Chapter 21 Geologic Time
21.1Measuring Geologic Time
Geological time simply means the time Earth has existed. They can use the law of superposition and the concepts of radioactive decay & half-lives to determine geologic time.
Rate of Sedimentation – a method once used to determine geologic time. This was not accurate. It involved measuring the size of sedimentary layers & estimating how long it took to be deposited. It could not account for several factors.
Law of Superposition – in undisturbed sedimentary rocks, younger rocks are on top of older rocks. This can give you a relative age of a rock & any fossils they may contain.
Radioactive Decay – a method to determine absolute ages of rocks & fossils in them. They discovered radioactive elements in 1896. A radioactive element will decay into a stable element called a decay element. This happens at a constant measurable rate.
Half-life – the amount of time it takes a 1\2 of a radioactive element to decay to a decay element.
Ex. If you have 1 kilogram of element A & it has a half-life of 100 years after 100 years .5 kilograms of element A will have decayed into stable element B. After 200 years you will have .25 kg of A & .75 kg of B and so on.
When scientists use radioactive decay & half- lives to determine a rocks age it is called radioactive dating.Carbon 14 is used to date rocks & fossils from the last 50,000 years. Uranium 238 is used to date older rocks. (U 238 has a half-life of 4.5 billion years.) Age of the oldest rocks on earth is 4.6 billion years old. Found in South Africa. Moon rocks were found to be approximately the same age as Earth’s.
21.2 Divisions of Geologic Time.
Divisions of geologic time divided into units based on fossil records of extinct organisms and the law of superposition.
- Eras– the largest divisions of geological time
1.Precambrian Era- earliest time period. We know the least about this Era because there are only a few fossils.The longest of all Eras. Lots of mountain building & only simple marine fossils. 4 billion years long.
2.Paleozoic Era–began 570 million years ago & lasted 345 million years.Lots of fossils of many plants & animals. The most common fossils are invertebrate fossils (mostly Trilobites & Brachiopods).
Vertebrate fossils found toward the end of this Era(fish, amphibians, reptiles)
Large fern-like tree dominated the landscape and all of the continents were believed to be together in one large supercontinent called Pangea.
3.Mesozoic Era– started 225 million years ago 7 lasted 160 million years. “The age of reptiles”. Dinosaurs dominated this Era but birds & mammals show up here.
*Talk about dinosaurs, Archaeopteryx, extinction.
Giant mosses & fern trees were dominant plants but the following plants were all introduced in this Era: Cycads(palms), Conifers(pines, spruce,cedars), oaks, maples, elms & others.
There were lots of mountains built in this Era(Rockies, Appalachians, etc.). During part of the Era North America was covered in water. Pangea began to break up.
4. Cenozoic Era– started 65 million years ago & we are still in this Era.
“The age of mammals” There were 4 ice ages in this Era & we are now in a period between ice ages called an Interglacial. Human appeared late in the fossil record of this Era.
Continents now in current positions, but still moving.
**The end of each Era is usually marked by the extinction of the Era’s major organism.
Eras are divided into smaller units of time called Periods. The Precambrian Erahas no Periods.
In the Cenozoic Era the Periods are divided into even smaller time units called Epochs.
***Look at Periods & Epochs on Geologic Time Scale.