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
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• Some or all of the original materials have been
replaced by minerals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