Rock Record

Spring 2013

Determining Relative Age

• Geologists estimate that the earth is 4.6 billion years old

• James Hutton– Principle of uniformitarianism- current geologic

processes are the same processes that were at work in the past • This principle is one of the foundations of geology.

Determining Relative Age

• Relative age- indicates that one layer is older or younger than another layer based upon its position – Scientists can determine the order in which rock

layers were formed • Commonly done in sedimentary rocks

Determining Relative Age

• Law of Superposition– States that an underformed sedimentary rock layer is older

than the layers above it and younger than the layers below it. • Unconformities- this is when layers of rock are not in

their original position – Nonconformity- stratified rock lies on stratified rock – Angular Unconformity- boundary between the tilted layers

and the horizontal layer – Disconformity- boundary between the older eroded surface,

and the younger overlying layers is nearly horizontal

Determining Relative Age

• Crosscutting Relationships – Law of crosscutting relationships • States that a fault or an intrusion is always younger

than the rock layers it cuts through

Determining Absolute Age

• Rates of Erosion & Deposition – Studying rates of erosion allows scientists to

determine how old a feature is • Ex: Niagra Falls– the banks of Niagra Falls are eroding at

about 1.3 m/year so it is estimated that it is 9000 years old • This allow good approximations only for

features10,000 to 20,000 years old

Determining Absolute Age

• Rates of Erosion & Deposition – The rate of sediment deposition can be calculated

using data collected over a long period of time. • On average 30 cm of sedimentary rock are deposited

over a period of 1000 years

Determining Absolute Age

• Varve Count – Varve- an annual layer of sedimentary deposit on

a lake bed • Consist of light colored and a dark colored band

– A coarse summer layer & a fine winter layer form one varve

– Usually formed in glacial lakes

Determining Absolute Age

• Radioactive Decay – Radioactive isotopes have nuclei that emit particles and

energy at a constant rate. • The natural radioactivity found in rocks gives scientists an accurate

way to find the absolute age of rocks.

– As an atom emits particles and energy, the atom changes into a different isotope of the same element or an isotope of a different element.

– Scientists measure the concentrations of the original radioactive isotope & the newly created isotopes & then compare the proportion of the two to determine the age of the rock.

Determining Absolute Age

• Half-Life – The time it takes for half the mass of a given

amount of a radioactive element to decay into its daughter elements • Ex: Uranium-238 decays into Lead-206: uranium is the

parent element, lead is the daughter element

Determining Absolute Age

• Carbon Dating – Carbon dating is used when rock samples are

recently formed • Rocks contain carbon-14 • Carbon-14 decays into carbon-12

Fossil Record

• Paleontologists- scientists that study fossils – Fossils- remains or traces of animals or plants

from a previous geologic time. • Provide clues about events, climates, & evolution• Evolution- change of living things over time.

• Paleontology- study of fossils • Fossils are almost always found in sedimentary

rock

Fossil Record

• Kinds of Fossils– Bones– teeth– Shells– Rarely an entire organism– Replicas – Imprints

Fossil Record

• Preservation of Organisms– Only dead organisms that are buried quickly or

protected from decay can become fossils. – Mummification- drying • Mummified organisms are often found in desert caves

and buried beneath desert sands

– Amber- hardened try sap • Insects are preserved this way when they get trapped

in the sap and it hardens

Fossil Record

• Preservation of Organisms cont…– Tar Seeps – formed by thick petroleum oozing to

the earth’s surface• Animals become trapped in the sticky tar and tar

eventually covers them preserving them

– Freezing• Low temperatures of frozen soil & ice protect &

preserve animals because most bacteria cannot survive freezing temperatures

Fossil Record

• Petrification – Petrification- mineral solutions remove original

organic materials and replace them with new materials

– Often results in a near perfect replica of the original organism

Fossil Record

• Traces of Organisms– Trace fossils- tracks, footprints, borings, & burrows

• Formed when sand or silt fills the hollowed out area and hardens

– Imprints, Molds, & Casts • Imprints are usually of leaves, stems, flowers, & fish and are

formed from soft mud or clay that has hardened • Molds empty cavities left by the shells of snails & parts of trees ;

retains the surface markings & shape of the original organism • Casts form when a mold fills with mud or sand; it is a replica of

the original organism

Fossil Record

• Trace Organisms cont…– Coprolites• coprolites- fossilized dung or waste materials from

ancient animals

– Gastroliths • Gastroliths- stones that were found in the digestive

systems of dinosaurs to help grind their food

Fossil Record

• Interpreting the Fossil Record– Provide clues to the relative ages of rocks– Reveal environmental changes & how they have

affected organisms through the geologic history of the earth

Fossil Record

• Index Fossils – Fossils found exclusively in rock layers of a particular

geologic age – To be considered an index fossil…

• Must be present in rocks scattered over a wide area of the earth’s surface

• Index fossils must have features that clearly distinguish them from all other fossils

• Organisms from which the index fossil formed must have lived during a relatively short span of geologic time

• Must occur in fairly large numbers within the rock layers

Fossil Record

• Fossil Clues to the Past – Fossil tells us about different climate and

environmental changes that occurred in the past. • Ex: tropic plant and animal fossils have been found in

both Canada and the Arctic