Coastal Processes Lab

What actually moves the material

Coasts change, and the shapes they end up in get most of the attention: the caves, the arches, the spits. This module is about what happens before any of that, because a landform is only ever the result of material being broken off, carried somewhere and put down. Get the processes right and the landforms explain themselves later; get them vague and every landform answer becomes a description rather than an explanation. There are five kinds of process to know, and they do different jobs. Weathering breaks rock down where it stands, without moving it. Mass movement is material sliding or falling downslope under gravity. Erosion is the sea actively wearing the coast away, and it works in four distinct ways that your specification names separately. Transportation carries the loose material along the coast. Deposition drops it again when the water no longer has the energy to carry it. The one worth the most attention is erosion, because four named processes are easy to list and hard to tell apart, and exam questions ask which one is acting rather than asking for the list. ⚠️ A note on what this module deliberately leaves out. Caves, arches, stacks, beaches and spits are studied in the next module on this topic, and coastal management in the one after. Here we stay with the processes themselves, because they are what the other two are built from.

Words for wearing and carrying

Six terms. The first three are commonly confused with each other, and the difference is always about whether the material moves and what moves it.

Match each way the sea erodes

Thrown at the rock, or worn down

Pebbles on a beach are noticeably rounder and smaller than the rock of the cliff behind them. Which process explains that?

• Attrition, because the pieces collide with one another as they are moved and are gradually worn smaller and rounder
• Abrasion, because the pebbles are being worn away by the sea
• Hydraulic action, because water is forced between the pebbles
• Solution, because seawater dissolves the pebbles

The four ways the sea erodes

Your specification names four erosion processes, and questions ask which one is acting rather than asking you to list them, so learn each by what it acts ON. Hydraulic action. Waves force water and trapped air into cracks in the rock under great pressure. As the wave retreats the pressure is released, and repeated over and over this widens the cracks until pieces break away. It acts on the rock face, and needs no material other than the water itself. Abrasion. The sea picks up sand, pebbles and larger material and hurls it against the cliff. It acts on the coast, and it needs the loose material as its tool - which is why a coast with plenty of sediment can erode faster than one without. Attrition. The pieces of material being carried collide with one another and with the seabed, and are gradually worn smaller and rounder. ⚠️ This is the one that catches people out, because it acts on the MATERIAL rather than on the coast. Attrition does not wear the cliff away at all; it changes the sediment, which is why beach pebbles are rounded while the cliff behind them is angular. Solution. Some rock, particularly limestone and chalk, is slowly dissolved by the chemical action of the water. It depends on the rock type in a way the other three do not. ⚠️ The test to apply: abrasion shapes the coast, attrition shapes the sediment. If a description is about what happened to the pebbles, it is attrition. If it is about what happened to the cliff, it is not.

Which two describe attrition

Select the TWO statements that are true of attrition.

• It acts on the material being carried, rather than on the cliff
• It makes pieces smaller and rounder over time as they collide
• It wears the cliff away by throwing material against it
• It depends on the rock type of the cliff being soluble

The zig-zag along the beach

Put the stages of longshore drift into order. Notice which stage is the one that makes the material travel sideways.

• Prevailing wind drives the waves towards the shore at an angle rather than straight on
• The swash carries material up the beach at that same angle
• The wave loses energy and the water drains back down the beach
• The backwash runs straight down the slope under gravity, not back at the original angle
• Each wave therefore moves the material a little way along the coast in a zig-zag

Coastal processes in words

The breakdown of rock where it stands, without being moved, is called _____. Material sliding or falling downslope under gravity is _____ movement. Of the four erosion processes, water and air forced into cracks under pressure is _____ action, while material hurled against the cliff is _____. Material is then carried along the coast by longshore drift, and _____ occurs when the water no longer has the energy to carry it.

In place, or carried away

Weathering and erosion are the pair students swap most often after abrasion and attrition, and the difference is simple once you have it: one moves the material and one does not.

Naming the process from a description

Exam questions on this topic rarely ask you to list the processes. They give you a description and ask which process it shows, so here is that skill worked through on four short descriptions. ⚠️ The method: ask what is being acted ON, and what is doing the acting. "Repeated waves force water into a crack in the cliff, and over time a piece breaks off." Acted on: the cliff. Acting: the water itself, under pressure. Hydraulic action. "Sand and shingle carried by the waves are thrown against the base of the cliff." Acted on: the cliff. Acting: the loose material, used as a tool. Abrasion. "The stones on the beach have become noticeably rounder and smaller than the rock they came from." Acted on: the material. Acting: the pieces against each other. Attrition. "Cracks in a chalk cliff have widened where seawater has slowly dissolved the rock." Acted on: the rock, chemically. Acting: the chemistry of the water, and it depends on the rock being chalk. Solution. Now a harder one, because it mixes two processes. "Heavy rain soaks into a cliff, and a section slips down onto the beach, where the sea then carries it away." That is mass movement first, because gravity moved the material down the slope, and then transportation, because the sea moved it along afterwards. The sea did not cause the slip. ⚠️ Answers lose marks by naming one process when a description contains two, so check whether anything happened before the sea got involved.

The description with the wrong name

Four statements about coastal processes. Select the ONE that names the wrong process.

• Water and air forced into cracks under pressure until rock breaks away is hydraulic action.
• Pebbles becoming rounder and smaller as they knock together is abrasion.
• Material being hurled against the base of a cliff by the waves is abrasion.
• Chalk being slowly dissolved by the chemical action of seawater is solution.

Name the process precisely

Assemble a sentence that names a process and says what it acts on.

Reading a stretch of coast

You are shown a stretch of coast and asked which processes are at work. Take each observation in turn.

• The base of the cliff is worn into a notch, and there is loose shingle at its foot. What is the likely process?
• Beach material is noticeably coarser at one end of the bay and finer at the other. What does that suggest?
• A section of cliff has slipped down onto the beach after a period of heavy rain. What happened?
• You are asked to explain why one stretch is eroding faster than another nearby. What is the strongest kind of answer?

Explain the coastal processes

Explain the processes that shape a coastline, from the breakdown of rock to the deposition of material. Say what each process acts on.

• Explain what weathering is and why it does not move material
• Explain what mass movement is and what causes it
• Name and explain all four erosion processes
• Explain clearly the difference between abrasion and attrition, and what each acts on
• Explain how longshore drift moves material along a coast
• Explain why the backwash returns straight down the beach
• Explain when deposition occurs
• Finish by explaining why a description can contain more than one process