Practical 3 — Ocean Floor Topography
Two investigations. The first is the labelling exercise CAPS builds the whole block around. The second hands you a fishing line and a lead sinker and asks you to sound the depth exactly the way navigators did for three hundred years — including discovering, the hard way, why it took a century to collect 7 000 readings.
"Use a plumb line with press stick on a lead weight to measure water depth, and collect a sediment sample."
Resources: "Fishing line · Lead sinker · Videos · Maps · Charts."
And the Depth column, which is the reason Investigation 1 matters: "Learners must be able to label a diagram of the features found on the ocean bed and have an understanding of the processes which led to their development."
Investigation 1 needs only a pencil. Investigation 2 needs fishing line, a lead sinker and a lump of press stick (Prestik) — under R100 at any tackle or stationery shop, and this learner probably already owns the first two.
No water deep enough to sound? A swimming pool works perfectly for the method and the accuracy questions, and Investigation 2 supplies a full harbour data set if you cannot get to water at all.
Aim
To identify and label the features of the ocean floor on a basin profile, and to explain why an Atlantic profile and a Pacific profile look different.
Part A — label it from memory
Below is the Atlantic profile from Section 2 of your topic page with every label stripped off. Write the correct feature name against each number. Cover the topic page first. This is the exact task CAPS says you must be able to do.
| # | Feature name | What it is / how it formed (one line) |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 | ||
| 5 | ||
| 6 | ||
| 7 | ||
| 8 | ||
| 9 | ||
| 10 | ||
| 11 | ||
| 12 | ||
| 13 | ||
| 14 |
Part B — Atlantic versus Pacific
Two real basin profiles, described. Read both, then answer the comparison questions.
📊 Two profiles, described
Profile A — Africa to South America (the Atlantic). From the African coast: a shelf about 90 km wide falling gently to 150 m, then a break, then a slope down to 3 000 m, then a broad sediment rise, then abyssal plain at ~5 000 m. A mid-ocean ridge rises to 2 500 m in the centre with a rift valley along its crest. Then plain, rise, slope and shelf again, mirrored, to the South American coast. No trench anywhere on the section. Total width ~6 000 km.
Profile B — Peru to the mid-Pacific. From the South American coast: a shelf only 15 km wide, then an extremely steep slope dropping straight into the Peru–Chile Trench at 8 000 m, just 100 km offshore. No rise at all. Beyond the trench the floor climbs to abyssal plain at ~4 000 m, dotted with seamounts and guyots. The Andes run along the coast behind, and the region has frequent earthquakes and active volcanoes.
Analysis
- 14Name all fourteen numbered features on the blank profile. (1 each.)
- 6For features 4, 10 and 12, add one line on the process that formed it — CAPS asks for the processes, not just the names. (2 each.)
- 4Classify each margin in Profile A and Profile B as active or passive, and give the single piece of evidence you used.
- 4Explain why Profile B has no continental rise, while Profile A has a broad one.
- 3Profile B has earthquakes, volcanoes and a mountain range on the coast; Profile A has none of these. Explain why, in terms of plate boundaries.
- 3South Africa's margin resembles which profile? State your evidence, and name the feature off our south coast that demonstrates it.
- 2On Profile A, the seafloor gets older the further you travel from the centre in either direction. Explain what that tells you about the ridge.
Extension
Find the Heezen–Tharp World Ocean Floor map online — the 1977 painting that showed the public what the seafloor looks like for the first time. Trace the Mid-Atlantic Ridge from Iceland to the Southern Ocean and find the fracture zones cutting across it at right angles.
Aim
To measure water depth by plumb line, to collect a bottom sample using the historical grease-on-the-weight method, and to evaluate the accuracy of the technique.
Hypothesis
Write your own prediction about how repeatable your readings will be, and what will make them vary. You will test it directly.
