Coastal & Arid Geomorphology

Lab 9

Coastal GeomorphologyConcepts

Principal Forces for Erosion (5)

Landforms Barrier Island/Lagoon Barrier Spit Beach Coral Reefs Tombolo Wave-cut arch & platform

1. Tides 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

in ocean Creates emergence (above water) and submergence

(below water) coastlines

Coastal GeomorphologyPrincipal Forces for Erosion

Coastal GeomorphologyPrincipal Forces for Erosion (con’t)

3. Waves Most important erosional force

4. Currents 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 Spills sediments out into the ocean and adds material to

beaches Deltas often form at the mouth of rivers – why?

Coastal GeomorphologyLandforms: 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 GeomorphologyLandforms: Barrier Spit

Barrier Spit

• Coastal barriers thatextend into open water,attached to themainland at one end. • Can develop into abarrier island if itbecomes separatedfrom the mainland.

Coastal GeomorphologyLandforms: Beaches

Beaches Made of eroded continental material (sand, gravel, rock fragments) that are

washed to the sea by streams.

Sediment gets suspended in sea water and is often transported further down the coastline by longshore currents.

Longshore currents provide a continual onshore-offshore movement which pushes the sand along the beach edge. Often called “littoral drift.”

• 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 GeomorphologyLandforms: Coral Reefs

Coastal GeomorphologyLandforms: Tombolo

Tombolo• 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 GeomorphologyLandforms: 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.

ARCH

PLATFORM

Deserts & Arid GeomorphologyConcepts Locations

Typical Conditions (4)

Principal Forces (2)

Landforms Alluvial Fan/Bajada Inselberg Playa Buttes, Pinnacles, Mesas Dunes

Deserts & Arid GeomorphologyLocations

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Desert & Arid Geomorphology4 Typical Conditions Sparse cover of vegetation and soil

Bedrock is exposed

Impermeable surface layers This increases the erosion potential of rainstorms!

Rain is infrequent, but short-lived & intense Causes dry streambeds to fill quickly & flow rapidly Flash flooding = fast erosion!

Interior drainage = centripetal Ephemeral streams flow to the bottom of basin Alluvium is deposited and water evaporates,

sometimes leaving behind a salt layer

Deserts & Arid GeomorphologyPrincipal Forces for Erosion1. Water

Water may be scarce, but rain/thunderstorms are often intense.

Rivers or floodwaters cut into the landscape, often creating canyons.

2. Wind Often removes sand/silt

from areas, exposingbedrock to the erosionalpower of water.

Desert & Arid GeomorphologyLandforms: Alluvial FanAlluvial Fan

When streams come out of steep canyons… 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 joinalong a mountain front

Desert & Arid GeomorphologyLandforms: Inselbergs & PlayasInselberg

Isolated hill/ridge, steep-sided Formed when less-resistant

material is eroded away frommore-resistant rock (often anintrusive formation).

Example: Uluru (Ayers Rock), AU

Playa Dry lake bed Found at lowest point

of basin Recognizable by dried

mud, often covered w/crust of salt

Desert & Arid GeomorphologyLandforms: Pinnacles, Buttes, MesasPinnacles 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 Larger than a pinnacle, flat top Example: Chimney Rock, NE

Mesas Larger than a butte, smaller than a

plateau. Flat top.

Desert & Arid GeomorphologyLandforms: Sand Dunes Shape & formation depends on…

Amount of sand available Persistence & direction of wind Presence of vegetation

With little vegetation: can be movement over time

4 main shapes Barchan Transverse Longitudinal Blowout

Desert & Arid GeomorphologyLandforms: Sand DunesBarchan

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 GeomorphologyLandforms: Sand DunesTransverse

Less uniform than Barchan

More supply of loose sand

Maintains general crescent shape

Usually forms interconnected ridges of sand

Desert & Arid GeomorphologyLandforms: Sand DunesLongitudinal

a.k.a. “Seif” Wind directions shifts back & forth Long parallel ridges

Egypt

Desert & Arid GeomorphologyLandforms: Sand DunesBlowout

Wind erosion Scoops out sand

and rock,creating closeddepressions

Depressionsmay fill withwater