Chapter 3 The Rock and Fossil Record

Sections 1-5Pages 58-89

Principle of Uniformitarianism

• The Principle of Uniformitarianism – a principle that states that geological processes that occur in the past can be explained by current geological processes

• James Hutton wrote Theory of Earth about the processes that we observe (erosion and deposition) remain uniform (do not change) over time

Uniformitarianism vs. Catastrophism

• Uniformitarianism did not gain acceptance until Charles Lyell published Principle of Geology in 1833 to prove Hutton correct with his new evidence

• Catastrophism: principle that states geological change occurs suddenly and or rapidly

• Remained Geology’s guided principle until 1830’s

Modern Geology

• Late 20th century: Stephen Gould indicated that while most of Earth’s history is gradual, sometimes catastrophes play a part

( asteroid striking Earth, change global climate or cause extinction) • Mixture of both

uniformitarianism and catastrophism

Paleontology- study of past life (fossils)

• http://www.youtube.com/watch?v=B7zo2zY1Zqg

Relative Dating

• Determining whether an object is older or younger than other objects or events

• Geologists use rocks and fossils

Principle of Superposition

• The principle that states that younger rocks lie above older rocks in undisturbed sequences

• However, sometimes the oldest rocks are not on the bottom due to geological forces (tilting, folding and breaking)

Geological Column

• Used to help geologists, they combine data from all the known undisturbed rock sequences around the world

• Geologists use the geological column to interpret rock sequences and to identify layers in puzzling rock sequences

Disturbed Rock Layers

• Geologists use the relationship between rock layers and the features that cut across them to assign relative ages to the features and the layers

• Faults, intrusions, folding and tilting are examples of features that cut across rock layers

Gaps in the Record-Unconformities

• Missing rock layers create breaks in rock-layer sequences called unconformities

• An unconformity is a surface that represents a missing part of the geological column

• Unconformities also represent missing time

• Nondeposition- stoppage of deposition when a supply of sediment is cut off

• Erosion- (wind and water) allow deposition

Types of Unconformities

• Disconformities: part of a sequence of parallel rock is missing. (thousand to millions of years)

• Nonconformities: found where horizontal sedimentary rock layers lie on top of eroded surface of older intrusive ingenious or metamorphic rock. (millions of years)

• Angular unconformities: found between horizontal and sedimentary rock layers and rock layers that have been tilted or folded. (millions of years)

Absolute Dating

• The process of establishing the age of an object by determining the number of years it has existed is called absolute dating.

Radioactive Decay

• Unstable isotopes: radioactive• Radioactive isotopes tend to

break down into stable isotopes of the same or other elements in a process called radioactive decay.

• Occurs at a steady rate, scientists use the relative amounts of stable and unstable isotopes present in an object to determine the object's age.

Dating Rocks• After the unstable isotope breaks

down into a stable isotope, the stable isotope may be of the same element or more often a different element.

• The unstable radioactive isotope is called the parent isotope.

• The stable isotope produced is called the daughter isotope.

• Rate of decay is constant.• Scientists compare the amount of

parent isotopes to the daughter isotopes to determine the age of rock. The more daughter material, the older the rock is.

Radioactive Decay

• http://www.furryelephant.com/player.php?subject=physics&jumpTo=re/15Ms1

Radiometric Dating• If you know the rate of decay for a

radioactive element, you can determine the absolute age of the rock.

• Based on a parent to daughter ratio, you can determine the absolute age, this process is called radiometric dating.

• A half life is the time that it takes one half of the radioactive sample to decay.

• After every half-life, the amount of parent material decreases by one-half.

½, ¼, 1/8, 1/16

Try this example: Carbon 14’s half-life is 5, 730 years. How old will the object containing this be when ¼, 1/8, 1/16, 1/32 of it’s Carbon 14 remains.

Types of Radiometric Dating

• Potassium Argon Method: used for rocks older than 10,000 years old.

• Uranium Lead Method: used for rocks more than 10 million years old

• Rubidium Strontium Method: used for rocks over 10 million years old

• Carbon 14 Method: used for dating living things that lived within the last 50,000 years

Fossilized Organisms

• A fossil is the remains or physical evidence of an organism preserved by geological processes

• Fossils in rock: sometimes decaying organisms get quickly buried by sediment (rock), then the sediment becomes a rock. The harder parts of the animal (bones) become well preserved in this newly formed rock.

• Fossils in amber: Organisms (insects) get caught in soft sticky tree sap which hardens and preserves them.

Fossilized Organisms

• Petrifaction: process in which minerals replace an organism’s tissues.

• Permineralization is when pore space of an organism’s hard tissue bone or wood) is replaced with minerals.

• Replacement is when the organism’s tissues are completely replaced by minerals. (Petrified wood)

Fossilized Organisms

• Fossils in Asphalt: thick, sticky pools of asphalt on Earth’s surface (La Brea in Los Angeles), trapped and preserved organisms from 38,000 years ago!

• Frozen Fossils: the last ice age has preserved many types of fossils since the extreme cold drastically slows down decay (Wholly Mammoth, Siberian tundra)

Trace Fossils

• Any naturally preserved evidence of animal activity is called a trace fossil.

• Tracks• Burrows• Coprolite (animal dung)

Molds and Casts

• A mold is a cavity in rock where a plant or animal was buried.

• A cast is an object when sediment fills a mold and becomes rock.

Using Fossils to Interpret the Past

• The fossil record is incomplete because most organisms never became fossils…and some have not been discovered.

• The fossil record helps reveal a history of environmental change. Using the fossils of plants and land animals, scientists can reconstruct past climates.

• To fill in the missing information about changes in organisms in the fossil record, paleontologists look for similarities between fossilized organisms or between fossilized organisms and their closest living relative.

Using Fossils to Date Rocks

• Index fossils are fossils of organisms that lived during a relatively short, well-defined geological time span.

• Ammonites: tropites (mollusk/squid like), 230-208 mya

• Trilobites: phacops (horseshoe crab) 400 mya

Geologic Time

• The layers in the Grand Canyon represent nearly 2 billion years of Earth’s history.

• Well preserved plant and animal fossils are common in Green River because of fine-grained lake-bed sediments.

Geological Time Scale

• The Geological Time Scale accounts for Earth’s entire history.

• It is divided into 4 major parts called eons.

• Dates are estimates. • Represents about 4.6

billion years of Earth’s history.

Divisions of Timein size order (greatest to smallest)

• Eons: largest division of time, 4 major eons- Hadean, Archean, Proterozoic, and Phanerozoic.

• Era: there are 3 eras in the Phanerozoic Eon

• Periods: the 3 eras are divided into periods

• Epochs: the periods are divided into epochs

Extinction

• The death of every member of the species.

• Can be caused by global change and/or change in ocean currents

Paleozoic Era

• Marine life flourished• Land plants appeared• Amphibians and reptiles• Insects• Largest mass extinction

in Earth’s history….cause? Ocean currents?

Mesozoic Era

• Age of the Reptiles• Small mammals• Dinosaurs and birds• Extinction….cause?....

Global climate?

Cenozoic Era

• Age of Mammals• Mammals competed

with dinosaurs • Due to climate change…

mammals better suited for environment