Geologic Time

The Geologic Time Scale  A summary of major events in Earth’s past that are preserved in the rock record  Divisions of Geologic Time  Eras  Periods  Epochs

5 Major Eras of Geologic Time 1. Archean ↓ Precambrian (largest Era, lasted almost 4 Precambrian (largest Era, lasted almost 4 billion years) billion years) 2. Proterozoic ↑ 3. Paleozoic – “Age of the Invertebrates” 4. Mesozoic – “Age of the Reptiles” 5. Cenozoic – “Age of the Mammals”

Reading the Rock Record  Two different methods are used to measure geologic time 1. Relative Time 2. Absolute Time

A. Relative Time  Places events in a sequence but does not identify an actual date of does not identify an actual date of occurrence occurrence

Methods for determining relative age: 1. Law of superposition – states that in a sequence of undisturbed sedimentary rocks the oldest rocks will be at the bottom of the sequence and the youngest will be on the top.

2. Law of cross-cutting relationships – states that an igneous rock is younger than the rocks it has intruded or cut across. (this can also apply to a fault)

3. Law of included fragments – states that pieces of one rock found in another rock must be older than the rock in which they are found.

 Identifies the actual date of an event  An actual number can be given when determining absolute time B. Absolute Time

Methods for determining absolute time:  Tree Rings – count tree rings, each ring = one year each ring = one year

 Varves – any sediment showing a yearly cycle, can be found in any body of water (most clearly seen in glacial lakes)

 Radiometric Dating – uses radioactive isotopes to determine older dates (radioactive isotopes are atoms of elements that give off radiation from their nuclei) –Each time a ray is released from an atomic nucleus, the atom changes into a new, lighter element. This process is called radioactive decay. –These radioactive elements decay at characteristic constant rates. –Half-life is the time it takes for half of the atoms of the radioactive element to decay to a stable end product. (ex. : Uranium-238 has a half-life of 4.5 billion years)

 Radiocarbon Dating – uses Carbon-14 to date living things (has a half-life around 5700 years)

Fossils  Any evidence of life preserved in a rock  A scientist that studies fossils is studies fossils is called a called a paleontologist paleontologist

How are fossils formed? 1. An organism dies 2. The organism is buried in sediment 3. The sediment changes to sedimentary rock containing a fossil 4. Over time, erosion exposes the fossil at the surface

Types of fossils: 1. Original remains: very rare, it is the actual unchanged remains of a plant or animal (example: woolly mammoth frozen) Woolly Mammoth

2. Amber: a hardened resin, which is sticky sap from pine trees

3. Replaced remains: the soft parts of the original remains have decayed and the hard parts have been replaced by minerals Replaced remains of clams

4. Mold: the hollow depression in a rock formed when a fossil, shell, or bone is dissolved completely out of the rock 5. Cast: when new mineral fills in the mold

Cast vs. Mold Mold Cast

6. Trace fossil: evidence of life other than the remains of a plant or animal (example: footprints)

7. Carbonaceous film: thin film or carbon residue, forming an outline of the organism

Fossils and Relative age of rocks  Index fossils can be very useful to geologists because they can identify the relative age of the rock in which they occur.  What makes a fossil an index fossil? A. Can easily be told from other similar fossils B. Widespread in occurrence C. Only existed for a short period of time

Example of an index fossil: Example of an index fossil:  The Trilobite

Rock Correlation  Described as the matching of rock layers from one area to another  Layers can be matched by characteristics like appearance, color, composition, or some unique feature (key beds – a layer of volcanic ash)  The best method for rock correlation is using index fossils

Unconformity  A place in the rock record where a layer is missing Unconformity

Fossils as evidence for evolution  Fossil records indicate that the first organisms were very simplistic in structure  As time passed, life forms increased in size and complexity  This change that produces new life forms over geologic time is called evolution

More on Evolution  The theory of evolution provides a scientific explanation for the past and present diversity on Earth  Charles Darwin developed the theory of natural selection. His theory states that organisms that survive to produce offspring are those who inherited the most beneficial traits for surviving in a particular environment  We call this survival of the fittest!