Earth Science: Fossils, Superposition of Rock Layers, and Radioactive Dating

Glencoe Earth Science c1999Chap 12

Fossils—what are they

• Fossils are the remains, imprints, or traces of once-living organisms

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?)

Petrified Wood and Bones

Petrified Wood

Petrified Bones

Types of Fossils (2)

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

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)

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)

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