© 2011 Pearson Education, Inc. Geologic Time Chapter 8.
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Transcript of © 2011 Pearson Education, Inc. Geologic Time Chapter 8.
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© 2011 Pearson Education, Inc.
Geologic TimeChapter 8
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Historical notes
• Catastrophism• Landscapes developed by catastrophes • James Ussher, mid-1600s, concluded
Earth was only a few thousand years old (created in 4004 B.C.)
• Modern geology• Fundamental principle of geology• Uniformitarianism aka “The present is the
key to the past”• James Hutton
• Theory of the Earth• Published in the late 1700s
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• “Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists.
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Geologic Time• There are two ways of dating geological
materials.– Relative ages – Based upon order of formation.
• Qualitative method developed 100s of years ago.
• Permit determination of older vs. younger relationships.
– Numerical ages – Actual number of years since an event.
• Quantitative method developed recently.
• Age is given a number.
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Relative vs. Absolute
• Relative ages
assign order
to events.
• Numerical ages
assign exact
dates to events.
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Relative dating
• Law of superposition• Developed by Nicolaus Steno in 1669• In an undeformed sequence of
sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom
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Superposition is well-illustratedby the strata in the Grand Canyon
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• Principle of original horizontality• Layers of sediment are generally deposited
in a horizontal position• Rock layers that are flat have not been
disturbed
• Principle of cross-cutting relationships• Younger features cut across older features
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Cross-cutting relationships
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• Inclusions• An inclusion is a piece of rock that is
enclosed within another rock• Rock containing the inclusion is younger
• Unconformity• An unconformity is a break in the rock
record produced by erosion and/or nondeposition of rock units
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• Unconformity• Types of unconformities
• Angular unconformity—Tilted rocks are overlain by flat-lying rocks
• Disconformity—Strata on either side of the unconformity are parallel
• Nonconformity—Metamorphic or igneous rocks in contact with sedimentary strata
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Mygeoscienceplace.comanimation
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Angular Unconformity
• “Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists.
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Fossils: Evidence of past life
• Fossil—traces or remains of prehistoric life now preserved in rock
• Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)
• Paleontology—study of fossils
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• Geologically fossils are important because they
• Aid in interpretation of the geologic past• Serve as important time indicators• Allow for correlation of rocks from different
places
• Conditions favoring preservation• Rapid burial• Possession of hard parts (skeleton, shell,
etc.)
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• Types of fossils• Petrified - turned into stone• Replacement – cell walls and and solid material replaced
with minerals• Mold and cast• Impressions• Amber• Coprolite• Tracks• Burrows
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Trilobite showing mold and cast preservation
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Petrified wood from Arizona
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Fossils and correlation
• Matching of rocks of similar ages in different regions is known as correlation
• Correlation often relies upon fossils• William Smith (late 1700s) noted that
sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content
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• Principle of fossil succession—Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content
• Index fossil—Geographically widespread fossil that is limited to a short span of geologic time
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Dating rocks using overlapping fossil ranges
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Dating with radioactivity
• Reviewing basic atomic structure• Nucleus
• Protons—positively charged particles with mass
• Neutrons—neutral particles with mass• Electrons—negatively charged particles that
orbit the nucleus
• Atomic number• Element’s identifying number• Equal to the number of protons
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• Reviewing basic atomic structure• Mass number
• Sum of the number of protons and neutrons
• Isotope• Variant of the same parent atom• Differs in the number of neutrons• Results in a different mass number than the
parent atom
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• Radioactivity• Spontaneous changes (decay) in the
structure of atomic nuclei
• Types of radioactive decay• Alpha emission
• Emission of 2 protons and 2 neutrons (an alpha particle)
• Mass number is reduced by 4 and the atomic number is lowered by 2
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• Types of radioactive decay• Beta emission
• An electron (beta particle) is ejected from the nucleus
• Mass number remains unchanged and the atomic number increases by 1
• Electron capture• An electron is captured by the nucleus and
combines with a proton to form a neutron• Mass number remains unchanged and the
atomic number decreases by 1
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• Parent—an unstable radioactive isotope
• Daughter product—the isotopes resulting from the decay of a parent
• Half-life—the time required for one-half of the radioactive nuclei in a sample to decay
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• Radiometric dating• Principle of radioactive dating
• The percentage of radioactive atoms that decay during one half-life is always the same (50 percent)
• However, the actual number of atoms that decay continually decreases
• Comparing the ratio of parent to daughter yields the age of the sample
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Radioactive decay curve
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• Dating with Carbon-14 (Radiocarbon dating)
• Used to date very recent events (half-life of 5,730 years) and historic events
• Continually produced in the upper atmosphere from cosmic-ray bombardment
• All living things have carbon-14 in them• Anthropologists, archeologists, and historians use
carbon-14 dating
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• Radiometric dating• Sources of error
• A closed system is required• To avoid potential problems, only fresh,
unweathered rock samples should be used
• Importance of radiometric dating• Rocks from several localities have been
dated at more than 3 billion years• Confirms the idea that geologic time is
immense
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The geologic time scale
• The geologic time scale—A “calendar” of Earth history
• Subdivides geologic history into units• Originally created using relative dates
• Structure of the geologic time scale• Eon—The greatest expanse of time
• Phanerozoic (“visible life”)—The most recent eon, began about 540 million years ago
• Proterozoic• Archean• Hadean—The oldest eon
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• Structure of the geologic time scale• Era—Subdivision of an eon• Eras of the Phanerozoic eon
• Cenozoic (“recent life”)• Mesozoic (“middle life”)• Paleozoic (“ancient life”)
• Eras are subdivided into periods• Periods are subdivided into epochs
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The geologic
time scale
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• Precambrian • Nearly 4 billion years prior to the Cambrian
period• Not divided into smaller time units because
the events of Precambrian history are not known in great enough detail
• First abundant fossil evidence does not appear until the beginning of the Cambrian
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• Difficulties in dating the geologic time scale
• Not all rocks can be dated by radiometric methods
• Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed
• The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed
• Datable materials (such as volcanic ash beds and igneous intrusions) are often used to bracket various episodes in Earth history and arrive at ages
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Bracketing sedimentary ages using igneous rocks
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Questions?