ch13 geologictime
TRANSCRIPT
Chapter 12
Geologic Time
Outline• Geologic time: perspective & a bit of history
• Dating geologic materials-General: relative & absolute dating-Relative dating:
-7 Principals & their application to a geologic history-Fossil successions
• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column
• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth
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Chapter 12
Geologic Time
• Magnitude of Earth’s past is amazing.• Discovering this forever altered our perception of
ourselves within nature & the Universe.
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Geologic Time
• Understanding time permits assigning ages to…• Rocks.• Fossils.• Geologic structures.• Landscapes.• Tectonic events.
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Geologic Time• Prior to late 1600s, geologic time was thought to =
historical time. • Archbishop James Ussher, Ireland, 1654.
• He added up generations from Old Testament.
• Determined Earth formed on Oct 23, 4004 BCE.
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Geologic Time• Scientists began to find clues to a more ancient Earth…
• Nicolaus Steno (1638–1686; danish physician).• Observed marine fossils high in mountains. • Deduced they were ancient animals.• Processes of lithification & uplift suggested long time periods.
fossilsharktooth
Outline• Geologic time: perspective & a bit of history
• Dating geologic materials-General: relative & absolute dating-Relative dating:
-7 Principals & their application to a geologic history-Fossil successions
• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column
• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth
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Geologic Time2 ways to date geological materials:
1. Relative age – Based on order of formation.• Qualitative method developed 100s of yrs ago.• Determine older vs. younger relationships.
Chapter 12
Geologic Time2 ways to date geological materials:
1. Relative age – Based on order of formation.• Qualitative method developed 100s of yrs ago.• Determine older vs. younger relationships.
2. Numerical (absolute) age – # of yrs since an event.• Quantitative method developed recently. • Numerical age assigned.
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Relative vs. Absolute
1. Relative ages
assign event order.
2. Numerical ages
assign dates to events.
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Relative Age
• Logical tools are useful for defining relative age.• Principles of:
1. Uniformitarianism
2. Superposition
3. Original horizontality
4. Original continuity
5. Cross-cutting relations
6. Inclusions
7. Baked contacts.
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Geologic Time1. Uniformitarianism – “The present is key to the past”.
• Physical processes we observe today have always operated the same way (ie. in the geological past).
• Modern processes help us understand ancient events.
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Defining Relative Age
2. Superposition. • In an undeformed sequence of layered sed. rocks…
• Younger rocks on top; older rocks below.
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Relative Age
3 & 4. Horizontality and continuity.• Seds deposited in horizontally extensive layers.• Erosion then dissects once-continuous layers.• Flat-lying layers unlikely to have been disturbed.
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Relative Age5. Cross-cutting relations.
• Younger features cut across older features.• Faults, dikes, erosion etc., must be younger than the
material that is faulted, intruded, or eroded.• E.g. a volcano cannot intrude rocks that aren’t there yet.
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Relative Age6. Inclusions – a rock fragment within another.
• Inclusion is older than surrounding material.
• Eg.:• Igneous xenoliths – Country rock that fell into magma.
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Relative Age
7. Baked contacts.• Thermal (contact) metamorphism occurs when
country rock is intruded by igneous rock. • Baked rock is older than the intruded rock.
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Relative Age• Determining relative ages empowers geologists to
unravel complicated geologic histories.
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Geologic History
• Deposition of horizontal strata below sea level in order 1 8 (old to young). *Horizontality & continuity*
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Geologic History
• Igneous intrusion of a sill. *baked contact*
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• Intrusion solidified into sill
• Tectonic compression
Geologic History
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• Compression results: • Folding (inference: layers had to exist to be folded).• Uplift (above sea level) & erosion.
• Intrusion of a pluton. *baked contact/cross-cutting*
Geologic History
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• Extension -> normal faulting.• Faulting cross-cuts pluton & rock layers.
Geologic History
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• Dike intrusion.• Dike cross-cuts everything (even normal fault).
Geologic History
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• Erosion to present landscape.• Removed volcano and cuts down the dike top.
Geologic History
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Geologic History• Relative ages help to unravel a complicated history.• Those rules permit one to decipher this diagram!
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Geologic History
• Test yourself at home:
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Geologic History
A cross-section through the earth reveals the variety of geologic features. View 1 of this animation identifies a variety of geologic features; View 2 animates the sequence of events that produced these features, and demonstrates how geologists apply established principles to deduce geologic history. For more information, see Section 12.4 Principles for Defining Relative Age starting on p.418 and Figure 12.5 in your textbook.
Geologic History
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Fossil Succession
• Fossils (organism traces) can be preserved in sedimentary rocks. • Useful for relative age determination.
• Several fossil types will occur as an assemblage.• Fossils are time markers.
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Fossil Succession• Species evolve, exist, and then go extinct.
• 1st appearance to extinction dates rocks. • Fossils succeed one another in a known order.• A time period is recognized by fossil assemblage.
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• Fossil range – first to last appearance.• Each fossil has a unique time range.• Overlapping ranges provide
distinctive time markers.• Also index fossils (unique).
• Permit correlation of strata.• Locally to globally.
