Download The Unstable Landscape: California Plate Tectonics

Physical Geography:
Landforms of California
Overview
•
•
•
•
•
•
Geologic Time
Movements of the Continents
Earth Materials
Tectonic Forces
Weathering and Erosion Processes
Erosional Agents and Deposition
Geologic Time
Pretend the age of the earth (4.6+ billion years) is
compressed into one calendar year.
January 1 - Earth and planets formed
Early March - liquid water stands in pools.
Late March - earliest life
July - oxygen is important part of atmosphere
October 25 - multicellular organisms
Late November - plants and animals abundant
December 15 to 25 - dinosaurs arise and disappear
11:20 pm, December 31 - Humans appear
One second before midnight - Automobile invented
The Earth’s Interior
• General trends: temperature, density
• Horizon composition, behavior
Distance: 6730 km (3963 miles)
Earth Materials
• Three major rock types
– Igneous
– Sedimentary
– Metamorphic
Igneous Rocks
• Igneous (ignus = fire)
• Formed from the cooling of molten rock
(magma/lava), a process called crystallization.
– Slow cooling  larger crystals > dense rock
– Rapid cooling  small crystals > lighter rock
• Two classes of igneous rocks
– intrusive: formed inside the Earth
– extrusive: formed at Earth’s surface
Igneous Intrusive Rocks
• Cools slowly (thousands of years)
• Visible crystals
• Examples
- granite
- diorite
- gabbro
Igneous Extrusive Rocks
• Cools rapidly - exposed to surface
• No visible crystals
• Examples
- rhyolite
- andesite
-basalt
Typical Igneous Intrusions
Know: Batholith and Dike
Exposed Batholiths
Sierra Nevada, CA
Sedimentary Rocks
Sedimentary
Rocks
Relative Abundance by
Type
Compaction
Cementing
Formation
Limestone (CaCO3)
Shale (fine grains)
Sandstone (larger grains)
Where do Sedimentary Rocks
Form?
Terrestrial environments
(non-marine)
Marine environments
 Continental shelf
 Rivers and floodplains
(fluvial environment)
 Continental slope and
rise (deep sea fans)
 Lakes
 Abyssal plain
 Deserts (aeolian
environment)
 Beach and barrier
islands
Metamorphic Rocks
or That’s very Gneiss, but I
don’t give a Schist!
Schist (narrow foliation)
Gneiss (broad foliation)
The Unstable Landscape:
California Plate Tectonics
• Crustal Processes
– Destruction (subduction)
– Creation (volcanism )
– Alteration / deformation (folding and faulting)
Introduction
• Plate boundaries: main location for
Earth’s volcanic and earthquake activity.
• Type of plate boundary determines activity.
• 3 types
– diverging (spreading)
– converging (colliding)
– transform (sliding past each other)
Convergent Plate Boundaries
• Action:
– collision; destructional or constructional
• Activity:
– depends on type of convergence
– 3 types: ocean-continent, ocean-ocean, cont.-cont.
Convergent: Ocean-continent
• Action:
– collision; destructional (subduction of ocean plate)
• Activity:
– shallow to deep earthquakes; volcanism (continental)
• Features:
– ocean trench; volcanic mtns on continental margin
Volcanoes: Explosive
• Composite cones (stratovolcano)
– pointed, steep-sided, tall volcanoes
– “Composite”: layers of pyroclastics and lava
(mostly felsic)
– Explosive and dangerous; found near subduction zones
Volcanoes: Explosive
Arenal, Costa Rica
Mt. Shasta, California
Mt. Lassen, California
Crustal Deformation:
Folding, Faulting, and Earthquakes
Introduction
• Crustal Processes
– Destruction (subduction)
– Creation (volcanism - convergent/divergent)
– Alteration / deformation (folding and faulting)
Crustal Deformation
• Outcome / result of “battle”:
Stress v. strain (force v. resistance)
– Stress: force imposed on the rock
(tension, compression and shear)
– Strain: how the rock responds to the stress
(folding / bending or faulting / breaking)
Is the rock brittle or ductile?
Figure 12-7
Faulting
• Definition: fractures where some type of
displacement (movement) has occurred
along a break in rock.
