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