Transcript Yosemite National Park

Geology of National Parks
Yosemite
Instructor:
Jack Pierce
Yellowstone
Grand Canyon
Bryce Canyon
Lassen Peak Volcanic
Geology of National Parks Overview
“More than a Park Ranger Discussion of National Parks”
 This class surveys basic geologic principles which include the
physical and historical geological characteristics of each
national park.
 Hands-on examples of various rock types will be presented for
each national park.
 Discussion will include various geological interest points within
each national park.
National Park Geologic Background
• The Rock Cycle - Igneous, Sedimentary, Metamorphic Rocks
• Plate Tectonic Overview
• The Geologic Time Scale
Geologic Time Scale
The Rock Cycle
Plate Tectonics
Pangaea
Alfred Wegener
Alfred Wegener – German Meteorologist – geophysicist
•Proposed that continents drift - observed Africa and South America
(middle 1920’s)
•Proposed the super-continent Pangaea
•Found evidence to support his hypothesis of Continental Drift
•fossil evidence, rock and climate correlations
• Continental Drift hypothesis was NOT accepted --- Wegener the
laughing stalk of the science world
• Could not answer HOW the continents move
Seafloor spreading
Magma rises from the mantle “spreading apart” and
creates new ocean floor – discovered in the 1960’s
oldest
ocean floor
Mantle
Youngest ocean floor
Magma
oldest
ocean floor
Mantle
The Theory of Plate Tectonics
solid rigid lithospheric plates riding along a “plastic”
(hot wax) material called the asthenosphere
continental and ocean floor material
lithosphere
upper mantle
Asthenosphere – hot mantle rock that behaves
like “hot wax”
lithosphere
Asthenosphere
Continents Ride on Plates
Plate
Africa
3-Types of Plate Boundaries
• plates move relative to one another
• all major interactions of deformation occur at plate boundaries
• most volcanic and EQ activities occur at plate boundaries
Convergent Boundaries
Ocean-Cont
Divergent Boundary:
• new ocean floor is produced (2-3 cm/yr)
• ocean floor “grows” equally on both
sides of the ridge’
• Example: Mid-Atlantic Ridge
Ocean-Ocean
Cont-Cont
Transform Boundary
• plates slide past one another
• common on the ocean floor
• Example: San Andreas Fault
The Geologic Time Scale:
• Created during the nineteenth century in Western
Europe and Great Britain
• Sub-divides the 4.6 billion-year history of the earth
• Eons
Eras
Larger
Time Frame
Periods
Epochs
Smaller
Time Frame
Age of Reptiles
Amphibians
Age of fish
Invertebrates
Yosemite National Park
Established in 1890
California
Yosemite
National
Park
Yosemite Valley
Intrusive Igneous Rocks of Yosemite
• Magma cools below the surface
• Magma cools slowly
• Visible mineral grains
• quartz, orthoclase, plagioclase, hornblende,
biotite
• Various forms of granite
K, Al, Na, Ca, SiO2
• granodiorite, tonalite, quartz-monzonite
NEVER TAKE GRANITE FOR GRANITE !
Diorite
increasing Fe, Mg
decreasing SiO2
Gabbro
How did the granite form to produce Yosemite?
The Yosemite Tectonic Setting
Ocean to Continent Convergence
A
A. convergence of plates
D
B
Yosemite
Granite
C
B. subduction of oceanic
plate material
C. oceanic plate melts
and less dense magma
rises
D. formation of composite
volcanoes (explosive)
on the continent
El Capitan
Monolith
Cathedral Rocks
granite
Half Dome
Quartz Monzonite
granite
The land continued to push up, the water continued
to erode down. The V-shaped valley got deeper.
The mountains were pushed up higher from an
increase in activity below the earth’s surface, causing
the Merced river to dig deeper (erode) into its valley.
25-2 m.y.
Uplift and erosion stripped away most
of the sedimentary layers, leaving the
cooled magma (granite) exposed.
80-25 m.y.
220-80 m.y.