• Three types
– normal
– reverse/thrust
– transform
(strike-slip)
Carmel Valley Fault, CA
Normal Faults
• Tensional stress
• Earthquake and displacement along fault plane  fault scarp
Landforms - Normal Faulting
Owens Valley, CA
Sierra Nevada, CA
Grand Tetons, WY
Basin and
Range
• Horst and graben
(“hill” and “grave”)
Death Valley/
Panamint
Ranges
Why
saline?
Landforms: Normal Faulting
• Grabens (“Graves”)
Basin and Range
Transform Plate
Boundary
• Action:
– shear (lateral motion)
– no loss/gain of
plate material
San Andreas fault system
– How long is it? About 1000 km
– Relative motion of the Pacific
Plate? @ 2 inches (5 cm) northwest per year. In 10
million years Los Angeles will be off of San
Francisco .
San Andreas Fault System Southern California
Transform Plate Boundary
• Activity:
– shallow to moderate earthquakes
– little to no volcanism
Tremblor
Range
Dragon’s
Back
Carrizo Plain, CA (view to the east)
http://quake.usgs.gov/recenteqs/
Transform Plate Boundary
•
Features:
– shallow, linear
rift valleys
– sag ponds
Carrizo Plain,
central CA
San Andreas Lake
(Crystal Springs Reservoir)
- looking south along fault
- San Francisco water supply
- geology  vegetation
Transform Plate Boundary
• Features:
– offset streams, objects
Stream channel offset,
Carrizo Plain,
central CA
1906 earthquake offset,
Point Reyes, CA
The Geography of Earthquakes
• USA: 1977-1997 earthquake events
• USA: every state except ND, FL
The Geography of Earthquakes
• Globally: primarily at plate boundaries
• Intraplate earthquakes do occur!
Mag 6.5
Earthquakes
• Earthquakes are the shaking or vibration of
the ground as a result of rocks suddenly
breaking along a fault.
• Focus (hypocenter) = rupture point
• Epicenter = point on surface above focus
• Foreshocks
• Aftershocks
Process: the earthquake cycle
(elastic rebound theory)
• Earthquakes are a ‘release of energy’ in the
form of a seismic wave (vibrates the crust).
• Plate movement  strain builds rocks
“locked together” (frictional bond)
• Rocks bend  hit limit --> rupture/break
• Cycle repeats ”start-stop” motion along
fault
Seismic waves
• Some of the waves that are generated by an
earthquake travel within the earth and other
travel along the surface, creating surface waves.
• Waves traveling within the earth are known as
body waves.
Surface Waves
• Surface waves cause the most damage to buildings
during an earthquake.
• Surface waves can set up liquefaction in wet
alluvium. This is where the most extensive
damage to buildings occurs.
– Liquefaction: wavelike, almost liquid, rolling of
surface
– Alluvium: fine material deposited by water over
many years.
Measuring Earthquakes
• seismograph: records the vibrations
of the crust
• Richter Scale measures
vibration, not damage.
• seismogram: tracing record
Major California Earthquakes
• Fort Tejon, 1857 - 8.0 magnitude
• San Francisco, 1906 - 7.9 magnitude
• 1933 Long Beach - 6.3 magnitude
Destroyed Glendale College Buildings!
• San Fernando, 1971 - 6.6
• Northridge, 1994 - 6.7
• Hector Mine, 1999 - 7.1
• TIME: January 9, 1857
Fort Tejon,
1857
• LOCATION: 35° 43' N, 120° 19' W
• about 72 km (45 miles) northeast of San
Luis Obispo
about 120 km (75 miles) northwest of
Bakersfield,
as shown on the map (epicenter location
uncertain).
• MAGNITUDE: Mw 8.3 (approx.)
• TYPE OF FAULTING: right-lateral
strike-slip
• FAULT RUPTURED: San Andreas
fault
• LENGTH OF SURFACE RUPTURE:
about 360 km (225 miles)
• MAXIMUM SURFACE OFFSET: about 9
meters (30 feet)
San Francisco Aftermath, 1906
Magnitude: 7.9
San Francisco, 1906
Magnitude: 7.9