The Rock and Fossil Record Chapter 6 The Rock and Fossil Record

I. The Principle of Uniformitarianism Many people believed in catastrophism, or sudden events called catastrophes, changing earth’s landscape In 1788, James Hutton wrote Theory of the Earth, in which he developed the thought of uniformitarianism - Processes that we observe today remain uniform, or do not change over time

II. Modern Geology Scientists today realize that neither uniformitarianism nor catastrophism accounts for all geologic change throughout Earth’s history - Most geologic change occurs gradually - however, catastrophes have also occurred

III. Paleontology The science involved with the study of past life Data collected comes from fossils Invertebrate paleontologists study animals without backbones Vertebrate paleontologists study animals with backbones Paleobotanists study fossils of plants

IV. Relative Dating A. The Principle of Superposition Younger rocks lie above older rocks in undisturbed sequences Some rocks however, are disturbed by forces within the Earth

B. The Geologic Column An ideal sequence of rock layers Contains all the known fossils and rock formations on Earth

C. Events That Disturb Rock Layers Scientists assume that sediment has always been deposited in horizontal layers If rock layers are not horizontal, something must have disturbed them after they formed Disruptions create features that cut across existing rock

1. Faults are breaks in the Earth’s crust - Blocks slide relative to one another

2. Intrusions occur when molten rock forms in the Earth’s interior and squeezes into existing rock and cools

3. Folding occurs when rock layers bend and buckle from Earth’s internal forces

4. Tilting occurs when internal forces in the Earth slant rock layers

D. Gaps in the Record – Unconformities Missing rock layers create breaks in the geologic column called unconformities Geologists must question whether the “missing layer” was never present or whether it was somehow removed

E. Types of Unconformities 1. Disconformities Younger layers at the top of the sequence are removed by erosion and are deposited elsewhere Deposition resumes at a later time Represents thousands to millions of years of missing time

2. Nonconformities Horizontal sedimentary rock layers lie on top of an eroded surface of older intrusive or metamorphic rock

3. Angular Unconformities Found between horizontal layers of sedimentary rock and layers of rock that have been tilted or folded

V. Absolute Dating A. Radioactive decay Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons

Isotopes of radioactive elements are used to determine the absolute ages of fossils and rocks Unstable, or radioactive, isotopes tend to break down into stable elements by radioactive decay Radioactive decay occurs at a steady rate so scientists can use unstable isotopes to determine an object’s age

B. Radiometric Dating Unstable isotopes, or parent isotopes, decay and form stable isotopes, or daughter isotopes Unstable isotopes have known half-lifes, the time it takes for half of the parent isotopes to decay to daughter isotopes

Scientists determine the age of substances by calculating the ratio of parent isotopes to daughter isotopes Example – if a rock contains an element with a half-life of 10,000 years, it will have half parent material and half daughter material when it is 10,000 years old 10,000 years 20,000 years 30,000 years 40,000 years 0 years

C. Types of Radiometric Dating 1. Potassium-Argon Method Used to date rocks older than 100,000 years Potassium-40 has a half-life of 1.3 billion years and decays to argon

2. Uranium-Lead Method Used to date rocks older than 10 million years Uranium-238 has a half-life of 4.5 billion years and decays to lead-206 in a series of steps

3. Rubidium-Strontium Method Used to date rocks older than 10 million years Radidium-87 has a half-life of 49 billion years and decays to Strontium-87

4. Carbon-14 Method Used to date things that lived within the last 50,000 years Carbon is found in constant ratios of carbon-14, a radioactive isotope, and carbon-12

Carbon-14 has a half-life of 5,730 years Theses isotopes combine with oxygen to form carbon dioxide (CO2) Carbon dioxide is taken in by plants for photosynthesis as long as it is alive

Animals eat these plants and thus take in carbon dioxide as well When a plant or animal dies, it no longer takes in carbon dioxide and the amount of carbon-14 begins to decrease

VI. Fossils A. Fossilized Organisms 1. Fossils in Rocks Most often preserved in sedimentary rock Hard parts of organisms are more resistant to decay

2. Fossils in Amber Hardened tree sap Best preserves insects

3. Petrifaction Process in which minerals replace an organism’s tissues Example: petrified wood

4. Frozen Fossils Cold temperatures slow down decay Many frozen fossils are preserved from the last ice age

B. Other Types of Fossils 1 B. Other Types of Fossils 1. Trace Fossils Example: tracks - Footprints are filled with sediment and preserved in rock

2. Molds and casts

C. Using Fossils to Interpret the Past 1. The information in the Fossil Record Fossil record is incomplete because most organisms never became fossils - More is known about organisms that had hard body parts and organisms that lived in environments that favored fossilization

2. History of Environmental Change Marine fossils help scientists reconstruct ancient coastlines Scientists can tell whether the climate in an area was cooler or wetter than it is at present

3. History of Changing Organisms Older rock layers contain organisms that often differ from the organisms found in younger rock layers

D. Using Fossils to Date Rocks Certain types of fossils appear only in certain layers of rock Scientists can determine the time span organisms lived by dating rock layers above and below these fossils

1. Index Fossils Fossils of organisms that lived during a relatively short time span found in layers throughout the world Ammonites and trilobites are index fossils that lived 208 and 400 million years-ago respectfully When scientists find these fossils, they know how old the rock is around these fossils

III. Geologic Time Scale Scientists study 4.6 billion years of Earth’s history Geologists have created a geologic time scale that divides Earth’s history into distinct intervals of time

A. Divisions of Time Represent shorter intervals in which visible changes have taken place on Earth Largest divisions of geologic time are eons - Hadean eon, Archean eon, Proterozoic eon, and Phanerozoic eon

The Phanerozoic eon has three eras - Paleozoic era, Mesozoic era, and Cenozoic era Eras are broken down into periods which are broken down into epochs

1. The Paleozoic Era – Old Life 542 to 251 million years ago First era well represented by fossils Marine life flourished Land plants, amphibians, reptiles, and insects appeared at the end of the Paleozoic era

2. The Mesozoic Era – The Age of Reptiles 251 million years ago Dinosaurs inhabited the land Birds appeared at the end of the Mesozoic era and small mammals appeared

3. The Cenozoic Era – The Age of Mammals 66.5 million years ago and continues to the present Unique traits, such as regulating body temperature internally and bearing young that develop inside the mother, may have helped mammals survive the environmental changes that probably caused the extinction of the dinosaurs

B. The Appearance and Disappearance of Species An increase in the number of species on Earth is a result of either a relatively sudden increase or decrease in competitions A decrease in the number of species on Earth is a result of extinction