Learning target: I can use fossil, rock, shape and seafloor evidence to argue whether South America and Africa were once joined and are still moving apart, and explain why independent lines of evidence make the argument strong.
You are on a review panel that must decide whether the evidence supports a bold claim: that South America and Africa were once a single landmass, split apart, and are still moving away from each other today. Read the six evidence items. One of them is not useful evidence for the claim; part of your job is to spot it. Answer the warm-up questions, which help you organize the evidence. Then write your Claim, Evidence and Reasoning in the boxes. A strong response uses at least three evidence items from at least two different families (fossils and rocks, shapes, seafloor) and explains why each one is expected if the continents moved and hard to explain if they did not.
You will need: This worksheet, Pencil, Scrap paper for the expected-or-not table, Calculator
Evidence A: Fossils (real distributions). The table lists three organisms whose fossils occur in rocks of the same age on both continents.
| Fossil | What it was | Where found | Could it cross 3,000 km of open ocean? |
|---|---|---|---|
| Mesosaurus | Reptile about 1 m long that lived in shallow coastal and inland waters, about 280 million years ago | Southern Brazil and Uruguay; South Africa and Namibia | No; built for shallow water, not open sea |
| Glossopteris | Extinct seed plant (a tree) with tongue-shaped leaves, 250 to 300 million years ago | South America, Africa, India, Australia, Antarctica | No; its seeds were too large and heavy to blow across an ocean |
| Cynognathus | Dog-sized land reptile with mammal-like teeth, about 240 million years ago | Argentina; South Africa | No; it lived on land and did not swim |
Evidence B: Rocks (real). Belts of old rock on the two coasts match in type, age and the order of their layers. Rock layers in eastern Brazil and in Gabon, in West Africa, were laid down in the same kind of lakes at the same time, about 130 million years ago, and contain the same fish fossils. Both continents also hold deposits left by ice sheets about 300 million years ago, with scratches on the bedrock showing which way the ice moved. When the continents are placed side by side, the scratches point outward from one common center, as if a single ice sheet had covered the joined lands.
Evidence C: Shapes (real). The coastlines of the two continents fit roughly, but coastlines move whenever sea level changes. In 1965 a team at Cambridge University used a computer to fit the continents along the edge of the continental shelf, at a water depth of about 900 meters. That edge is the true rim of each landmass. The shelf edges of South America and Africa fit with only small gaps and overlaps along more than 5,000 kilometers.
Evidence D: Seafloor ages (simulated drill cores). A drilling ship took cores along a line from the Brazilian shelf to the West African shelf. The dataset gives the age of the oldest sediment resting on the volcanic rock at each site. These numbers are a teaching dataset; the pattern is the one real drilling found.
| Site | Distance from ridge crest (km) | Side | Age of seafloor (Ma) |
|---|---|---|---|
| B1 | 1400 | west (toward Brazil) | 70 |
| B2 | 1000 | west | 50 |
| B3 | 600 | west | 30 |
| B4 | 200 | west | 10 |
| R0 | 0 | ridge crest | 0 |
| A4 | 200 | east (toward Africa) | 10 |
| A3 | 600 | east | 30 |
| A2 | 1000 | east | 50 |
| A1 | 1400 | east | 70 |
Evidence E: Seafloor structures (real). Sonar mapping shows a continuous mid-ocean ridge running down the middle of the South Atlantic, about halfway between the two continents, and its bends follow the bends of both coasts. A narrow valley runs along the ridge crest. Nearly all the earthquakes in the South Atlantic happen in a thin belt along that crest. Satellite measurements show the two continents moving apart today at about 3 to 4 centimeters a year.
Evidence F: Climate today (real). Both the Amazon basin in Brazil and the Congo basin in Africa have hot, wet climates and tropical rainforests today, and both lie near the equator.
Before writing, make a table on scrap paper with the evidence letters A to F down the side and two columns: 'Expected if the continents moved apart?' and 'Easy to explain if they never moved?' Fill each cell with yes or no and three or four words of reason. Every row with yes in the first column and no in the second is a piece of evidence you can use. The row with yes in both columns is the item to leave out. Your Reasoning section is simply those rows written as sentences.
Write the strongest case a careful scientist in 1926 could make against the claim using only Evidence A, B and C, including the mechanism problem. Then write a reply from a scientist in 1970 that uses Evidence D and E to answer each objection point by point. Finish with two sentences on what the exchange shows about why scientists ask for a mechanism and a testable prediction before accepting an idea.