Transcript Lab 9 Lecture

Coastal & Arid
Geomorphology
Lab 9
Coastal Geomorphology
Concepts
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Principal Forces for Erosion (5)
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Landforms
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Barrier Island/Lagoon
Barrier Spit
Beach
Coral Reefs
Tombolo
Wave-cut arch & platform
Coastal Geomorphology
Principal Forces for Erosion
1. Tides
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Oscillations of ocean water: gravitational pull of the Moon
and Sun
24 hours: High-Low-High-Low tide periods
2. Sea-level changes
 Rising/Falling results from tectonic activity or amount of water
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in ocean
Creates emergence (above water) and submergence
(below water) coastlines
Coastal Geomorphology
Principal Forces for Erosion (con’t)
3. Waves
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Most important erosional force
4. Currents
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Large volumes of water moving horizontally
Consistent winds create currents
“Longshore” currents transport sediment down a coastline
(helps in formation of beaches)
5. Stream Outflows
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Spills sediments out into the ocean and adds material to
beaches
Deltas often form at the mouth of rivers – why?
Coastal Geomorphology
Landforms: Barrier Islands/Lagoons
Barrier Island
• Long, narrow, low island
that lies parallel to a
shoreline.
• Buffers the mainland from
storms and large waves.
Lagoon
• Nearly isolated body of
water, separated from sea
by the barrier island.
• Low-energy waves and
relatively calm area, so
sediments may accumulate.
•Mudflats, marshes, swamps
Coastal Geomorphology
Landforms: Barrier Spit
Barrier Spit
• Coastal barriers that
extend into open water,
attached to the
mainland at one end.
• Can develop into a
barrier island if it
becomes separated
from the mainland.
Coastal Geomorphology
Landforms: Beaches
Beaches
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Made of eroded continental material (sand, gravel, rock fragments) that are
washed to the sea by streams.
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Sediment gets suspended in sea water and is often transported further down the
coastline by longshore currents.
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Longshore currents provide a continual onshore-offshore movement which
pushes the sand along the beach edge. Often called “littoral drift.”
Coastal Geomorphology
Landforms: Coral Reefs
• Reef formation first determined by Charles Darwin, mid-1800s.
• Occurs in tropical zone – approx 30oN to 30oS.
• More coral species live in the Pacific than anywhere else!
Fringing Reef
• Forms along shoreline of volcanic island (hot spot) – it likes the warm waters!
Barrier Reef
• Island begins to sink or erode, but reef continues to grow upward. Lagoon is created between the top of
the reef and the sinking island.
Atoll
• Island sinks/erodes below sea level, reef continue to grow upward.
• If a “broken circle” of reef, it is likely due to storm action.
Coastal Geomorphology
Landforms: Tombolo
Tombolo
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A narrow piece of land between the shore and an island, or between two
islands.
Forms because wave refraction around islands causes sand and sediment to
build up in a linear formation where the waves meet (around the backside of the
island).
Goat Rock Beach, CA
Coastal Geomorphology
Landforms: Wave-cut Arches & Platforms
Wave-cut Arches
• Formed by wave action which erodes less-resistant rock from an
outcrop.
Wave-cut Platforms
• Formed after waves hit against a cliff face, causing undercutting.
• Most obvious at low tide when they become visible as huge areas of
flat rock.
• An “extreme” environment (for marine life) because of continual
wave action.
PLATFORM
ARCH
Deserts & Arid Geomorphology
Concepts
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Locations
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Typical Conditions (4)
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Principal Forces (2)
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Landforms
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Alluvial Fan/Bajada
Inselberg
Playa
Buttes, Pinnacles, Mesas
Dunes
Deserts & Arid Geomorphology
Locations
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Desert & Arid Geomorphology
4 Typical Conditions
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Sparse cover of vegetation and soil
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Impermeable surface layers
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This increases the erosion potential of rainstorms!
Rain is infrequent, but short-lived & intense
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Bedrock is exposed
Causes dry streambeds to fill quickly & flow rapidly
Flash flooding = fast erosion!
Interior drainage = centripetal
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Ephemeral streams flow to the bottom of basin
Alluvium is deposited and water evaporates,
sometimes leaving behind a salt layer
Deserts & Arid Geomorphology
Principal Forces for Erosion
1. Water
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Water may be scarce, but rain/thunderstorms are often
intense.
Rivers or floodwaters cut into the landscape, often
creating canyons.
2. Wind
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Often removes sand/silt
from areas, exposing
bedrock to the erosional
power of water.
Desert & Arid Geomorphology
Landforms: Alluvial Fan
Alluvial Fan
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When streams come out of steep canyons…
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the velocity of the stream drops dramatically, and…
the slope of the river
decreases.
Therefore, alluvium is
deposited!
Edges of the fans may
be clearly defined.
Bajadas are formed when
multiple alluvial fans join
along a mountain front
Desert & Arid Geomorphology
Landforms: Inselbergs & Playas
Inselberg
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Isolated hill/ridge, steep-sided
Formed when less-resistant
material is eroded away from
more-resistant rock (often an
intrusive formation).
Example: Uluru (Ayers Rock), AU
Playa
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Dry lake bed
Found at lowest point
of basin
Recognizable by dried
mud, often covered w/
crust of salt
Desert & Arid Geomorphology
Landforms: Pinnacles, Buttes, Mesas
Pinnacles
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Isolated hill with steep sides and pointed or flat
top
“Caprock” (resistant) remains while softer rock
is eroded away
Typical of southwest U.S.
Buttes
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Larger than a pinnacle, flat top
Example: Chimney Rock, NE
Mesas
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Larger than a butte, smaller than a
plateau. Flat top.
Desert & Arid Geomorphology
Landforms: Sand Dunes
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Shape & formation depends on…
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Amount of sand available
Persistence & direction of wind
Presence of vegetation
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With little vegetation: can be movement over time
4 main shapes
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Barchan
Transverse
Longitudinal
Blowout
Desert & Arid Geomorphology
Landforms: Sand Dunes
Barchan
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Limited supply of loose
sand
Crescent-shaped
“Horn” points downwind
Wind blows constantly
in a single direction
Migrates downwind
over long time
Which way is the wind blowing?
Desert & Arid Geomorphology
Landforms: Sand Dunes
Transverse
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Less uniform than
Barchan
More supply of loose
sand
Maintains general
crescent shape
Usually forms
interconnected ridges of
sand
Desert & Arid Geomorphology
Landforms: Sand Dunes
Longitudinal
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a.k.a. “Seif”
Wind directions shifts back & forth
Long parallel ridges
Egypt
Desert & Arid Geomorphology
Landforms: Sand Dunes
Blowout
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Wind erosion
Scoops out sand
and rock,
creating closed
depressions
Depressions
may fill with
water