I. History -Earth is believed to be somewhere around 4.6 billion years old. -Geologists use clues recorded in rock to reconstruct the events from the past. -Geologists use two ways to understand Earth’s history:

A. Uniformitarianism: -Process that remain uniform or do not change over time ex. Erosion and deposition

B. Catastrophism: -states that all earth’s changes occur suddenly ex. Mountains, canyons, and seas

C. Sudden Event - Today scientists think that sudden events are responsible for some changes during Earth’s past. ex. Asteroids

D. Past Life * Paleontology-the study of past life ex. Fossils *history of the Earth would be incomplete without the knowledge of the organisms that have lived on our planet and the conditions where they lived.

II. Rocks -geologist rely on rocks and fossils to help them place Earth’s events in order. A. Relative Dating -determining whether an object or event is older or younger than other objects or events.

B. Superposition -younger rocks lie above older rocks in undisturbed order. -not all rocks are arranged with older layers on bottom, some are disturbed by forces within the Earth.

C. Geological Column -an ideal order of rock layers that contain all the known fossils and rock formations on Earth. -scientists rely on this to interpret rock order.

D. Disturbing Rock Layers *if rock layers are not horizontal then something must of disturbed them such as: 1. Faults- a break in Earth’s crust. 2. Intrusion- molten rock from the Earth’s interior.

3. Folding-occurs when rock layers bend and buckle. 4. Tilting-occurs when internal forces in Earth slant rock layers.

* All make dating rock a challenge. E. Unconformity -missing rock layers that create breaks in rock layer sequences. -represent missing time (millions of years) -caused by erosion or disposition

*Types of unconformity 1. Disconformities -most common, found where part of a sequence of parallel rock is missing.

2. nonconformities -are found where horizontal rock sediment layers lie on top of an eroded surface of older rock

3. angular unconformities -found between horizontal layers of sedimentary rock and layers that have been tilted or folded.

III. Time -scientists determine an objects age by: A. Absolute Dating - the process of establishing the age of an object by determining the number of years it has existed.

1. Isotopes -same number of protons but different number of neutrons -helps to find the absolute age of rocks and fossils. -some can be unstable or radioactive. -some unstable isotopes can break down to stable isotopes.

B. Radiometric Dating -scientists use different types: 1. Potassium-Argon Method: mainly use to date racks older than 100,000 years 2. Uranium-Lead Method: used for rocks more than 10 million years 3. Rubidium-Strontium Method: used to date rocks older than 10 million years. 4. Carbon-14 Method: used to date fossils/living things that lived within the last 50,000 years.

IV. Fossils -remains of an organism that lived long ago. -often reserved in sedimentary rock.

A. Fossils in Rock After an organism dies it is quickly buried by sediment This slows down decay Hard parts of animals, such as bones and shells are more resistant to decay, so they are more commonly preserved.

B. Fossils in Amber When an insect gets covered by sap then hardens and preserves the insect inside.

C. Petrifaction -process in which minerals replace an organism’s tissue. Petrified wood-all wood has been replaced by minerals.

D. Fossils in Asphalt Places where asphalt wells up at Earth’s surface in thick, sticky pools This has trapped and preserved many kinds of organisms for the past 38,000 years Ex. La Brea in California

E. Frozen Fossils Cold temps. Slow down decay Ex. Ice age-mammoth

IV. Other types of Fossils Rare soft parts that organisms may leave behind.

A. Trace Fossils Naturally preserved evidence. ex. tracks

B. Molds and Casts Mold: a cavity in a rock where a plant or animal was buried. Cast: an object created when sediment fills a mold and becomes rock. Shows what the outside of an organism looked like.

V. Fossils and the Past Offer a rough sketch of the history of life on Earth. Fossils reveal a history of environmental change. How life has changed over time Ex. Fossils differ in rock layers

Fossils are used to date rocks. Certain types of fossils appear only in certain types of rock By dating rock layers above and below the fossils, scientists can determine the time span in which the organisms lived. Index fossils-fossils of organisms that lived during a relatively short geological time span. Ex. Ammonites and Trilobites

IV. Geological Scale -represents the billions of years that have passed since the first rock formed on Earth. The scale was created to make Earth’s years easier to understand. Earth’s history is divided into sections of time and its shown on the scale.

A. Eons Largest division of geological time. There are four

B. Era Includes two or more periods Second largest division of geological time Three Eras

C. Periods -Third largest 3 eras equal a period

D. Epochs Periods are divided epochs Fourth largest of geological time

* boundaries between geological time are marked by the disappearance of index fossils and others by paleontological studies.

V. Geological Eras A. Paleozoic Era-Old Life First era well represented by fossils. Marine life flourished at the beginning and a few land organisms In the middle of this era land plants had appeared At the end of the era amphibians, retiles, and insect lived on land Largest mass extinction in Earth history-it was believed that ocean changes were likely the cause

B. Mesozoic Era-Age of Reptiles Dinosaurs/Reptiles dominated this time period Small mammals and birds appeared late in this time period Dinosaurs became extinct-global climate change may have been the cause

C. The Cenozoic Era-Age of Mammals began about 65.5 million years ago and continues to the present. Regulating body temperature helped mammals survive.