The Sea Geography Notes JC-Learn. JC-Learn. Geography Notes The Sea. 1 P a g e

Transcription

1 JC-Learn Geography Notes The Sea 1 P a g e

2 The Sea *Here, you can choose to study the chapter on glaciation or the one on the sea, or even both, because whenever these topics come up in the Junior Cert as short questions or long questions you always will have an option between the two. So, here we will just study The Sea as it is much easier to study and to answer exam questions on. Our coasts are constantly under attack due to the action of the sea. Waves are responsible for the erosion, transportation and deposition of material along our coasts. Waves occur when wind moves across the surface of the sea. The open area of sea that a wave travels is called its fetch. When waves reach shallower water at the coast, they break When the wave breaks onto the shore, there is a rush of water known as the swash When the water moves back again into the sea, it is known as the backwash The swash and backwash are responsible for the movement of material up and down the shore Types of wave Constructive waves - Waves, which deposit more than they erode, are constructive waves. During calm weather the wave is low and the swash covers more ground up the beach. The weak backwash means there is time for much deposition Destructive waves - Waves which erode more than they erode are called destructive waves. During stormy weather the wave is high and crashes on the beach leaving little swash up the beach. The strong backwash leaves quickly without soaking into the sand so little material is deposited and erosion is active. 2 P a g e

3 Coastal Erosion Hydraulic action - It is the force of the water, i.e. waves bashing against the coastline. Compressed air - Air gets trapped in cracks in rocks. The air is compressed, which increases the pressure. As the wave moves out, the air expands. This creates an explosive effect in the confined crack. This weakens the rock and eventually breaks it. Abrasion/ corrassion - Abrasion occurs when rocks, pebbles etc. are hurled against the coastline. This breaks off rock from the coastline causing erosion. Attrition - The stones that are carried along by the sea are in collision with each other and with the coast. They are worn down, rounded and smoothened over time. Solution - Acids in the water often dissolve rocks. This is possible where the seawater is along a coastline of chalk (limestone). Features of Coastal Erosion Bay and Headland A bay is a hollowed out area where the sea has cut inland. A headland is an area of hard resistant rock that juts out into the sea. The processes involved in the formation of bay and headlands are compression and abrasion. Coastlines consist of areas of different rock types. Soft rocks are eroded faster and a bay is formed. The harder rocks are more resistant to erosion and erode slower. They stand out as headlands. Cliff A cliff is a steep slope on the coast. They are formed as a result of hydraulic action, compression and abrasion. 3 P a g e

4 The hydraulic action of the waves forms a notch in the coast at a line of weakness. With continued erosion and undercutting, the notch gets deeper and bigger. The rocks above the notch are left unsupported, and eventually collapse forming a cliff. As the waves hit the cliff face, air gets trapped in fissures. When the wave moves out, the trapped air escapes and small explosions follow. This leads to shattering of rock, which falls into the sea. Compression has occurred. The sea uses these shattered rock pieces to further erode the cliff using abrasion. The rock surface that is left after cliff has retreated is called the wave-cut platform. The deposited material found below low tide as a result of cliff erosion is known as the wave-built terrace. Sea Cave A sea cave is a tunnel at cliff base. It gets narrower and lower on inside. Waves will erode at weak spot on cliff face. Hydraulic action and abrasion will aid the formation of the cave. 4 P a g e

5 Sea Arch A sea arch is a tunnel that stretches right through a headland. Continued erosion deepens a cave until it eventually reaches the other side of the headland. A sea arch may also form when two sea caves on the opposite sides of a headland meet. Sea stack A sea stack is rock, which stands out in isolation in sea away from coastline. When erosion continues to widen sea arch, roof may collapse. This then leaves former tip of headland separated from headland forming a sea stack. Sea stumps A sea stump is the eroded remains of a sea stack. These stumps are visible at low tide. Blowhole A blowhole occurs when a hole forms on the surface of the cliff top and joins with the roof of a sea cave. Waves trap air in sea cave. The build up of pressure loosens and shatters rock in roof of cave. The rock eventually collapses and resulting feature is a blowhole. 5 P a g e

6 Sea Transport The sea s load refers to all material that is carried along by the waves. Waves carry their load in two main ways: Up the shore in the swash, and back down the shore in the backwash This movement results in the process of longshore drift Longshore Drift This occurs when waves reach the shore at a sideways angle. Material is transported along the shore in a zigzag pattern. 1. Waves approach shore at angle 2. The swash from wave carries load up and along the shore and deposits some of its material on shore. The backwash brings remaining material out to sea following slope of beach. 3. The next wave coming in may go over outgoing backwash and carry its own material and material from backwash up the shore. 4. The process is repeated and material is transported along shore in a zigzag fashion. Longshore drift has a huge impact and influence on the shaping of our coastlines. Features of Coastal Deposition Beach A beach is an area between high tide and low tide mark. It develops in sheltered bay and consists of sand and shingle that is deposited between high and low tide levels. When waves break they cannot carry material further, so they deposit it. The swash is usually powerful and so carries and deposits large, coarse material up the beach. The backwash is less powerful and carries back out smaller, finer material and then deposits some of this at the lower end of the beach. Over time this material builds up to form a beach. A storm beach forms when waves are strong enough to push boulders and rocks above high tide mark. 6 P a g e

7 Sand Dune This is a hill of sand that forms when sand on beach is dried by onshore wind and blown inland to area above high tide. If obstruction exists, the sand will be trapped and form sand dunes. Sand Spit A sand spit is a long narrow ridge of deposited sand and shingle. It is connected to the costs at one end. Sand spits form in bays that are sheltered and where longshore drift has been interrupted resulting in deposition of sand. Over time, material builds up and eventually rises above tide level. The sand spit may curve inwards as result of wind and wave action. A beach can form on side facing out to sea. Sand Bar A sand bar forms when a sand spit extends across a bay and eventually connects the two land masses. Lagoon A lagoon forms as a result of a sand spit extending across a bay to form a sand bar. The lagoon is the area of water that is cut off from sea behind the sand bar. The lagoon is really a lake and over time the lagoon may fill with mud and silt deposited by rivers. Tombolo The process of longshore drift can lead to a sand spit extending out to an island and connecting the island to the mainland. This leads to the formation of a tombolo. An example of a tombolo is Howth, Co. Dublin. 7 P a g e

8 The Sea and Human Activity Human activities interact in a variety of ways with the natural landscape processes: Looked to sea for food, source of energy, for transport and leisure activities Coastlines provide scenic destinations for holiday makers Oil spillages threaten coastal environment Fishing in our coastal water (benefits humans, but leads to loss of life too) Protection from the sea Seawalls are built to protect coastal towns from sea attack. The walls break the power of incoming waves. Large boulders put at cliff base/ sand dunes. This rock armour is put there to reduce erosive power of the waves. Groynes are concrete walls/ wooden structures that are built down a beach extending out into the sea. They are built at right angle to sea to help slow down or prevent the process of longshore drift. They have advantage of helping build up beach in one area, but disadvantage of starving an area of sand further along the coast. Gabions are wire cages containing small stone. They are put in front of beaches or in front of sand dunes. They absorb and reduce the erosive power of the swash. 8 P a g e