Glencoe Earth Science c1999 Chap 12 Earth Science: Fossils, Superposition of Rock Layers, and Radioactive Dating Glencoe Earth Science c1999 Chap 12

Fossils—what are they Fossils are the remains, imprints, or traces of once-living organisms Need pictures of all three: remains, imprints, or traces

Conditions Needed Fossilize The dead organism must be protected from scavengers and microorganisms, so it must be buried quickly by sediment or something else. Organisms are more likely to fossilize if they have hard parts like bones, shells, or teeth. Fossils are usually found in sedimentary rocks (This is because the sediments that cover them become sedimentary rocks, and the heat and pressure needed to form igneous or metamorphic rocks often destroy fossils.)

Types of Fossils 1. Petrified Remains: bones, wood, etc. Hard and rock-like Some or all of the original materials have been replaced by minerals For example, water flowing over and through a bone may dissolve the calcium in the bone and replace it with quartz. (Scientific name for this?) NEED PICTURE

Petrified Wood and Bones Petrified Bones Petrified Wood

Types of Fossils (2) Carbonaceous Films—outlines of organisms Most tissues contain carbon Buried in sediment Over time, more sediment cause more pressure and heat on the dead tissue This forces gases and liquids from the tissue, leaving an outline of the original organism NEED PICTURE

Carbonaceous Film

Types of Fossils (3) 3. Molds and Casts Seashells or other hard parts fall into soft sediment, which eventually turns into rock. Pores in the rock let water and air reach the shell, which then decays, leaving a void in the rock called a mold The mold can be filled with other sediments which harden, creating a cast

Casts

Types of Fossils (4) Original Remains When the actual remains of an organism are found Examples Insect trapped in amber Human frozen in glacier Animal found in tar pit

Fossils: Original Remains Insect trapped in amber

Types of Fossils (5) Trace Fossils Fossilized tracks or other evidence of animal activity

Trace Fossils Worm trace burrow Dinosaur Track

Index Fossils Index fossils are from species that existed on Earth for a relatively short period of time Were abudndant Were widespread geographically Used to help date rock layers

Fossils and Ancient Environments The types of fossils we find tells us about what the environment used to be like at that location Examples: Antarctica used to be tropical Iowa was once covered by a warm, shallow, sea (limestone and brachiopods fossils)

Brachiopod Fossils

Relative Dating (not your cousin!) Relative dating is determining which rocks are the oldest, then the next oldest, etc.—the order of age Relative dating does NOT tell you the actual age of the rocks, only which rocks are younger or older

Principle of Superposition States that in an undisturbed layer of rock, the oldest rocks are on the bottom and the rock layers become younger going up.

Unconformities Unconformities are GAPS in the rock layers Three types Angular Disconformities Nonconformities

Angular Unconformities Rock layers are tilted by tectonic forces The tilted layers are eroded New rock layers form on top of the tilted layers

Angular Unconformities

Disconformities Rock layers form The top layer is eroded, and may disappear altogether New rock layers form on top, leaving a gap in the record

Disconformities

Nonconformities When two different rock types meet

Summary: Types of Unconformities

Absolute Dating Used to determine the age, in years, of a rock or other object Uses the idea of Radioactive Decay All elements are determined by the number of protons in their nucleus When the number of protons change, the element changes.

Half-Lives Example: Unstable isotope Uranium-238 eventually becomes Lead-206 by emitting protons and neutrons The Half-Life of a radioactive isotope is the amount of time it takes for half of the atoms in the isotope to decay into a different element The half life of Carbon-14 is 5730 years

Radiometric Dating The original element is called the parent When it decays into another element, it is called a daughter Radiometric Dating: measuring and comparing the amounts of parent to daughter elements

Parent-Daughter Radioactivity

Radiocarbon Dating Carbon-14 is useful for dating fossils, bones, and wood up to 50,000 years old Carbon-14 exists in the atmosphere, and we breathe it in. When we die, we don’t take in any more Carbon-14, and the Carbon-14 decays into Nitrogen-14 gas. The amount of Carbon-14 left in a sample can help determine its age

Principle of Uniformitarianism 1700’s—James Hutton (Scotland) and Charles Lyell (England) Uniformitarianism: Earth processes occurring today are similar to those that occurred in the past. He observed that rocks form very slowly now, so he concluded that rocks have always formed very slowly

Catastrophism Is the opposite of Uniformitarianism States that large catastrophes can quickly influence how land is shaped and when organisms become extinct. Example: Mt. St. Helens, Washington