class 3 geol u4

_{The Coastal Zone}

_Shoreline

• _{Shoreline = line that marks the contact between land and sea} 

_{– Low-tide shoreline} 

_{– High-tide shoreline}

_{Coastal Processes: Wave Refraction}

• _{Wave refraction is the bending of a wave}

• _{Waves generally do not approach the shoreline parallel to the shore but bend as they enter shallow waters (primarily due to changes in their velocity)}

• _{As the wave base touches the bottom, waves slow down}

• _{The part of a wave in shallow water moves slower than the part in deeper waters}

• _{Hence, when the depth under a wave crest varies with respect to the crest, waves bend (refract)}

_{Coastal Processes: Wave Refraction}

• _{Consequences of wave refraction:} 

_{– Wave energy is concentrated against the sides and ends of headlands}

• _{They erode away} 

_{– Wave energy is weakened in bays} 

_{– Over time, wave refraction straightens irregular coastlines}

_{Coastal Processes: Wave Refraction}

• _{As these waves approach nearly straight on, refraction causes wave energy to be concentrated at headlands (resulting in erosion) and dispersed in bays (resulting in deposition).}

• _{Waves travel at original speed in deep water}

• _{Waves “feel bottom” and slow down in surf zone}

• _{Result: waves bend so that they strike the shore more directly}

_{Coastal Processes: Longshore Currents}

• _{Even though they bend and become nearly parallel to the shore, most waves still reach the shore at a small angle} 

_{– as each wave arrives, it pushes water along the shore, creating a longshore current within the surf zone (the areas where waves are breaking)}

• _{Longshore currents (water!) easily move fine suspended sand along the coast}

_{Coastal Processes: Beach Drift}

• _{Because waves reach the shore at an angle, when they wash up onto the beach, they do so at an angle} 

_{– The upward-moving water, known as the swash, pushes sediment particles along the beach}

• _{When that same wave water flows back down the beach, it moves straight down the slope of the beach} 

_{– The downward-moving water, the backwash, brings them straight back}

• _{With every wave that washes up and then down the beach, particles of sediment are moved along the beach in a zigzag pattern}

_{Coastal Processes: Beach Drift}

• _{Sediment transported along the beach face in a zigzag pattern called beach drift}

• _{The movement of particles on a beach as a result of swash and backwash}

_{Coastal Processes: Longshore Drift}

• _{The combined effects of (1) sediment transport within the surf zone by the longshore current and (2) sediment movement along the beach by swash and backwash is known as longshore drift}

• _{Longshore drift moves a tremendous amount of sediment along coasts (both oceans and large lakes) around the world} 

_{– responsible for creating a variety of depositional features}

• _{Beach drift occurs as incoming waves carrying sand at an angle up the beach, while the water from spent waves carries it directly down the slope of the beach. Similar movements occur offshore in the surf zone to create the longshore current.} 

• _{These waves approaching the beach at a slight angle near oceanside, California, produce a longshore current moving from left to right.} 

_{Coastal Processes: Rip Currents}

• _{Rip currents flow in the opposite direction of breaking waves} 

_{– Most backwash from waves moves back to the open ocean as sheet flow along the ocean bottom} 

_{– Rip currents are concentrated movements of backwash on the ocean surface}

_{Coastal Processes: Rip Currents}

• _{Rip currents flow straight out from the shore and are fed by the longshore currents}

• _{They die out quickly just outside the surf zone} 

_{– can be dangerous to swimmers who get caught in them}

• _{If part of a beach does not have a strong unidirectional longshore current, the rip currents may be fed by longshore currents going in both directions}

_{Coastal Processes: Rip Currents}

•  _{Look for:} 

_{– Deeper and/or darker water} 

_{– Fewer breaking waves} 

_{– Sandy colored water extending beyond the surf zone} 

_{– Debris or seaweed}

_{– Significant water movement} 

• _{Rip currents on Tunquen Beach in central Chile}

_{Coastal Features}

• _{Features vary depending on several factors, including:} 

_{– The proximity to sedimentladen rivers} 

_{– Degree of tectonic activity} 

_{– Topography and composition of the land} 

_{– Prevailing winds and weather patterns} 

_{– Configuration of the coastline}

_{Coastal Processes: Coastal Erosion}

• _{Wave erosion} 

_{– Breaking waves exert a great force}

• _{Atlantic winter waves average 10,000 kilograms per square meter}

• _{Abrasion (grinding action of water with rock fragments) is another erosional force of waves} 

