Sediments
On 18 November 1929, an earthquake off Newfoundland triggered a wall of mud that raced hundreds of kilometres across the Atlantic seabed at up to 80 km/h, snapping the transatlantic telegraph cables one after another, in a precise sequence. Scientists later worked backwards from the cable-break times and discovered something new: sediment moves — sometimes catastrophically. This topic is about the material that blankets the entire ocean floor, where it comes from, and why it is a history book, a habitat, and a treasure chest.
South Africa mines some of the world's finest diamonds — and not all of them come from the famous Kimberley "Big Hole". Some of the richest deposits are mined off the seabed near Kleinzee on the north-west coast. How did diamonds end up in ocean sediment, hundreds of kilometres from the volcanic pipes they formed in?
Sediment transport. The diamonds formed deep underground in volcanic Kimberlite pipes on the interior subcontinent. Over millions of years, rivers — chiefly the Orange River — eroded those pipes and washed the loose material, including the diamonds, all the way down to the Atlantic as alluvial sediment. The river dumped its load into the sea; waves and currents then sorted and concentrated it along the West Coast seabed. Today, mining ships off Kleinzee and southern Namibia recover those diamonds straight out of the marine sediment. The CAPS curriculum names this exact example: "these diamonds were washed down from Kimberlite cores on the subcontinent as alluvial sediment."
That is what sediment is: material such as sand, organic matter and stone, suspended in water, which normally settles out and accumulates on the seabed. The continental margins and ocean basins are constantly being supplied with it. Scientists classify it two completely different ways — by where it came from (its origin) and by where it ends up (its situation) — and the sorted layers it forms are the planet's most complete history book.
The Blanket on the Seafloor
Sediment is solid material — sand, organic matter, stone — that is suspended in water and normally settles out onto the seabed. It is classified two ways: by origin (where it came from) and by situation (where it is deposited).
Every grain of sediment was once part of something else — a mountain, a shell, a volcano, a meteor. Rivers, wind, waves, glaciers and gravity move these particles around until they finally settle. There they accumulate, layer upon layer, very slowly: in the deep open ocean, only about 1 cm every 1 000 years. Near the continents, where rivers dump huge loads, it builds up far faster and thicker — up to several kilometres.
CAPS asks you to know both classification systems. By origin there are four types: lithogenous (from rock), biogenous (from living organisms), hydrogenous (precipitating out of seawater), and cosmogenous (from outside Earth's atmosphere). By situation — the pattern of where it is found — there are three: terrigenous (near its source on the coastline), neritic (coastal, on the continental shelf), and pelagic (out in the deep). The golden rule that ties it all together: finer particles travel further and settle in calm water; coarse particles drop out fast in turbulent water near their source.
Where Sediment Ends Up — the "Situation"
Sediment isn't spread evenly. Coarse, heavy particles drop near the shore where the water is energetic; the finest particles drift far out before they settle in the calm deep. Tap each zone of this ocean-floor cross-section to see what collects there, which CAPS "situation" category it is, and why.
Classification 1 — by Origin
CAPS classification system one: where did the material originally come from? Four origins. Mnemonic — L-B-H-C: Land, Bio, "H₂O" chemistry, Cosmos.
① Lithogenous
~ from rockFrom the weathering and erosion of rock (and volcanic ash). Carried to the sea by rivers, wind, glaciers and landslides. Mostly quartz, clay and feldspar. The most abundant type; dominates near continents.
~ river silt · desert dust · volcanic ash · glacial flour
② Biogenous
~ from living organismsFrom the hard remains of marine organisms. Forms the deep-sea "oozes": calcareous (CaCO₃ — foraminifera) and siliceous (SiO₂ — radiolarians, diatoms). The direct link to Topic 18.
~ foram & radiolarian shells · coral fragments · diatom frustules
③ Hydrogenous
~ precipitates out of seawaterForms when dissolved chemicals precipitate directly out of seawater, often extremely slowly. Includes manganese nodules, phosphorite and metal sulphides at hydrothermal vents (Topic 11).
~ manganese nodules · phosphorite · evaporite salts
④ Cosmogenous
~ from outside Earth's atmosphereFrom space — micrometeorites, cosmic spherules and tektite glass that constantly rain down and settle on the seabed. The smallest fraction by volume, but it never stops falling.
~ micrometeorites · cosmic spherules · tektite glass
Biogenous ooze = Topic 18, revisited
You already met biogenous sediment without knowing it. The foraminifera with calcium-carbonate shells make calcareous ooze; the radiolarians with glassy silica shells make siliceous ooze (the "radiolarian ooze" of Topic 18). When those single-celled drifters die, their shells are the sediment. Roughly half the deep ocean floor is covered in one ooze or the other.
Classification 2 — by Situation
CAPS classification system two: where is the sediment actually found? This is a different question from origin — and a sediment has both labels at once. Three "situation" patterns:
Terrigenous
~ near its sourceSediment found near its source on the coastline — it hasn't travelled far. Typically coarse (sand, gravel) because it was dropped quickly in turbulent water close to where the land delivered it.
