Learning target: I can work with a team to decide what kind of information each piece of evidence gives (an age in years, an order, or what happened), place the evidence on a timeline of Earth's history, and defend which clues matter most.
Directions
Work in home groups of four. Step 1, expert round (12 minutes): each member takes one expert set of three cards (Set A: cards 1-3; Set B: cards 4-6; Set C: cards 7-9; Set D: cards 10-12), reads them, and meets the other students holding the same set to complete q1 together. Step 2, home round (20 minutes): return to your home group, teach your three cards in one minute each, and complete the group task and the group record. Step 3, defense (8 minutes): present your 'three strongest clues' choice to a neighboring group, who will challenge it. Step 4, individual (10 minutes): answer Part C alone.
You will need: Twelve cards per home group, cut from the card sheet, Masking tape strip with five bins marked, Small sticky notes or slips marked Y, O and W, Group record table, Pencils
Twelve evidence cards (real findings, rounded values)
1 Card 1: Meteorite agesPieces of the iron meteorite that blasted out Meteor Crater in Arizona, and many stony meteorites, give mineral-clock ages of about 4.56 billion years. Meteorites are fragments of asteroids, leftover pieces from the cloud that formed the sun and planets. No rock found on Earth is this old.
2 Card 2: StromatolitesIn Western Australia, rock about 3.5 billion years old contains layered, dome-shaped mounds. Their layers match the shape of living microbial mounds growing today in the shallow water of Shark Bay, Australia, where mats of microbes trap sediment. They are the oldest widely accepted fossils.
3 Card 3: Scratched bedrock and the Great LakesAcross Canada and the northern United States, bare bedrock carries long parallel scratches and a polished surface, and boulders of Canadian rock types lie scattered as far south as Illinois. The basins of the Great Lakes were gouged out by moving ice sheets and filled with meltwater as the ice retreated, between about 14,000 and 10,000 years ago.
4 Card 4: Banded iron formationsSeafloor rocks between about 2.5 and 1.8 billion years old show thin rust-red layers rich in iron alternating with gray layers. Iron dissolved in seawater settles out only when it meets oxygen. Rocks like these mostly stop forming after about 1.8 billion years ago.
5 Card 5: Matching fossils on separate continentsFossils of the same seed fern, Glossopteris, and of the same freshwater reptile, Mesosaurus, about 280 to 250 million years old, are found in South America, Africa, India, Antarctica and Australia, continents now separated by oceans these organisms could not cross. Rock belts in eastern North America also match belts in Scotland and Norway.
6 Card 6: Zircon grainsIn the Jack Hills of Western Australia, a sandstone about 3 billion years old contains tiny zircon crystals with mineral-clock ages up to about 4.4 billion years. Zircon survives weathering and transport, so the grains outlived the original rock they formed in. They are the oldest known pieces of Earth.
7 Card 7: The iridium layer and a buried craterAt more than 100 sites worldwide, a thin clay layer is unusually rich in iridium, a metal rare in Earth's crust but common in asteroids. Below the clay, fossils include non-bird dinosaurs and many kinds of sea life; above it, those kinds are absent. Volcanic layers near the clay date to 66.0 million years, and a buried crater about 180 km wide under Chicxulub, Mexico, has the same age.
8 Card 8: The Siberian lava pileIn Siberia, stacked lava flows cover about 2 million square kilometers and reach several kilometers thick. Their mineral-clock ages cluster at about 252 million years, the same age as the level in rock strata worldwide where most kinds of sea fossils disappear in the most severe mass extinction known.
9 Card 9: Deep-sea mud coresLong cores of mud drilled from the ocean floor contain shells of tiny organisms whose chemistry records the temperature of the water they grew in. Reading down the core, the shells show about 50 swings between colder and warmer conditions during the last 2.6 million years, each swing lasting tens of thousands of years.
10 Card 10: Moon rock chemistryThe oldest rocks brought back by Apollo astronauts are about 4.4 to 4.5 billion years old. Their chemistry, including a fingerprint of oxygen types that differs from one body in the solar system to another, is nearly identical to rock from Earth's mantle and unlike meteorites from elsewhere.
11 Card 11: Sea fossils near the top of the worldThe limestone near the summit of Mount Everest contains fossils of sea lilies and other sea animals roughly 470 million years old. Satellite positioning shows India still moving north into Eurasia at about 4 to 5 centimeters per year, and the collision that began lifting the Himalayas started about 50 million years ago.
12 Card 12: Human fossils in AfricaFossil skulls and stone tools found at Jebel Irhoud in Morocco were dated to about 300,000 years by measuring how long burnt flint tools had been storing energy from natural radiation. They are the oldest known fossils of our own species. No human fossil of any kind is older than about 7 million years.