_{– Very intense in the surf zone}

_{Coastal Processes: Coastal Erosion}

• _{Wave action operates like a horizontal saw cutting at the base of the cliff like this one in Oman}

• _{Smooth, rounded rocks along the shore are an obvious reminder that abrasion can be intense in the surf zone}

_{Landforms and Coastal Erosion}

• _{Bays and headlands created in areas where:} 

1. _{geology or rock type runs at right angles to the coastline and}

2.  _{there are alternating bands of harder and softer rock}

• _{Hydraulic action, abrasion and corrosion more effective at eroding the softer rock} 

_{– Softer rock will erode further inland than the harder rock}

_{Landforms and Coastal Erosion}

• _{The formation of bays and headlands}

1. _{A coastline faces the wave attack with discordant beds of sediment}

2. _{Wave attack causes hydraulic action and attrition which causes the cliff to retreat.} 

3. _{Less resistant rocks are eroded at a faster rate to create bays, more resistant rocks stick out to sea as headlands.} 

4. _{During calm periods, the sheltered bays allow deposition of beaches}

_{Landforms and Coastal Erosion}

• _{When waves approach an irregular shore, they are slowed down to varying degrees, depending on differences in the water depth, and as they slow, they are bent or refracted}

_{Landforms and Coastal Erosion}

• _{A headland is a cliff that sticks out into the sea and is surrounded by water on three sides} 

_{– formed from hard rock, that is more resistant to erosion, such as limestone, chalk and granite}

_{Landforms and Coastal Erosion}

• _{The red arrows represent wave energy}

• _{Wave energy is evenly spaced out in the deep water} 

_{– because of refraction, the energy of the waves— which moves perpendicular to the wave crests—is being focused on the headlands} 

• _{The approach of waves (white lines) in the Cox Bay area of Long Beach, Vancouver Island. The red arrows represent wave energy; most of that energy is focused on the headlands of Frank Island and Cox Point.}

_{Landforms and Coastal Erosion}

• _{On irregular coasts, the headlands receive much more wave energy than the intervening bays, and thus they are more strongly eroded}

_{– result is coastal straightening}

• _{An irregular coast will eventually become straightened, although that process will take millions of years}

_{Landforms and Coastal Erosion}

• _{Wave erosion is greatest in the surf zone, where the wave base is impinging strongly on the sea floor and where the waves are breaking}

• _{The result is that the substrate in the surf zone is typically eroded to a flat surface known as a wave-cut platform} 

_{– Wave-cut platforms are flat, bench-like surfaces}

• _{A wave-cut platform typically extends across the intertidal zone}

• _{An uplifted wave-cut platform is a marine terrace}

• _{A wave-cut platform in bedded sedimentary rock on Gabriola Island, B.C. The waveeroded surface is submerged at high tide.}

_{Landforms and Coastal Erosion}

• _{Sea Caves, Sea Arches and Sea Stacks} 

_{– Headlands are the focus of wave erosion due to wave refraction}

• _{Rocks in headlands do not erode at the same rate} 

_{–Soft and fractured rocks erode faster than hard rocks, forming sea caves}

• _{A sea arch forms when two sea caves meet}

• _{A sea stack forms when the arch of a sea arch falls}

• _{Sea caves develop in zones of weakness}

• _{Sea caves enlarge to form sea arch}

• _{Arch collapses leaving a sea stack}

1. _{A headland juts out to sea where waves can attack it. Headlands has faults and cracks that can be attacked}

2. _{The process of wave pounding and hydraulic action (where waves compress air into cracks) enlarge cracks into a crevice}

3. _{Waves continue to enlarge crevice through wave pounding and hydraulic action, coupled with stones thrown against the cliff face in the erosion process of attrition, which creates a cave.}

4. _{Once the cave is hollowed out enough and is connected on sides by the sea an arch has formed. Weathering processes such as freeze-thaw action and salt crystalization attack the weaknesses along the bedding planes}

5. _{Eventually, gravity takes hold and the weakened arch collapses leaving a vertical upstanding stack}

6. _{The stack is attacked by erosion and weathering to eventually collapse and leave behind a stump} 

_{Top: An arch in tilted sedimentary rock at the mouth of the Barachois River, Newfoundland, July 2012}

• _{Bottom: The same location in June 2013 – The arch has collapsed and a small stack remains}

