Interactions between oceans and coastal places practice questions.

A beach faces south, and the prevailing wind is from the south-west. Waves therefore approach at an angle from the south-west. In which direction will longshore drift move sediment along this coast?

• Eastwards, in the direction of the swash
• Westwards, against the direction of the swash
• Directly offshore, at right angles to the beach
• Sediment will not move, because the waves are oblique

Answer: Eastwards, in the direction of the swash

Swash runs up the beach at the angle of wave approach while backwash returns straight down the steepest gradient, so each wave shifts material a little in the direction the waves are heading - here from the south-west towards the east.

Reef-building corals are largely restricted to shallow, clear tropical water. Which set of conditions explains this restriction?

• Warm water, strong light for the symbiotic algae, and low sediment
• Cold nutrient-rich water with a high plankton concentration
• Deep water with stable temperature and no wave action
• Brackish water at a river mouth with a high sediment supply

Answer: Warm water, strong light for the symbiotic algae, and low sediment

The coral depends on zooxanthellae living in its tissues, and those algae need light, which limits reefs to shallow clear water; suspended sediment both shades them and smothers the polyps. This is why deltas and turbid coasts have no reefs.

The Grand Banks cod fishery off Newfoundland was closed by moratorium in 1992 after stocks collapsed. Which explanation best fits the collapse?

• Catch rates were held up by better technology while the stock was falling
• Cod migrated permanently into deeper water beyond the fishing grounds
• A sudden natural cooling of the water killed the adult population
• The fishery had never been regulated or monitored in any way

Answer: Catch rates were held up by better technology while the stock was falling

Sonar, larger vessels and factory processing kept landings high even as the underlying biomass fell, so the catch figures gave a false signal of health until the stock crashed. It is the classic case of effort masking depletion, and of a fishery managed on the wrong indicator.

On a cliff face well above the high-water mark, spray dries in the pores and joints of the rock, salt crystals grow there and grains are prised loose. Which process is this?

• Hydraulic action
• Abrasion
• Salt weathering
• Attrition

Answer: Salt weathering

The rock is broken down in place by a stress acting within it, with no agent transporting material, which makes it weathering; hydraulic action, abrasion and attrition are all erosional and all need moving water or moving sediment.

At Lulworth Cove on the Dorset coast of England the rock bands run parallel to the shoreline. The sea has cut a narrow gap through resistant Portland limestone and opened a wide, almost circular bay behind it, which stops at a wall of chalk. Which explanation fits this shape?

• Wave refraction concentrated energy on headlands to either side, and longshore currents then swept the bay out.
• Once the resistant band was breached, the weaker rock behind was cut back quickly in all directions until a resistant band halted it.
• The bay is a river mouth that was drowned when sea level rose at the end of the last glacial period.
• The bay was hollowed out by a rotational slump of saturated cliff material after heavy rainfall.

Answer: Once the resistant band was breached, the weaker rock behind was cut back quickly in all directions until a resistant band halted it.

On a concordant coast the resistant band shelters everything behind it, so erosion is slow until the band is breached and then very fast in the weak rock, which is why the cove is round rather than the alternating bay-and-headland pattern of a discordant coast.

The Mississippi builds a bird's-foot delta of long, finger-like distributaries into the Gulf of Mexico, while the Nile delta ends in a smooth, arc-shaped seaward edge. Which set of conditions produces the bird's-foot form?

• A small sediment load entering water with strong waves that spread sand along the shore
• A large sediment load entering a sea with a very large tidal range that scours the channels
• A small sediment load entering a sheltered sea in which distributaries cannot form
• A large sediment load carried by strong distributary flow into water with weak waves and a small tidal range

Answer: A large sediment load carried by strong distributary flow into water with weak waves and a small tidal range

Delta shape records the balance between the river and the sea: where waves and tides are weak the river wins and builds channels out beyond the coastline, whereas wave-dominated deltas such as the Nile have their sediment redistributed into a smooth arc.