Fossil Succession
Outline• Geologic time: perspective & a bit of history
• Dating geologic materials-General: relative & absolute dating-Relative dating:
-7 Principals & their application to a geologic history-Fossil successions
• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column
• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth
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Unconformity
• An unconformity is a time gap in the rock record.• Causes: non-deposition or erosion.
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Unconformities3 Types:
1. Disconformity – parallel strata bracketing non-deposition.• Due to an interruption in sedimentation.• Can be difficult to recognize.
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DisconformitiesDisconformities
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Unconformities3 Types:
2. Nonconformity – Metamorphic/igneous rocks overlain by sedimentary strata.
• Igneous/metamorphic rocks exposed by erosion.• Sediment deposited on eroded surface.
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NonconformityNonconformity
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Unconformities3 Types:
3. Angular unconformity – represents a big gap in time.• Horizontal rocks deposited, then deformed (i.e. titled/folded).• Then eroded.• Then sediments horizontally deposited on erosion surface.
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Types of Unconformity
This animation shows the stages in the development of three main types of unconformity in cross-section, and explains how an incomplete succession of strata provides a record of Earth history. View 1 shows a disconformity, View 2 shows a nonconformity and View 3 shows an angular unconformity.
Types of Unconformity
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Unconformities• Earth history is in
strata.• Missing strata =
missing history. • The Grand Canyon:
• Thick strata.• Many gaps (red).• Partial record of
geologic past.
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Stratigraphic Correlation
• In 1793, William “Strata” Smith noted strata could be matched across distances. • Similar rock types in a similar order. • Rock layers contained same distinctive fossils.
• He made the 1st geologic map (of the UK).
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Stratigraphic Correlation• Stratigraphic columns depict strata in a region.
• Drawn to portray relative thicknesses.• Rock types depicted by fill patterns. • Divided into formations (mapable rock units).• Formations separated by contacts.
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Stratigraphic Correlation• National Parks of Arizona & Utah.
• Formations can be traced long distances.• Overlap in rock type sequences.• Overlapping rock columns are used to build a composite.
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The Geologic Column• A composite global stratigraphic column exists.
• Constructed from incomplete sections across the globe.• It brackets almost all Earth history.
Outline• Geologic time: perspective & a bit of history
• Dating geologic materials-General: relative & absolute dating-Relative dating:
-7 Principals & their application to a geologic history-Fossil successions
• Gaps in the geologic record (unconformity)-3 types of unconformities-Stratigraphic correlation & the global geologic column
• Numerical (absolute) dating-Radioactive decay-Meaning of a radiometric date-Other numerical dating methods-Dating the geologic column, geologic time scale, & age of Earth
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Numerical (Absolute) Dating
• Based on radioactive decay of atoms in minerals.• Radioactive decay proceeds at a known, fixed rate.• Radioactive elements act as internal clocks.
• Numerical dating is called geochronology.
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Radioactive DecayIsotopes
• Atoms with same # of protons, different # of neutrons. • Have similar, but different mass numbers.
Some are stable – never change (i.e., 13C).
Some are unstable (radioactive) – Spontaneously change to something else (decay) at a fixed rate (i.e., 14C).
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Radioactive Decay• Decay process has 2 main components:
• Parent – isotope that decays.• Daughter – decay product isotope.
• Decay process can: • Have 1 step (parent -> daughter)• Have many steps (parent -> daughter -> etc…)
• Decay product is also unstable and hence also decays.
• Eventually proceeds to a stable endpoint.
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Radioactive Decay Time• Half-life (t½) – time for ½ unstable parent to decay.
• t½ is unique for each isotope.
• After one t½ - ½ original parent remains.
• After three t½ - 1/8th original parent remains.
• Parent disappears (nonlinear), daughter accumulates.
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Radiometric Dating• Mineral age can be determined by:
• Measuring parent/daughter isotope ratio.• Calculating time by using the known t½.
• Must pick the right mineral & isotope. • Geochronology requires analytical precision.
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What Is a Radiometric Date?
• Time since a mineral began to retain all parent & daughter isotopes. • Requires cooling below “closure (blocking) temperature.”
• Daughter retained only below closure T.• Daughter leaks out above closure T.• Thus, if rock is reheated above closure T, the radiometric clock
can be reset to zero.
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Other Numerical Ages
• Numerical ages are possible without isotopes. • Growth rings – Annual layers from trees or shells. • Rhythmic layering – Annual layers in seds or ice.
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Other Numerical Ages• Magnetostratigraphy – Magnetic signatures in strata
are compared to global reference column.
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Other Numerical Ages• Decay process can cause scars (tracks) in minerals.
• Decay by fission (explosion) produces scar (track).• Daughter isn’t another isotope, it’s a damage zone.
• Fission Tracks!
• Track density (daughter) is proportional to age.
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Dating the Geologic Column• Use geochronology to:
• Date specific strata OR • Bracket those that can’t be dated directly.
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The Geologic Time Scale
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Age of the Earth
• Oldest rocks are 3.96 Ga.• Zircon minerals in some sandstones are 4.1-4.2 Ga.• Earth is ~4.57 Ga based on correlation with…
• Meteorites, moon rocks.