Apparatus
- 🟡 tackle shop Fishing line — at least 15 m, and braid if you have it (it stretches far less than monofilament, which turns out to be the whole point)
- 🟡 tackle shop A lead sinker, 50–100 g, with a flat-ish base
- 🟢 kitchen Press stick / Prestik — a lump pressed onto the bottom of the sinker
- 🟢 kitchen A permanent marker and a tape measure, to mark the line every 50 cm
- 🌊 site A jetty, harbour wall, boat, or a swimming pool
Method
- Calibrate the line. Lay it out along a tape measure and mark it clearly every 50 cm with the permanent marker. Number the metre marks. This is your measuring instrument — if the marks are wrong, everything is wrong.
- Press a lump of press stick onto the base of the sinker, flattened but not smoothed. This is the modern stand-in for the grease or wax that early navigators used.
- Lower it slowly until you feel the line go slack — that slackening is the weight touching the bottom. Do not let it free-fall; you will overshoot.
- Pinch the line at the water surface, lift straight up, and read off the depth from your marks.
- Repeat at the same spot five times. Record every reading, including any that disagree.
- Examine the press stick. What stuck to it? Sand, mud, shell fragments, weed, nothing? Describe grain size and colour — this is your bottom sample, and it tells you the substrate type.
- Now move along the wall or jetty and take a reading every 2 m for at least 8 positions. This is a transect — you are building your own depth profile.
| Reading | 1 | 2 | 3 | 4 | 5 | Mean | Range (max − min) |
|---|---|---|---|---|---|---|---|
| Depth (m) |
| Distance along (m) | Depth (m) | Bottom sample — what stuck to the press stick |
|---|---|---|
| 0 | ||
| 2 | ||
| 4 | ||
| 6 | ||
| 8 | ||
| 10 | ||
| 12 | ||
| 14 |
📊 Supplied results — a real sounding transect, Port Elizabeth harbour wall
Five repeats at one spot: 4.2 · 4.4 · 4.1 · 4.6 · 4.3 m. Mean 4.32 m, range 0.5 m. The line was monofilament, and there was a light cross-current.
| Distance (m) | Depth (m) | On the press stick |
|---|---|---|
| 0 | 2.1 | coarse sand, a few shell fragments |
| 2 | 2.8 | coarse sand |
| 4 | 3.6 | fine sand |
| 6 | 4.3 | fine sand, trace of dark mud |
| 8 | 5.0 | dark grey mud |
| 10 | 5.4 | dark grey mud, slight smell |
| 12 | 5.6 | dark grey mud |
| 14 | 5.7 | dark grey mud |
Plot your profile
Depth on the y-axis, distance on the x-axis — and invert the y-axis so depth increases downward, the way a real bathymetric profile is drawn. Apply the seven graph rules from Topic 5.
Analysis
- 3Calculate the mean and the range of your five repeat readings.
- 4Your five readings at one fixed spot were not identical. Give four reasons why not.
- 7Plot your transect as a depth profile, y-axis inverted, following all seven graph rules.
- 3Describe the shape of your profile. Does the bottom slope evenly, or is there a break of slope? Which feature from the topic page does it most resemble in miniature?
- 4Describe how the bottom sample changed along your transect, and explain the pattern. (Hint: think about where fine particles settle and where coarse ones do.)
- 4CAPS concept 1 says early sounding was inaccurate because the rope stretched. Explain which direction that error pushes the reading — too deep or too shallow — and why. Then say what replaced rope, and why that helped.
- 3You took 8 readings in about half an hour. Before sonar, only 7 000 soundings existed for the entire world ocean, none deeper than 2 000 m. Using your own experience, explain why so few.
- 3Name the technology that replaced the plumb line in the 1920s, explain in one sentence how it works, and state one advantage it has beyond speed.
Extension
Repeat one spot with braided line instead of monofilament and compare the range of five readings. Braid stretches very little — you have just run the same experiment that persuaded 19th-century surveyors to switch to piano wire.
✅ Memo & marking guide — try it first
Total: 67 marks — Investigation 1: 36 · Investigation 2: 31.
This memo does not print with the worksheet.
