Unit 8: Geologic Time and Earth's History
How geologists read a rock's age from where it sits and from the atoms decaying inside it, and what the fossil record shows about life through deep time.
11 topics · 33 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Relative Dating: The Law of Superposition
- How it worksHow does each new layer of sediment settle on top of the layer laid down before it, oldest on the bottom?
- Common mistakeWhy doesn't the law of superposition tell you a rock layer's exact age in years?
- Going furtherWhat happens to the normal top-to-bottom order of layers once the whole sequence gets flipped by folding?
Your turn1 of 2
A cliff face shows rock layers stacked one atop another with nothing disturbing their order. Why can you tell a rock layer's rough age just by knowing whether it's near the top or bottom of a cliff?Relative Dating: Cross-Cutting Relationships
- How it worksHow does the principle of cross-cutting relationships let you date a fault relative to the rock around it?
- How it worksWalk me through ordering a fault, a lava dike, and the layers they both cut through from oldest to youngest.
- Common mistakeWhy can't a rock layer be older than a fault that cuts straight across it?
Your turn1 of 2
A fault slices cleanly through several rock layers, offsetting each one by the identical amount. Why must a fault that slices through several rock layers be younger than every layer it cuts through?Relative Dating: Unconformities
- How it worksHow does an unconformity form when erosion strips away rock before new layers get deposited on top?
- Common mistakeWhy isn't an unconformity just an unusually thin rock layer instead of a missing chunk of time?
- Ties togetherHow does recognizing an unconformity change the story you'd tell from superposition alone?
Your turn1 of 2
A rock sequence shows tilted, eroded layers directly overlain by flat, much younger layers, with a sharp boundary between them. Why does a gap in the rock record mean millions of years of Earth's history are simply missing?Putting Relative Dating Together in One Rock Sequence
- How it worksWalk me through reading a sequence with tilted layers, a cutting fault, and an intrusion to reconstruct the order of events.
- Worked exampleFind the oldest-to-youngest order for three sedimentary layers, a cross-cutting dike, and an erosional unconformity above them.
- Ties togetherHow do superposition, cross-cutting relationships, and unconformities work together to date one whole rock sequence?
Your turn1 of 2
A rock sequence contains 5 sedimentary layers, 2 cross-cutting faults, and 2 unconformities, each a separately dateable event in the relative sequence. How many total dateable events does the sequence contain?Absolute Dating: Radioactive Decay and Half-Life
- How it worksHow does an unstable parent isotope decay into a stable daughter isotope at a fixed, unchanging rate?
- Worked exampleFind how much of a 100 gram sample of a radioactive isotope remains after 3 half-lives.
- Common mistakeWhy doesn't heat, pressure, or a chemical reaction speed up or slow down a radioactive decay rate?
Your turn1 of 2
A radioactive isotope has a half-life of 363 million years. Starting with 147 grams, how many grams remain after 5 half-lives have passed?Absolute Dating: Radiometric Dating Methods
- How it worksHow does comparing the ratio of parent to daughter isotope in a sample calculate its absolute age?
- Worked exampleFind the age of a rock sample with 25 percent of its original potassium-40 remaining, given a half-life of 1.25 billion years.
- Ties togetherHow does radiometric dating fill in the exact ages that relative dating alone can never provide?
Your turn1 of 2
A rock sample has 66 percent of its original potassium-40 remaining, and potassium-40 has a half-life of 1.324 billion years. Using age equals half-life times log base 2 of 100 divided by percent remaining, what is the rock's age in billion years?The Geologic Time Scale
- How it worksHow do eons, eras, periods, and epochs nest inside each other to organize all of geologic time?
- Worked exampleFind what fraction of the entire geologic time scale is taken up by the roughly 300,000 years humans have existed.
- Common mistakeWhy isn't the Mesozoic Era the same length of time as the much shorter Cenozoic Era that followed it?
Your turn1 of 2
Humans have existed for roughly 409 thousand years, out of Earth's full geologic history of about 4.66 billion years. What percentage of Earth's total history do humans represent?How Fossils Form
- How it worksHow does an organism's buried remains get replaced mineral by mineral until they turn to solid stone?
- How it worksWalk me through a shell being buried in mud, dissolving away, and leaving behind a mold and cast fossil.
- Common mistakeWhy isn't a fossil the actual original bone or shell of the organism it came from?
Your turn1 of 2
Museums display countless fossilized shells and bones, but fossils of jellyfish or worms are extraordinarily rare. Why is finding a fossil of a soft-bodied animal so much rarer than finding one with a hard shell or bone?Index Fossils and the Fossil Record
- How it worksHow does a species that lived briefly but spread widely make an ideal index fossil for correlating rock layers?
- Going furtherWhat happens to an index fossil's usefulness if the species turns out to have survived for 50 million years instead of 1?
- Ties togetherHow does the fossil record combine with relative dating to match up rock layers from different continents?
Your turn1 of 2
A particular trilobite species existed for only a million years but is found on nearly every continent. Why can one particular fossil species tell you the age of a rock layer on the other side of the world?Earth's Early History and the Origin of the Atmosphere
- How it worksHow did volcanic outgassing build Earth's early atmosphere out of gases trapped inside the young planet?
- How it worksWalk me through photosynthetic organisms slowly filling the atmosphere with oxygen over a billion years.
- Ties togetherHow does the rise of atmospheric oxygen connect to the rock record left behind by the Great Oxidation Event?
Your turn1 of 2
Earth's earliest atmosphere formed from gases released by volcanoes, long before any life existed to produce oxygen. Why did Earth's original atmosphere have almost no free oxygen in it at all?Mass Extinctions
- How it worksHow could a single asteroid impact trigger a chain reaction that wipes out most life on Earth?
- Going furtherWhat happens to the surviving groups of organisms in the millions of years right after a mass extinction?
- Ties togetherHow do mass extinctions show up in the fossil record as a sudden layer where diversity crashes?
Your turn1 of 2
Dinosaurs thrived for over 150 million years, yet vanish abruptly from the fossil record at a single, sharp boundary. Why did the dinosaurs, after surviving for over 150 million years, disappear in a geological instant?