Layered sediments covered
the sea floor west of the North
American continent.
470-220 m.y.
The sediment eventually filled Lake Yosemite and
turned it into the meadows and forests of today.
Today
The glaciers melted out of Yosemite Valley.
10,000 yrs
An ice age occurred. Glaciers from the highest
parts of the Sierra Nevada grew so big that they
flowed down through Yosemite Valley, widening and
cutting the Merced river’s path.
2 m.y.
Geologic Forces that Shaped Present Day
Yosemite National Park
Glacial Processes
• Strong erosive glacial forces produce the striking topography
observed today throughout Yosemite Valley
Weathering and Mass Wasting Processes
• Physical / Chemical processes work 24/7 to reduce
topography of the Yosemite Valley
• Mass wasting processes involve the down-slope movement
of weathered material under the influence of gravity
Glacial Processes
Alpine (valley) glaciation
•Exists in high mountainous areas
•Occupies pre-existing river valleys
•Typically bounded by steep
mountain walls
Continental glaciation
•Extensively covers continents
•Greenland, Antarctica
•Flat broad large ice sheets
•Typically, lengths are longer than
width
Yosemite
Great Lakes U.S.
Evidence of past glacial activity in Yosemite – What do glaciers leave behind?
River valleys before
glaciation (note the “V”
shape valleys)
Glaciation takes place and
invades the “V” shape
valleys
Glacial Erosion – the forces of glacial activity
Erosional features left after
glaciation creating various
erosion features
U-shaped valley
horns
cirques
areates
glacial moraines
hanging valleys
Glacial Erosion – Glacial landforms from erosion
Plucking processes
Frost wedging
Ice body moves across bedrock
fracturing rock material.
Frost wedging processes migrate
through cracks and rock material
is plucked out on the low pressure
side.
Abrasion processes
Rock material embedded within
the glacier moves along the surface
and scratches, scours and produces
grooves and striations
Acts like sandpaper and “polishes”
the rock surface.
Yosemite
Can you see the U-shaped valley?
Can you see the hanging valley?
Yosemite Region
Glacial moraines: Layers of “rock debris” (piles of rock) left over from an
advancing / retreating glacier
Tuolumne Meadows
(lateral moraine)
Base of Half Dome
(medial moraine)
Beginning of Yosemite
Valley – terminal moraine
Erosional features (landforms) of Alpine glaciation
Roches Moutonnees
elongated hills that appear to be
asymmetrical bedrock knobs that
protrude upwards from the bedrock
Direction of glacier
plucking
Direction of glacier
plucking
Glacier moves over resistant bedrock
producing a gentle slope and plucking
of debris steepens the opposite side
Lembert Dome - Yosemite
flow direction of glacier
How did this boulder get here?
Erratic
• “huge” rock fragments “floating” in the moving glacier
• glacier melts and literally drops
and the boulder settles to
the ground.
Yosemite Erratic
Weathering Processes at Yosemite
Rocks can be broken down or chemically altered on the
earth’s surface.
Mechanical Weathering:
Rocks are physically broken down by various
surface processes into smaller parts without
changing physical properties.
Chemical Weathering:
Rocks are altered from one form to a
completely new form with a different
set of physical properties.
Yosemite Weathering
Frost Wedging
• Repeated cycles of freezing
and thawing.
• Ice expands 9% of its volume.
• Rock is continually broken
down over geologic time
• Part of the Yosemite
weathering cycle.
Frost Wedging – rocks are broken into
plates and split apart.
Yosemite Frost Wedging
Exfoliation
• unloading of overlying rock
material releases pressure
• expansion of granitic bodies
causing “sheeting” or slabbing
granite
• granite “sheets” break along
planes of weakness or joints
• exacerbated by frost wedging
• dominant weathering force
in Yosemite
Exfoliation Dome – granite is
“sheeting” from loss of pressure due
to unloading processes.
Exfoliation at Half Dome
Royal Arches - Exfoliation
Yosemite Exfoliation
Lembert Dome
Tunnel to view
the U-shaped Valley