_{Coastal Features: Depositional}

• _Beaches

• _Spits

• _{Baymouth Bars}

• _Tombolos

• _{Barrier Islands/Lagoons}

• _{Deltas Le tombolo de Sainte-Marie, Martinique (Caribbean)}

_{Coastal Features: Depositional}

• _{Beach – accumulation of sediment found along the landward margin of an ocean or lake} 

_{1. Foreshore slopes (dips) seaward} 

_{2. one or more berms »Berm = relatively flat platform composed of sand} 

_{3. Backshore slopes (dips) landward}

_{Coastal Features: Depositional}

• _Foreshore 

_{– Swash zone} 

_{– Located between high and low tide lines} 

_{– Dips seaward}

 _{– Strong shallow flows} 

_{– Upper flow regime planar beds dominate.} 

_{– Sometimes slightly cross-bedded}

•  _{Bartolome Island foreshore}

• _{A berm is a flatter region beyond the reach of high tides;this area stays dry except during large storms}

_{Coastal Features: Depositional}

• _Beach 

_{– composed of locally abundant material} 

_{– Examples:}

 _{–quartz sand beaches in Texas} 

_{–shell-fragment beaches in Florida} 

_{–black (basaltic) sand beaches in Hawaii Quartz sand beach} 

_{- Padre Island National Seashore, Texas}

_{Coastal Features: Depositional}

• _{Sand movement on the beach} 

_{– Movement perpendicular to the shoreline}

• _{Swash and backwash move sand toward and away from the shoreline, respectively}

 _{–Net loss or gain depends on the level of wave activity} 

_{»Summer typically has light waves and the beach widens (swash dominates)} 

_{»Winter typically has powerful storm waves that erode the beach (backwash dominates)}

• _{The difference between summer and winter on beaches in areas where the winter conditions are rougher and waves have a shorter wavelength but higher energy. In winter, sand from the beach is stored offshore}

_{Coastal Features: Depositional}

• _{The evolution of sandy depositional features on sea coasts is primarily influenced by} 

_{1. Waves}

 _{2. Currents (especially longshore currents)}

• _{As sediment is transported along a shore, it is either:} 

_{– deposited on beaches, or} 

_{– creates other depositional features}

_{Coastal Features: Depositional}

• _Spit 

_{– an elongated ridge of sand extending from the land into open water (mouth of an adjacent bay) in the direction of a longshore current} 

_{– Many spits are characterized by a hooked or curved end}

•  _{largely due to the refraction of waves around the mouth of the bay}

_{Coastal Features: Depositional}

• _{A spit can form when the coastline changes direction by more than 30 degrees, such as at the mouth of the K`ómoks Estuary, Vancouver Island, Canada}

• _{Dungeness Spit, Puget Sound, Washington. The mountains in the background are the Olympic Mountains} 

• _{Sand Spit and coastal development. San Diego, California}

_{Homer Spit, Alaska}

• _{juts 4.5 miles out into Kachemak Bay}

_{Coastal Features: Depositional}

• _{Baymouth Bars} 

_{– If a spit extends across a bay, it is called a baymouth bar}

• _{elongated sand or pebble banks lying parallel to (but separate from) the shoreline}

• _{form offshore bars (semi-submerged sand deposits) outside breaker zone} 

_{– exchange of water between the bay and the ocean is accomplished through the groundwater system}

_{Coastal Features: Depositional}

• _Tombolo 

_{– ridge of sand that connects an island to the mainland or another island} 

_{– natural bridge formed when sand is deposited between the shoreline and an island}

_{Coastal Features: Depositional}

•  _{In areas where there is sufficient sediment being transported, and there are near-shore islands, a tombolo may form}

_{Coastal Features: Depositional}

• _{Barrier Islands} 

_{– low ridges of sand that parallel the coast 3 to 30 kilometers offshore}

• _{In US, found mainly along the Atlantic and Gulf Coastal Coasts}

• _{Most are 1 to 5 kilometers wide and 15–30 kilometers long} 

• _{Galveston Island is a barrier island}

_{Coastal Features: Depositional}

• _{Barrier Islands – Probably form in several ways:} 

_{1. Some originate as spits} 

_{2. Some originate from sand piled up offshore} 

_{–In regions with an abundant sand supply, offshore bars rise above the mean high-water level and become barrier islands} 