~ beach sand · river-mouth gravel
Neritic
~ coastal / continental shelf"Coastal" sediment that comes from land and the coast, found under the shallow waters towards the edge of the continental shelf. Thicker and faster-accumulating than deep-sea sediment.
~ shelf sands & muds (e.g. the Agulhas Bank)
Pelagic
~ at depthSediment found out in the deep ocean, far from land. Only the very finest material reaches here: red clay and biogenous ooze, accumulating astonishingly slowly.
~ abyssal red clay · calcareous / siliceous ooze
Two labels, one grain — don't mix the systems up
A scoop of sand from the Agulhas Bank is lithogenous (origin — it came from eroded rock) and neritic (situation — it's on the continental shelf). A foraminifera shell on the deep abyssal plain is biogenous (origin) and pelagic (situation). Origin answers "what was it made from / where did it come from?"; situation answers "where did it end up?" Every CAPS sediment question expects you to keep these two ideas separate.
Turbidity Currents — Underwater Avalanches
Sediment doesn't only settle gently. Sometimes it moves with terrifying speed.
When too much sediment piles up on the steep continental margin, it can suddenly collapse and slide down into the ocean depths as a turbidity current — a dense, fast-moving slurry of saturated sediment (sediment mixed with water). It races downslope under gravity, sometimes for hundreds of kilometres, and finally spreads out and settles at the base of the slope, building a gently sloping wedge called the continental rise. Turbidity currents leave a tell-tale signature: a graded bed, coarse particles at the bottom (they settle first) and progressively finer particles towards the top.
The 1929 Grand Banks cable mystery
This is the CAPS-referenced example. On 18 November 1929, an earthquake off the Grand Banks of Newfoundland triggered a massive turbidity current. As the underwater avalanche raced south-east across the Atlantic seabed, it snapped twelve transatlantic telegraph cables one after another, each at a precisely recorded time. For years no one understood why the cables had failed in sequence. When scientists finally plotted the break times against the cable positions, the answer was astonishing: a wall of sediment had travelled ~600 km at speeds up to 80 km/h. It was the first direct proof of how powerful — and how fast — turbidity currents can be. Today, undersea internet cables landing at Cape Town and Gqeberha are still routed and armoured with exactly this hazard in mind.
Sorted by Size — and How We Sample It
The other way to describe sediment is by particle size. CAPS gives a specific classification: from boulders (minimum 256 mm across) down to silt and clay (less than 0.0039 mm). The key principle: the more energetic the water, the bigger the particle it can carry.
~ the Wentworth scale — the CAPS particle-size classification, coarsest → finest
CAPS full list: boulders · cobbles · pebbles · granules · very coarse sand · coarse sand · medium sand · fine sand · very fine sand · mud · silt & clay
Why the deep sea is all clay and ooze
Drop a mix of gravel, sand and clay-dust into a river. The gravel sinks at once. The sand travels a bit, then drops. The clay-sized particles are so tiny and light they stay suspended and travel thousands of kilometres — past the shelf, across the slope, only finally settling in the still water of the abyssal plain. That is why CAPS says "finer sediments are deposited offshore… coarse sediments are concentrated close to their source in turbulent waters" — and why the deep ocean floor is covered almost entirely in the finest material: clay and biogenous ooze.
How geological oceanographers sample the seabed
CAPS names four tools. Piston corers punch a long tube into the seabed and pull up an intact vertical column of layered sediment. Dredges drag a metal basket along the bottom to scoop up surface material. Grab samplers bite a single jawful of sediment from one spot. Acoustic profiling uses sound pulses to "see" the sediment layers below the seabed without touching them. To check water clarity (turbidity), researchers also lower a Secchi disc — the same simple black-and-white disc you met in the Term 2 Paper 1 — and record the depth at which it disappears.
Why Sediment Matters
Earth's history book
CAPS point 13: sediment cores show distinct layers that vary in colour, thickness and shape, as well as type and grain-size of particles — revealing the age, origin, past climate and biological content of each layer (links to Topic 12 stratigraphy).
Habitat & nutrients
Sediment houses vast burrowing "infauna" — worms, clams, brittle stars — and stores the nutrients that decomposer bacteria recycle into the food web (Topic 16).
Economic resource
Construction sand & gravel, oil & gas (Topic 18), manganese nodules — and South Africa's marine diamonds and heavy mineral sands.
Local connection — the CAPS South African examples
- Diamonds off Kleinzee & southern Namibia. CAPS names this directly: diamonds are mined off the north-west coast, having been "washed down from Kimberlite cores on the subcontinent as alluvial sediment" by the Orange River over millions of years. Mining ships recover them straight off the seabed.
- Heavy mineral sands along the Richards Bay coast. On the KwaZulu-Natal coast, the dune and beach sands are mined for heavy minerals (ilmenite, rutile, zircon — titanium and zirconium ores) concentrated by wave and current sorting. One of South Africa's biggest mineral-sand operations.
- Oil & gas, Southern Cape & Angola. Hydrocarbons (from organic-rich buried sediment, Topic 18) have been found beneath the ocean floor off the Southern Cape and off Angola — the source of the PetroSA gas at Mossel Bay.