Investigation 1
- Q1 (14): 1 wave-cut terrace · 2 continental shelf · 3 shelf break · 4 submarine canyon · 5 continental slope · 6 turbidity current · 7 continental rise · 8 abyssal plain · 9 abyssal hill · 10 seamount · 11 guyot · 12 mid-ocean ridge (with rift valley at its crest) · 13 fracture zone · 14 continental rise (South American side). Accept "continental margin" for the shelf–slope–rise group if labelled as a bracket.
- Q2 (6): 4 Submarine canyon — cut into the slope by turbidity currents, dense sediment-laden flows that scour their way down. 10 Seamount — an undersea volcano built by erupted lava, rising 1 000 m+ but never reaching the surface. 12 Mid-ocean ridge — a divergent plate boundary where plates separate and rising magma creates new seafloor, building a mountain chain with a rift valley along the crest.
- Q3 (4): Profile A both margins passive — evidence: a broad continental rise and no trench. Profile B active — evidence: a trench 100 km offshore and no rise.
- Q4 (4): On a passive margin, sediment carried down the slope accumulates undisturbed at its foot and builds a rise. On an active margin the trench intercepts that sediment — it falls into the trench or is dragged down with the subducting plate — so no rise can form.
- Q5 (3): Profile B sits on a convergent plate boundary where the oceanic plate subducts beneath the continent: friction and melting cause earthquakes and feed volcanoes, and compression pushes up the Andes. Profile A's margins sit in the middle of a plate, far from any boundary, so they are geologically quiet.
- Q6 (3): Profile A / passive. Evidence: no trench off our coast, and a wide shelf. The feature is the Agulhas Bank, up to 250 km wide off the south coast.
- Q7 (2): New crust is being created at the ridge and moves outward in both directions — seafloor spreading. The ridge is a divergent boundary.
Investigation 2
- Q8 (3): Supplied data — mean = (4.2+4.4+4.1+4.6+4.3)/5 = 4.32 m; range = 4.6 − 4.1 = 0.5 m.
- Q9 (4): Any four — the line stretched under the weight; a current pushed the line off vertical, so you measured a diagonal and over-read; waves/swell moved the surface reference up and down; difficulty judging the exact moment the weight touched down; the weight sank into soft mud; parallax or marker error when reading; the boat or your hand drifted between readings.
- Q10 (7): Axes labelled with units ✓ · y-axis inverted so depth increases downward ✓ · sensible scale using most of the grid ✓ · points accurate ✓ · line accurate ✓ · figure number ✓ · descriptive title ✓.
- Q11 (3): With the supplied data: it deepens steadily from 2.1 m to 5.7 m, but the gradient flattens after about 8 m — steep near the wall, then levelling. That flattening is a break of slope, the same shape as a shelf break in miniature, with the flatter deep part playing the role of a plain.
- Q12 (4): Coarse sand and shell near the wall → fine sand → dark mud in the deeper, further water. Explanation: water movement is strongest in the shallows (waves, wash, currents), so only heavy coarse grains stay put; fine particles stay suspended and are carried further out, settling only where the water is deeper and calmer. The dark colour and smell of the mud indicate low-oxygen, organic-rich sediment, typical of a sheltered harbour bottom.
- Q13 (4): A stretched line is longer than the true depth, so the reading is too deep — an over-estimate (2). It was replaced by thin non-stretch wire (piano wire), which barely extends under load, giving far more accurate readings (2).
- Q14 (3): Every single sounding required lowering and winching back a great length of line — up to 8 hours per reading at ocean depths. One ship could take only a handful a day, in one place, in good weather, and the ocean is vast. So a century of effort produced only ~7 000 points, none deeper than 2 000 m.
- Q15 (3): Echo sounding / sonar, from the mid-1920s (1). It sends a pulse of sound down and times the returning echo; knowing the speed of sound in water converts that time into depth (1). Advantage beyond speed: it records a continuous profile along the ship's track rather than isolated points — which is how the mid-ocean ridges were discovered at all (1). Accept also: it works at any depth; it does not disturb the bottom.