the rootlets.
•Life Processes
Humans are one of the most significant erosional forces on Earth, moving
more rock and sediment per year than almost any other single process. This is the Kennecoff Mine
in Utah, an open pit mine producing copper, molybdenum, gold and silver.
Ice Ages, modern human
•Life
Processes
Almost all species
that have ever
lived are now
Dinosaurs, flowering
extinct, and the
plants
creatures alive
Dinosaurs, conifers
today did not
Dinosaurs, mammals
exist just a
Reptiles, mammal-like
geological
reptiles
moment ago (that
Reptiles, giant insects,
is, we do not find
coal swamps
fossils of those
Amphibians and crinoids
creatures in rocks
from the
Sharks and armored fish
geological past).
Therefore, we
Land Plants and Eurypterids
reasonably
conclude that life
Fish and Coral
is a dynamic and
Trilobites
changing part of
Earth.
Hominids, whales,
Mammals
Eukaroytic life, single celled and simple organisms
Prokaryotes, blue green algae and cyanobacteria
•Life Processes
Example life from the Mesozoic, the Age of Reptiles. Percent of Mesozoic
species extinct > 99%. Clockwise from u-left, Tricerotops (a ceratopsian), Chasmosaur Belli (a ceratopsian),
Tyrannosaurus (a therapod), and Lambeosaurus (an ornithopod), all dinosaurs.
The following slides
are a preview of
plate tectonics for
our next section
•Tectonism
Illustration of the main types of
Tectonic Plate boundaries and their
features (including mountain belts,
volcanic chains, isolated volcanoes,
trenches, and high plateaus. Earth is
the only planet known with this style
of Plate Tectonics.
Space Station Earth Volcanoes in Mexico form
chains running parallel to the plate tectonic convergent
boundary.
•Tectonism Compressional forces at convergent plate boundaries can
thicken the crust in those areas, producing mountains. French Alps.
Lakes and volcanoes
in the rift valley.
Space Shuttle Topographic Map of East Africa Rift.
Gemini 11 (right),
Apollo 17 (above)
Plate tectonic rift, opening
of the Red Sea and Gulf
of Aden between Arabian
Peninsula and Horn of
Africa
•Tectonism Illustration of plate motions, plate boundaries, and changes in
continents from the end of the Precambrian until today; NAM = North America,
BAL=Baltic 1 sec = 10 million years.
after PGISMac by Malcolm Ross and
Christ Scotese and Paleomap Project by Chris Scotese.