Download Geologic Time

Geologic Time
Philosophies on Earth Dynamics
• Catastrophism (up to late 1700’s)
• Landscape developed by catastrophes
• Features such as mountains and canyons formed quickly by
catastrophic events
• James Ussher, a mid-1600s Anglican Archbishop, concluded
Earth was only a few thousand years old (4004 BC)
• Used the Bible to construct a chronology of Earth and human
history
• Widely accepted by scientists and religious leaders
Philosophies on Earth Dynamics
• Uniformitarianism
• James Hutton (father of modern geology)
• Theory of the Earth
• Published in the late 1700s
• Marked the beginning of modern geology
• Fundamental principle of geology
• The present is the key to the past
• The physical, biological, and chemical laws that operate
today have operated the same way throughout geologic
time
Uniformitarianism
• Hutton’s theory was based on scientific observations on
mountain building, and weathering and erosional features
• BUT, Hutton’s book was BORING and difficult to read, so
people gave up on reading it
– His ideas weren’t widely accepted.
– Contradicted the Church
– Playfair wrote a book that presented Hutton’s ideas
more clearly. That helped.
• Charles Lyell published Principles of Geology in 1830
– Much better publication
– Greatly advanced people’s knowledge of geologic
processes
Relative dating
 Placing rocks and events in sequence
 Principles and rules of
• Law of superposition – oldest rocks are on the bottom
• Principle of original horizontality – sediment is
deposited in horizontal beds
• Principle of cross-cutting relationships – younger
features cut through older features
Superposition illustrated by the strata
(layers) in the Grand Canyon
Cross-cutting relationships
Relative dating
 Principles and rules of
• Inclusions – fragments of one rock unit that are
contained within another (rock containing the
inclusions is younger than the inclusions)
• Unconformities
• An unconformity is a break in the rock record
– This means that a portion of geologic time is
“missing” from the rock record in that area.
– Indicates a period of weathering and erosion
– Old erosional surface
Relative dating
Types of Unconformities
• Angular Unconformity - tilted rocks are overlain by flat
layers of rocks
• Disconformity – Layers above or below are parallel
• Sometimes difficult to identify
• Rocks above and below could have significantly
different ages, OR
• The sequence of the rock TYPES isn’t “in order”
• Nonconformity - Older metamorphic or igneous rocks are
overlain by younger sedimentary rock layers
Relative dating
Types of Unconformities
Relative dating
Formation of an angular
unconformity
Relative dating
Unconformities of the Grand Canyon
Relative dating
Correlation of rock layers
• Matching rocks of similar age in different regions
• Often relies upon fossils
Fossils: evidence of past life
• Remains of prehistoric life, OR
• Anything other evidence that the organisms left behind (trace
fossils)
• Types of fossils
• Petrified – cavities and pores are filled with mineral matter
that was dissolved in underground water
• Formed by replacement – cell material is removed and
replaced with mineral matter
• Mold – shell or other structure is buried and then dissolved by
underground water, leaving a hollow space
• Cast – hollow space of a mold is filled with mineral matter
Fossils: evidence of past life
• Types of fossils
• Carbonization – organic matter becomes a thin residue
of carbon
• Impression – replica of the fossil's surface preserved in
fine-grained sediment
• Preservation in amber – hardened resin of ancient trees
surrounds an organism
Cast fossils of shelled invertebrates
Fossils: evidence of past life
• Trace fossils
• Indirect evidence includes
• Tracks
• Burrows
• Coprolites – fossil dung and stomach contents
• Gastroliths – stomach stones used to grind food by some
extinct reptiles
Dinosaur footprints near El Paso, at
Mt. Cristo Rey
Fossils: evidence of past life
• Conditions favoring preservation
• Rapid burial
• Possession of hard parts
• Fossils and correlation
• Principle of fossil succession
• Fossils succeed one another in a definite and
determinable order
• Proposed by William Smith – late 1700s and early
1800s
Determining the ages of rocks using fossils
Fossils: evidence of past life
Fossils and correlation
• Index fossils
• Widespread geographically
• Existed for a short range of geologic time
Absolute Dating
Absolute dating means finding the actual age in years
Atomic Structure Review
• Protons – positively charged particles in the nucleus
• Neutrons – particles in the nucleus with no charge. Formed
by protons and electrons colliding during supernovae
• Electrons – negatively charged particles that orbit the
nucleus
Radioactivity and Radiometric Dating
Atomic structure reviewed
• Atomic number
• An element's identifying number
• Number of protons in the atom's nucleus
• Mass number
• Number of protons plus the number of neutrons in
an atom's nucleus
• Isotope
• Atoms of the same element with different
numbers of neutrons, and different mass
numbers
• Many isotopes are radioactive
Radioactivity and Radiometric Dating
• Radioactivity
• Spontaneous breaking apart (decay) of atomic nuclei
• Radioactive decay
• Parent – an unstable isotope
• Daughter products – stable isotopes formed from the decay
of a radioactive parent
Radioactivity and Radiometric Dating
• Radioactivity
• Radioactive decay
• Types of radioactive decay
• Alpha emission
• Beta emission
• Electron capture
Half Lives of some Common Isotopes
Radioactive Decay and Half Life
Half life is the amount of time it takes for a radioactive isotope to
decay down to 50% of its original mass
 Carbon-14 dating
• Half-life of only 5730 years
• Used to date very recent events (not rock ages, usually)
• Used to find the ages of organic matter (must contain carbon
to begin with)
• Useful tool for anthropologists, archeologists, historians, and
geologists who study very recent Earth history
Geologic Time Scale
• Divides geologic history into units
• Originally created using relative dates
• Subdivisions
• Eon
• Greatest expanse of time
• Four eons
• Phanerozoic ("visible life") – the most recent eon,
plenty of fossils
• Proterozoic
• Archean
• Hadean – the oldest eon
(The Proterozoic, Archean, and Hadean are often referred to as just the Precambrian)
Geologic Time Scale
• Subdivisions
• Era (Subdivision of an Eon)
• Eras of the Phanerozoic eon
• Cenozoic ("recent life")
• Mesozoic ("middle life")
• Paleozoic (“early or ancient life")
• Eras are subdivided into periods
• Periods are subdivided into epochs
The Geologic Time Scale
Geologic Time
• Difficulties in dating the time scale
• Not all rocks are datable
• Sedimentary rock ages are often not reliable because
the sediment that comes together to form a rock is older
than the rock itself.
• Dateable materials are used to estimate the ages of events
and the rocks involved in the events.
• A combination of radiometric dating and field
observations are necessary to obtain accurate ages