- The JOIDES Project. The international deep-sea drilling programme (Joint Oceanographic Institutions for Deep Earth Sampling) has carried out exploratory coring off the coast of South Africa, recovering long sediment cores that read like Earth's diary.
When Sediment Becomes a Problem
So far sediment has been a history book, a habitat and a treasure chest. But too much sediment in the water — washed in by floods and eroding land, or stirred up by dredging, mining and trawling — harms living things. CAPS names three ways it does it.
It changes the temperature
Suspended particles absorb sunlight instead of letting it pass, and turn it into heat, so muddy surface water warms faster than clear water. Warmer water holds less dissolved oxygen, and a warm, muddy surface layer mixes less with the cooler water below — so less oxygen reaches the bottom.
It clogs gaseous-exchange organs
Fine particles lodge between a fish's gill filaments and coat them in mud and mucus, so less oxygen diffuses into the blood; sharp grains damage the thin gill surfaces. Filter-feeders — mussels, oysters, sponges — clog up, spend energy clearing their gills and starve, and settling mud can bury animals that cannot move.
It cuts the light for photosynthesis
Particles scatter and absorb light (attenuation — Topic 20), so the euphotic zone gets shallower. Seaweeds, eelgrass and the algae inside corals may then sit below the depth where they can make more food than they use, and slowly starve — even though nothing poisoned them.
Local connection — the brown plume
After heavy rain, rivers such as the Swartkops carry a brown plume of silt and clay into Algoa Bay. Clay is so fine that it can stay suspended for days (it "floats" down like a feather, as your booklet says), so the water stays murky long after the rain stops. A Secchi disk lowered in the plume vanishes within a metre or two, where on a calm week it might reach several metres. The same effects follow any activity that stirs up the seabed for weeks — which is why dredging and seabed mining near reefs and estuaries are studied and controlled.
In Practical 19 you can measure all three effects yourself: muddy water warming under a lamp, a coffee-filter "gill" clogging, and leaf discs photosynthesising more slowly behind a jar of muddy water.
In the Lab — the CAPS Investigations
CAPS says this topic "should be learnt with the view to apply the content to practicals and project work" — the only block in the course whose Depth column names practicals outright. It sets five investigations: a turbidity-current model, grain-size analysis, the Secchi disk, the 1929 cable breaks, and the harm sediment does to living things.
Five investigations · sieves, a box of water, a bottle and a core
- 🟢 kitchen Sieve a beach — dry a sample, sieve it with four kitchen sieves, and grade it on the Wentworth scale. Plus a settling race: why a flat shell falls slower than a pebble.
- 🟢 kitchen An avalanche in a box — pour a mud slurry down a ramp, time the current, and read the graded bed it leaves. Then time the real 1929 Grand Banks current from the cable breaks.
- 🟢 kitchen A Secchi tube — a mini Secchi disk in a bottle of muddy water, at five concentrations. How much light does mud steal?
- 🟡 pharmacy What mud does to living things — CAPS's three harms tested: muddy water warms faster, clogs a model gill, and slows photosynthesis in floating leaf discs.
- 🟢 desk Read a core — find all four origins, spot the turbidity-current layer, date the core and find an ice age. Plus the booklet's questions.
→ Open the Topic 19 practical sheet — printable, with supplied data for every task.
Key Vocabulary
Tap each card to flip it. These eight terms are examinable.
Material such as sand, organic material and stone that is suspended in liquid and normally settles out. Classified by origin and by the situation where it is found. The continental margins and ocean basins are constantly supplied with it.
Sediment from rock — the weathering and erosion of rock on land, plus volcanic ash. The most abundant type; dominates near continents. (Origin classification.)
Sediment from living organisms — shells and skeletons. Forms calcareous ooze (CaCO₃ — forams) and siliceous ooze (SiO₂ — radiolarians, diatoms). Covers ~half the deep seabed. (Origin classification.)
Sediment that forms by precipitating directly out of sea water as dissolved chemicals come out of solution. Includes manganese nodules, phosphorite and evaporite salts. (Origin classification.)
Sediment from outside the Earth's atmosphere — micrometeorites, cosmic spherules and tektite glass that constantly rain down and settle on the seabed. The smallest fraction by volume. (Origin classification.)
"Coastal" sediment that comes from land and the coast and is found in the shallow waters towards the edge of the continental shelf. (Situation classification — contrast with terrigenous, near its source, and pelagic, at depth.)
Sediment found at depth, far out in the deep ocean — only the very finest material (red clay, biogenous ooze) reaches here, accumulating about 1 cm per 1 000 years. (Situation classification.)
An underwater avalanche of saturated sediment that slides down the continental margin into the ocean depths under gravity — fast, dense, and far-travelling. It builds the continental rise and leaves graded beds. Famously snapped the transatlantic cables in 1929.
Quick Check
Question 1
Well done!
Next: Energy Transmission in Water — Light. How sunlight behaves once it enters the sea, and why the deep ocean is black.