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End
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Geologic Time• Deep time – The immense span of geologic time.• So vast, difficult to grasp.
• We think of time in terms of our lives…
• The lives of our parents and grandparents.
• The lives of our children or grandchildren.• Human history is tiny compared to geologic time.
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Geologic Time• James Hutton (1726-1797) – Scottish physician.
• Called “the Father of Modern Geology.” • Stated the Principle of Uniformitarianism.• Of time, He wrote: “we find no vestige of a
beginning; no prospect of an end.”
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Geologic Time• Principle of Uniformitarianism:
• “The present is the key to the past.”• Processes seen today are same as in the past.
e.g. Old mudcracks formed as mudcracks do today.• Geologic change = slow; large changes = long time.
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Stratigraphic Correlation• Lithologic correlation is based on rock type.
• Sequence – The relative order in which the rocks occur.
• Limited to correlation between nearby regions.• Fossil correlation – Based on fossils within rocks.
• Applicable to much broader areas.
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Correlation among rock strata in 3 national parks.Correlation among rock strata in 3 national parks.
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Angular Unconformity• James Hutton - 1st to realize the vast time-significance
of angular unconformities.• Mountains created, then completely erased.• Then new sed. deposition.
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Angular Unconformity
• “Hutton’s Unconformity” on Siccar Point, Scotland, is a common destination for geologists.
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• The composite column is divided into time blocks.• This is the geologic time scale, Earth’s “calendar.”
• The structure of the geologic time scale.
• Eons – The largest subdivision of time (100s to 1000s Ma).
• Eras – Subdivisions of an eon (65 to 100s Ma).
• Periods – Subdivisions of an era (2 to 70 Ma).
• Epochs – Subdivisions of a period (0.011 to 22 Ma).
Geologic Time
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• Time scale subdivisions are variously named.• The nature of life (“zoic” means life); i.e.,
Proterozoic.• A characteristic of the time period; i.e.,
Carboniferous.• A specific locality; i.e., Devonian.
Geologic Time
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Geologic Time and Life
• Life first appears on Earth ~ 3.8 Ga.• Early life consisted of anaerobic
single-celled organisms. • Oxygen from cyanobacteria
built up by 2 Ga.• ~ 700 Ma, multicellular
life evolved.• ~ 542 Ma marks the
1st appearance
of hard shells. Shells increased fossil Shells increased fossil
preservation.preservation.
Life diversified rapidly Life diversified rapidly ––the the ““Cambrian Explosion.Cambrian Explosion.””
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• Names of the eons.• Phanerozoic – “Visible life” (542 Ma to the present).
• Started 542 Ma at the Precambrian – Cambrian boundary.• Marks the 1st appearance of hard shells.• Life diversified rapidly afterward.
• Proterozoic – “Before life” (2.5 to 0.542 Ga). • Development of tectonic plates like those of today.• Buildup of atmospheric O2; multicellular life appears.
• Archean – “Ancient” (3.8 to 2.5 Ga).• Birth of continents.• Appearance of the earliest life forms.
• Hadean – “Hell” (4.6 to 3.8 Ga). • Internal differentiation.• Formation of the oceans and secondary atmosphere.
The Geologic Time Scale
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The Geologic Time Scale• Names of the eras.
• Cenozoic – “Recent life.”
• 65.5 Ma to present.
• The “Age of Mammals.”• Mesozoic – “Middle life.”
• 251 to 65.5 Ma.
• The “Age of Dinosaurs.”• Paleozoic – “Ancient life.”
• 542 to 251 Ma.
• Life diversified rapidly.
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The Age of the Earth
• Before radioactivity-based dating methods…• 20 Ma – From Earth cooling.• 90 Ma –Ocean salinization.
• Assumed oceans were initially freshwater.
• Measured the mass of dissolved material in rivers.
• Uniformitarianism and evolution indicated an Earth older than ~100 Ma.
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Geologic Time• The immensity of time is beyond comprehension.• Metaphors illustrate the scale of time.
• The age of Earth (4.6 Ga) can be compared to pennies.
• Lined up, 4.6 billion pennies would be 87,400 km long.
• More than twice around Earth.
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Geologic Time
What a Geologist Sees:
Unconformity
The photo shows the Siccar Point unconformity in Scotland, on the coast about 60 km east of Edinburgh; the sketch shows a geologist’s interpretation of the unconformity. For more information, see Section 12.5 Unconformities: Gaps in the Record starting on p.423 and Figure 12.8 in your textbook.
Zoomable Art:
The Record in Rocks: Reconstructing Geologic History
When geologists examine a sequence of rocks exposed on a cliff, they see a record of Earth history that can be interpreted by applying the basic principles of geology, searching for fossils, and using radiometric dating. For more information, see the Featured Painting on pp.426-427 in your textbook.
What a Geologist Sees:
Stratigraphic Column
The succession of rocks in the Grand Canyon can be divided into formations based on notable changes in rock type and changes in fossil assemblages. For more information, see Section 12.6 Stratigraphic Formations and Their Correlation starting on p.424 and Figure 12.11 on p. 429 in your textbook.