CHAPTER 8 GEOLOGIC TIME

FOSSILS Preserved remains or traces of living things Form when something dies and is covered by sediment that hardens into rock (Sedimentary Rock) Usually are hard parts of an organism – teeth, bones, shells, seeds, woody stems

Types of Fossils Molds – hollow area in sediment in the shape of the organism Cast – a solid copy of the shape of the organism Petrified fossils – “turned into stone”, minerals replace all parts of an organism Carbon film – thin coating of carbon on a rock, Trace Fossils – provide evidence of the activities of an organism, foot prints

Preserved Remains Little or no change to the organism Frozen remains Amber – hardened resin or sap from trees La Brea Tar Pits (CA) – animals were trapped in tar and died, tar soaked into their bones and preserved them

Change Over Time Paleontologist – study fossils Use fossils to determine what past life forms were like Classify organisms based on when they lived Information collected makes up the fossil record Fossil record provide evidence about past life and how life has changed. It also provides us evidence of past environments

Fossils and Past Environments Fossils provide evidence of past climates EX. Coal in Antarctica indicates that in the past it was much warmer Fossils are used to learn about changes to Earth’s Surface Ex. Fossils of past marine animals that are found in present day dry areas.

Fossils and Change Fossil record shows an order gives evidence to the Theory of Evolution Fossils record shows that many organisms have become EXTINCT – non longer exists

Relative Age of Rocks/ Fossils Geologist have two ways to express the age of rocks/fossils: Relative Age – age of rocks compared to the age of other rocks Absolute Age – number of years since the rock formed

Position of Rock Layers Absolute age can be hard to determine Relative age can be determined using the LAW OF SUPERPOSITION: In horizontal sedimentary rock, older layers are at the bottom and younger layers are closer to the top.

Determining Relative Age Geologist study faults, igneous rocks and gaps in the geologic record to determine relative age. Igneous Rock Forms when lava or magma cools Extrusion – lava that hardens on the surface Younger than the rocks below it Intrusion – magma that has pushed into bodies of rocks and cooled Younger than the rock layers around and beneath it

FAULTS Break in the Earth’s crust Rocks on opposite sides of faults move in different directions Faults are younger than the rocks they cut through Two types of faults: Dip slip – two pieces of land change their vertical position compared to one another Strike slip – two pieces of land move horizontally

Unconformity Surface where new rock layers meet a much older rock surface beneath them Occurs when some layers are eroded away, exposing older layers, then newer layers are deposited

Using Fossils to Date Rocks Index fossil – fossils of widely distributed organisms that lived for a known, short time EX. Ammonites Since these animals only lived for a short time, their remains can be used to determine the age of the layer in which it was found

Absolute dating Also called Radiometric Dating Puts an estimated number on the age of the rock layer Performed on Igneous rocks Cannot be performed on sedimentary rock Made of pieces of different rocks

Half Life Amount of time it take for half of the parent material to turn into (decay) daughter material. Must know half life to determine age of rock or fossil

Molten rock cools Inside rock is unstable radioactive atoms (parent atoms) Unstable atoms decay to stable atoms (daughter atoms) By comparing the amount of unstable atoms left to the amount of stable atoms, an estimate can be given about the age of the rock Carbon 14 and Potassium/Argon dating are types of absolute dating

The Geologic Time Scale Is a record of life forms and geologic events in Earth’s history GTS is used because Earth’s time span is so long Units used are called ERAS (long units of time) and periods Ceno – recent Meso – middle Paleo – ancient or early -zoic -- life

Major changes in life forms are used to mark a new unit geologic time Precambrian Era – 88 percent of Earth’s history Major Events Formation of: Sun and light Earth (4.6 bya) Atmosphere (from volcanic gasses) Oceans (water vapor condensing) life

Paleozoic Era Began with early invertebrates (no backbone)– arachnids, insects Then came vertebrates (fish, amphibians) First plants, then some seed plants Mass extinction to end the ERA

Mesozoic Era Reptiles dominated (including dinos) Small mammals and birds appeared Flowering plants developed Mass extinction ended the ERA

Cenozoic Era New mammals appeared Diversity of life increased Flowering plants the most common Human show up