Chapter 17: Telling Geologic Time

What are some different ways of telling geologic time?

B A relative time – sequence of events, not actual time Example: rock layer A is older than rock layer B

Pangea existed 200 million years ago absolute time – identifies actual date or time, specific number of years Example: Pangea existed 200 million years ago

catastrophism – the belief that changes on Earth have occurred in single catastrophic events

uniformitarianism – the belief that changes on the earth occurred gradually over time

All of these ideas apply to plant and animal species as well as Earth’s landscape.

What are the rules for relative time? How can we use them?

Law of Horizontality – strata (layers) are deposited horizontally

Law of Superposition – youngest strata (layers) are on top, oldest are on bottom

Intrusions, unconformities or folded rock layers are exceptions to the law of superposition.

unconformity – a break in the geologic record

nonconformity – stratified rock rests upon unstratified rock Sedimentary rock is stratified, or deposited in layers. Metamorphic and igneous rock are usually unstratified.

angular unconformity – when rocks deposited in horizontal layers are folded or tilted and then eroded

disconformity – boundary between horizontal layers of rock that have not been deposited upon continuously

Back to Rules of Relative Time: Law of cross-cutting relationships – rock that has intruded into existing rock layers is younger

Law of Included Fragments – rock found inside other rock is older

Determining Absolute Age Counting varves Radioactive decay Carbon dating

Counting varves – annual bands that can be found in glacial lakes

A coarse summer layer and the overlying fine winter layer make up one varve.

Radioactive Decay A radioactive isotope “decays” or breaks down at a constant rate from its “parent” element to its “daughter” element. constant

For instance, 10 grams of U-238 (parent) will decay into 5 grams of Pb-206 (daughter) in 4.5 billion years. This is considered its half-life, the time it takes for half the mass of a radioactive element to decay into its daughter elements.

By counting half-lives, the oldest rocks on Earth have been dated at 4 By counting half-lives, the oldest rocks on Earth have been dated at 4.6 billion years old.

Carbon Dating Carbon-14 (parent) decays into Nitrogen-14 (daughter) in 5,730 years and is great for dating organic material like wood, bones, shells, and the remains of early humans.

Paleontology is the study of fossils

Almost all fossils are found in sedimentary rock.

Why aren’t fossils found in metamorphic or igneous rock?

mummification – remains are dried and do not decay

amber – organism preserved in hardened tree sap

tar seeps – remains preserved in natural tar seeps (formed by thick petroleum oozing to the earth’s surface)

La Brea Tar Pits, Southern California

freezing – unchanged remains; no decay

Mastodons, woolly mammoths, woolly rhinoceroses – all which lived 40,000 to 5,000 years ago

petrification – atom by atom, remains are replaced by minerals Silica, calcite, and pyrite are common petrifying minerals

Petrified Forest National Park, Arizona

trace fossils – tracks, footprints, droppings, and burrows that provide information about an organism’s prehistoric life

imprints – displays the surface features of the organism

molds – empty cavities that once contained shells of snails, parts of trees, etc.

cast – when sand or mud fills a mold & hardens This creates a replica of the original organism

coprolites – fossilized waste material from ancient animals

gastroliths – stones from a dinosaur’s digestive system

index fossils – fossils that are typical of a particular time segment of Earth’s history

What are characteristics of an index fossil? Must be easily recognizable Must be widespread Must be limited in time (not found in every time period)