History of the Earth A matter-of-time scale… If the age of the Earth (4.6 billion years) were condensed into one year ... Jan. 1… Earth was born Early Feb… Oldest known rocks formed Late Mar… First primitive life formed Mid-Nov... First complex life with shells or skeletons formed Late-Nov… First land animals Dec 25… Extinction of the dinosaurs Dec 31… Humans evolved in the evening Dec 31… One second before midnight, humans first set foot on the Moon

BIG Ideas: Scientists use several methods to learn about Earth’s long history. The oceans and atmosphere formed and life began during the three eons of the Precambrian, which spans nearly 90% of Earth’s history. Complex life developed and diversified during the three eras of the Phanerozoic as the continents moved into their present positions.

HOW have scientists investigated and understood the history of Earth and the ages of rocks?

By studying rocks and fossils…

Uniformitarianism: geologic processes occurring today have been occurring since Earth formed the present is the key to the past

I. Determining Relative Age The age of a rock or an event as compared with some other rock or event A comparison (older vs. younger); does NOT indicate an exact age

Principle of Original Horizontality: most sedimentary layers of rock are deposited in a horizontal position

A. Law of Superposition: an undeformed sedimentary rock layer is older than the one above it and younger than the layer below it.

How would you explain this?

B. Unconformities: breaks in the geologic record

An UNCONFORMITY is… a GAP in the sedimentary rock record (like missing pages from a book). a BURIED erosional surface.

Is this an unconformity?

NO…just heavily eroded rocks (they remain HORIZONTAL)

Law of Crosscutting Relationships: a fault or intrusion is always younger than the rock layer it cuts through.

Put these rock layers in order, oldest to youngest…

II. Determining Absolute Age The actual age of the rock.

A. Varve Count How can you tell how old this tree is?

Varves: annual layers of sedimentary rock - consist of a light-colored band of coarse particles - and a darker band of fine particles

B. Radioactive Decay As elements age, they change into new elements… Uranium (U) will decay into lead (Pb) - in about 4.5 billion years

1. Half Life: the time it takes for half the mass of a given amount of a radioactive element to decay into its daughter elements.

U-238 decays into Pb-206 U-238 has a half-life of 4.5 billion years If we were to begin with 10g of U-238, after 4.5 billion years you would have 5g of U-238 left After another 4.5 billion years, how much U-238 would remain? How much Pb-206 would you have?

2. Carbon Dating Organic materials (remains of once-living things) decay at a measurable rate Can be used to estimate the ages of samples from 0 to 70,000 years old

Would you use half life or carbon dating to date this tooth? Why?

III. The Fossil Record Paleontologist: Scientist that studies fossils. Fossils: the remains or traces of animals or plants from a previous geologic time.

What type of rock are nearly all fossils found in? WHY?

Fossils may be preserved in a number of ways: Molds: sediment preserves the form of a fossil Casts: created by filling a mold with mineral or other material Imprints (trace fossils): tracks or footprints Original bone or shell

Trace Fossils:

Amber: the fossilized remaining part of tree resin (which is very thick and sticky when it first flows from a tree) resin runs down the trunk and may trap insects, spiders, and sometimes larger organisms (like lizards) these organisms can be preserved for millions of years with details of their soft tissue, such as muscles and hair-like bristles, still intact

Most Virginia fossils are of marine organisms. What does this suggest about the past environment of our state?

Large areas of the state have been periodically covered by seawater.

Where are most fossils found in Virginia?

Coastal Plain Valley and Ridge Appalachian Plateau

How are fossils used to determine the ages of rocks?

Index Fossils: fossils found exclusively in rock layers of a particular geologic age. Phacops (a species of Trilobite): lived 400 million years ago in oceans

Requirements for Index Fossils: Must be present in rocks scattered over a wide area of Earth’s surface Must have unique, distinguishing features Organisms must have lived during a relatively short span of geologic time Must occur in fairly large numbers within the rock layers

IV. The Geologic Time Scale Divided into units: EON: the LARGEST unit of geologic time Eons are divided into Eras Eras are divided into Periods Periods are divided into Epochs

PRECAMBRIAN The three earliest EONS make up 90% of geologic time Known together as ‘PRECAMBRIAN’ During this time, Earth was formed and became hospitable to modern life. 4.6 billion-540 million years ago

ERA: based on relative ages of life forms found in rocks Paleozoic (paleo = old): 540 million-248 million years ago Mesozoic (meso = middle): 248 million-65 million years ago Cenozoic (ceno = recent): 65 million years ago to present (zoic = of life)

During what ERA did humans ‘appear’ on Earth? What ERA are we in right now?