Chapter 13 CLUES TO EARTH’S PAST

Section 1: FOSSILS Types of fossils Mineral replacement Carbon films Coal Molds and casts Original remains Trace fossils Trails and burrows

Mineral Replacement Permineralized remains result from minerals in water or moist sediments replacing the cells of dead organisms, leaving a copy of the organism. Permineralized wood

Carbon Film Living organisms contain carbon. When they die and get covered by sediments, the pressure and heat squeeze out everything but a carbon “picture” of the organism.

Coal Coal is a FOSSIL fuel. Animation of coal formation: http://www.sceyencestudios.com/movies/coalformation.swf

Molds and Casts An organism falls into a soft sediment, like mud. More sediment covers the organism. The organism decays or becomes permineralized. The sediment hardens, and if the rock is cracked apart, a mold and cast are formed Mold: the impression—or indentation-- of the organism. Cast: the raised copy of the organism.

Original Remains The whole organism is preserved. In amber (tree resin) In ice In tar

In Ice 37,000 year old frozen remains of a baby wooly mammoth found in Siberia, Russia.

Iceman 5,000 year old remains of man found frozen in a glacier in the Alps.

Tar Mammoth Mastodon Saber-tooth cat Giant sloth Radiometric dating of preserved wood and bones found in the tar pits of La Brea, California has given an age of 38,000 years for the oldest known materials found there.

Giant sloth Saber-tooth cat

Trace Fossils Trace fossils represent activities of the animal while alive, rather than part of the dead creature. Dinosaur footprints

Trails and Burrows Trails and burrows are evidence of where animals have walked or lived.

Trails and Burrows Modern ant burrows

More Fossil Evidence . . . Fossilized nests Coprolite (fossilized feces)

Index Fossils Index fossils are fossils of organisms that lived for a short period of time that are found in many places. They are used to help date layers of Earth because only certain fossils will be found in certain layers.

More on Index Fossils Index fossils can help geologists correlate layers of Earth and determine the relative age of the rock they in which they are found. These layers are probably the same age. These layers are probably the same age.

Ancient Environments Fossils can help scientist determine what the environment was like in an area long ago. Antarctica has fossils of ferns, so it must have been warm and tropical at one time. Kansas has fossils of huge fish and other sea creatures. It is very likely it was once covered by a shallow sea.

Kansas Under Water Here is where Kansas is believed to have been located when covered by a shallow sea between 82 and 87 million years ago.

Section 2: RELATIVE AGES OF ROCKS The principle of superposition states that the oldest rocks are on the bottom.

Relative Ages Rock layers can be compared to each other in terms of older, younger, between, etc. They are compared, or RELATED, to each other by age. There is not an exact number of years used in relative dating. If an older index fossil is found in a top layer, and a younger fossil is found in a lower layer, a geologist would conclude that the layers may have been flipped over during compression.

UNCONFORMITIES Unconformities in rock layers can be difficult to see in reality. Unconformities are gaps in the rock sequence where some layers are missing. Three main types of unconformities Nonconformities Disconformities Angular unconformities

A Modern Unconformity Comparing layers in two trash pits at two universities.

How an Angular Unconformity Forms 2 3 1 2: The layers are uplifted by forces within Earth, then eroded. 3: New layers are then deposited over the angled layers, creating an angular unconformity where the new layers meet the angled layers. 1: Horizontal rock layers are deposited over time. A is the oldest layer.

Disconformities 3 1 2 2: Upper layers are eroded away. New layers are deposited on top of old layers, leaving a gap in the history where the other layers should be. 1: Horizontal rock layers are deposited over time. 1 is the oldest layer.

Nonconformity A nonconformity can occur when igneous rock invades rock layers that already exist.

Another Look at Nonconformity

AND . . .Another Nonconformity Layers A – D were uplifted from pressure. The top of C was eroded. E – G were then deposited on top. E is next to C in one place.

A look at all three . . . http://www.indiana.edu/~geol105b/images/gaia_chapter_6/unconformities.htm

Correlating Rock Layers Correlation means “matching up.” Rock layers in many places can match up.

Layers exposed in Zion National Park, Utah Layers exposed in Grand Canyon, Arizona Layers exposed in Bryce Canyon, Utah

Stratigraphic Matching Stratigraphic matching uses correlation to match patterns in layers in different regions. Correlation lines

Section 3: ABSOLUTE AGES OF ROCKS Absolute age of rocks is the age IN YEARS of the rock. The age is determined by using radiometric dating, which looks at how radioactive material decays within rock layers. Two types of radioactive decay: Alpha decay Beta decay

Alpha Decay In alpha decay, two neutrons and two protons (particles in the nucleus of an atom) break away from the parent atom as an alpha particle. The resulting daughter atom becomes a new type of element.

Beta Decay In beta decay, a neutron from an parent atom’s nucleus changes into a neutrino and a proton. The resulting daughter atom becomes a new type of element.

Half-Life in Radioactive Decay A radioactive element decays in a known amount of time. Half of all of the element will decay in a time period known as a “half-life.” Carbon-14 takes 5,730 years to break down through beta decay into nitrogen, but half of it will remain. After another 5,730 years, half of the remaining half will decay. Scientists can determine the age of the fossil by comparing the amount of carbon-14 to the amount of daughter nitrogen in the fossil sample.

How Does Carbon-14 Get Into Living Things?

More on Carbon-14 Since carbon is found in all living things, carbon-14 is a good element to use for dating fossils. The bad thing is, it can only date fossils up to 75,000 years old because it will decay completely into nitrogen by then. Other radioactive elements with longer half-lives are used to radiometrically date older rock.

Using Radiometric Dating to Find Ages of Rocks Aside from using Carbon-14 to date fossils, radiometric dating only works on igneous and metamorphic rock. Igneous rock is “new” rock, and when it cools, all the particles in a sample are the same age. Metamorphic rock is changed under heat and pressure. All particles in a sample will have changed at the same time. Sedimentary rock contains particles of rock from many different samples, so the age of the particles can be different within the sample.

Relative vs. Absolute Age Relative dating determines if one layer is older or younger than another. Absolute dating uses radioactive materials in igneous and metamorphic rock to determine their ages in years.

Oldest Rocks on Earth The oldest rocks on Earth are about 3.96 billion years old. Earth is about 4.5 billion years old. The rock older than 3.96 billion years probably eroded away or was subducted. Scientists use meteors that have landed on Earth to determine the age of Earth. Meteors that hit Earth had to have come from nearby in our galaxy, which is a little bit older than our Earth, since the galaxy began at the same time. Particles in the galaxy formed together into stars and planets due to gravitational pull. Meteors were a part of the debris.

A Galaxy

Uniformitarianism In the mid-1700s James Hutton estimated that Earth is much older than previously thought. He came up with the principle of uniformitarianism. It states that the Earth processes happening now are similar to those in the past—and those yet to come. The principle is often stated as “the present is the key to the past.”

Earth processes happen one of two ways . . . Earth processes can happen slowly over thousands or even millions of years. Plate movement Changes in environments are usually slow Earth processes can happen violently and quickly. Volcanic eruptions can build mountains quickly. Comets, asteroids, and meteorites can collide with Earth, changing things abruptly. Dinosaur extinction Weather changes due to sun being blocked by debris

Earth: What an amazing place!