_{3. Some are flooded sand dunes (sea level rise)}

_{Coastal Features: Depositional}

• _{A baymouth bar forms a barrier between two bodies of water}

• _{If the baymouth bar is broken, tidal inlets or passes can form}

• _{The separated segments become barrier islands}

_{Coastal Features: Depositional}

• _Lagoon 

_{– the water between a barrier island and the mainland becomes a lagoon} 

_{– relatively low-energy} 

_{– marshes, flats, seagrass beds, oyster reefs A lagoon is a body of shallow seawater separated from the open ocean}

_{Coastal Features: Depositional}

• _{Lagoon = body of quiet water isolated between the barrier and mainland}

• _{Sediment consists of fine-grained silts and clays, often laminated}

• _{Usually rich in microbial activity and/or vegetation}

• _{Water can be stagnant and anaerobic, forming organic-rich muds}

• _{Can be part of the tidal range and exhibit tidal flat deposits}

_{Coastal Features: Depositional}

• _Deltas 

_{– Term delta was coined by the Greek historian Herodotus (484–425 BC) after the Greek letter delta (∆) because of the deltoid-shape at the mouth of the River Nile, Egypt} 

_{– Form where rivers meet a standing body of water and the velocity drops}

• _{rivers carry a lot of sediment which is deposited to form deltas}

_{Coastal Features: Depositional}

• _{Deltas come in many shapes} 

_{– Those with the classic Δ shape form where waves or offshore currents redistribute sediment entering the standing water} 

_{– Where currents are stronger, less sediment accumulates so the edge of the delta is straight} 

_{– Where currents are weaker, more sediment accumulates so the edge of the delta arcs outward}

_{Coastal Features: Depositional}

• _{Deltas come in many shapes}

_{– Deltas that form where the strength of the river current exceeds that of ocean currents are called bird’s-foot deltas, because they resemble the scrawny toes of a bird}

_{Coastal Features: Depositional}

• _{Large deltas may consist of several distinct parts, or lobes, each of which formed during a different time interval}

• _{A stream builds a lobe by following a given course over a period of time}

• _{As the lobe grows seaward, the overall gradient of the stream as it crosses the delta decreases}

• _{When the gradient becomes too gentle for the stream to flow, the river overtops a natural levee upstream and begins to follow a new course, with a steeper gradient}

• _{Geologists refer to such events as avulsions}

_{Coastal Features: Depositional}

• _{The existence of several distinct lobes in the Mississippi Delta indicates that avulsions have happened several times during the past 9,000 years}

_{Coastal Features: Depositional}

• _{With time, the sediment of a large delta compacts, and the weight of the delta pushes down the crust below}

• _{As a consequence, the surface of a delta slowly sinks}

• _{Distributaries can provide sediment that fills the resulting space so that the delta’s surface remains at or just above sea level, forming a broad, flat area called a delta plain}

_{Causes of Coastal Variability}

• _{Plate Tectonic Setting}

• _{Relative Sea Level Changes} 

_{– Isostatic rebound as a result of deglaciation}

• _{Sediment supply}

• _Climate

_{Causes of Coastal Variability}

• _{Geologists refer to coasts where the land is currently rising or rose in the past, relative to sea level, as emergent coasts} 

_{– At emergent coasts, steep slopes typically border the shore, and in some locations, a series of step like terraces form}

 _{– These terraces reflect uplift of the coast as well as episodic changes in relative sea level}

_{Emergent Coasts: wave erosion produces a wave-cut platform along an emergent coast. As the land rises, the platform becomes a terrace, and a new wave-cut platform forms.}

_{Causes of Coastal Variability}

•  _{Coasts where the land has been sinking relative to sea level are submergent coasts}

•  _{At such places, landforms include estuaries and fjords that develop when the rising sea floods coastal valleys} 

• _{Submergent coastline: Sydney Harbor formed in a drowned river valley}

• _{Submergent coastline- a coast before sea level rises. Rivers drain valleys and deposit sediment on a coastal plain. As a submergent coast forms, sea level rises and floods the valleys, and waves erode the headlands.} 

_{Causes of Coastal Variability}

• _{Erosion and deposition occur on all coasts, although at different places} 

_{– erosion is the predominant process on the rocky headlands} 

_{– depositional processes predominate within the bays}

• _{Some coastal areas are dominated by erosion} 

_{– Pacific coast of Canada and the United States}

• _{Other coastal areas are dominated by deposition} 

_{– Atlantic and Gulf coasts of the United States}