EARTH SCIENCE

A TRIP THROUGH GEOLOGIC TIME FOSSILS

How a Fossil Forms BrainPOP Fossils-the preserved remains or traces of living things Fossil formation Living thing dies and is buried in sediment Sediments slowly harden into rock and preserve the shapes of the organism

Cast- a solid copy of the shape of an organism Mold Fossil --- Mold- a hollow area in sediment in the shape of an organism or part of one Cast- a solid copy of the shape of an organism Cast Fossil ---

Petrified Fossils- fossils in which minerals replace all or part of an organism Carbon Films- an extremely thin coating of carbon on a rock Trace Fossils- provide evidence of the activities of ancient organisms

Change Over Time Paleontologist- a scientist who studies fossils They use the fossil record to build a picture of Earth’s past environment The fossil record shows that older rocks contain simpler organisms while younger rocks contain more complex ones This supports the theory of evolution Scientific theory- a well tested concept that explains a wide variety of observations Evolution- the gradual change in living thihngs over time

Quick Check 1. What is a fossil? 2. In general, how does a fossil form? 3. What are the five different kinds of fossils? 4. What does the fossil record show about how life has changes over time? 5. What type of scientists studies fossils?

The Relative Age of Rocks

Relative vs. Absolute Age Relative age Absolute age Age compared to other rocks The number of years since a rock has formed

The Position of Rock Layers The law of superposition- In horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the one below it.

Determining Relative Age Geologists use clues from igneous rock and faults Clues from igneous rock: Igneous rock is formed from hardened magma Magma that hardens on the surface is called extrusion; this rock is always younger than the rock below it Magma that cools below the surface is called intrusion; this rock is always younger than the rock that surrounds it

Clues from faults: A fault is a break in Earth’s crust A fault is always younger than the rock it cuts through To determine the relative age of the fault, geologists determine the age of the lowest layer cut by the fault

Index Fossils An index fossil is one that is widely distributed and that represents an organism that existed only briefly Index fossils ell the relative ages of the rock layers in which they occur

Quick Check 1. In your own words, explain the difference between relative age and absolute age 2. What is the law of superposition? 3. What are the three types of clues that geologists use to find the relative age of rocks?

Radioactive Dating

Radioactive Decay BrainPOP All rocks are made of matter, all matter is composed of atoms Most elements are stable, but others are not Overtime these unstable elements break down during a process called radioactive decay During this process atoms of one element break down into atoms of another element

Half life All unstable(radioactive) elements have a “half life’ Half-life- the amount it takes for half of the radioactive element to decay

Radioactive Elements Elements commonly used in radio active dating: Carbon-14 Potassium-40 Rubidium-87 Thorium-232 Uranium-235

Quick Check 1. Define “radioactive decay 2. How does the composition of a radioactive element change over time? 3. If the half life of Carbon-14 is 20 years, how much carbon-14 will be left in a 100g sample after 100 years have passed? Show your work

The Geologic Time Scale

http://hereistoday.com/

Geologic time scale- a record of the life forms and geologic events in Earth’s history Divisions of the Geologic Time Scale: Eras Eras are divided into periods

Early Earth

1. Planet formation Rocks, ice and debris collided to become our planet. It was hot, fiery and hit by asteroids.

2. Atmosphere formation Earth was hot with volcanoes and magma. Eruptions released toxic gasses that became a protective layer (atmosphere)

Volcanoes and gasses

3. Rain and Land formation Volcanoes released water, which cooled and condensed in the atmosphere to make clouds. The clouds rained, and rain cooled the surface of the magma.

4. Ocean formation In the low areas, water accumulated. These become oceans.

Oceans

5. Life & Oxygen formation The first form of life developed in the oceans. These were one-cellular, plant-like bacteria that used photosynthesis to get energy.

Blue-Green Algae (Cyanobacteria)

Stromatolites

How do we know? A geyser in Nevada spews boiling water six feet. Cyanobacteria-ancient oxygen-producing algae-leave blue-green streaks on the rock, a moist, mineral-rich environment that may resemble the setting where the first life evolved.