Chapter 10: Geologic Time Flashcards
Geologic time provides a frame of reference for understanding:
- Rocks
- Fossils
- Geologic structures
- Landscapes
- Tectonic events
- Change
Explain James Hutton’s principle of uniformitarianism
“The present is the key to the past.”
- Processes seen today are the same as those of the past.
- Geologic change is slow; large changes require a long time.
- Therefore, there must have been a long time before humans.
What are the two ways of dating geological materials? Explain them.
Relative ages—based upon order of formation
- Qualitative method developed hundreds of years ago.
- Permit determination of older vs. younger relationships.
Numerical ages—actual number of years since an event
- Quantitative method developed recently.
What are the 6 physical principles of geologic time?
- The principle of original horizontality
- The principle of superposition
- The principle of lateral continuity
- The principle of cross-cutting relations
- The principle of baked contacts
- The principle of inclusions
Explain the principle of original horizontality
- Sediments settle out of a fluid by gravity.
- This causes sediments to accumulate horizontally.
- Sediment accumulation is not favored on a slope.
- Hence, tilted sedimentary rocks must be deformed.
Explain the principle of superposition
In an undeformed sequence of layered rocks:
- Each bed is older than the one above and younger than the one below.
- Younger strata are on top, older strata on bottom.
Explain the principle of lateral continuity
- Strata often form laterally extensive horizontal sheets.
- Subsequent erosion dissects once-continuous layers.
- Flat-lying rock layers are unlikely to have been disturbed.
Explain the principle of cross-cutting relations
- Younger features truncate (cut across) older features.
- Faults, dikes, erosion, etc., must be younger than the material that is faulted, intruded, or eroded.
- A volcano cannot intrude rocks that aren’t there
Explain the principle of baked contacts
- An igneous intrusion cooks the invaded country rock.
- The baked rock must have been there first (it is older).
Explain the principle of inclusions
Rock fragment within another
- Inclusions are always older than the enclosing material.
- Weathering rubble must have come from older rock.
- Fragments (xenoliths) are older than igneous intrusion.
What do physical principles allow us?
Physical principles allow us to sort out relative age.
This is possible even in complex situations.
What is the principle of fossil succession? Explain
- Fossils are often preserved in sedimentary rocks.
- Fossils are time markers useful for relative age-dating.
- Fossils speak of past depositional environments.
- Specific fossils are only found within a limited time span.
- Species evolve, exist for a time, and then disappear.
- First appearance, range, and extinction are used for dating.
- Global extinctions are caused by extraordinary events.
- Fossils succeed one another in a known order (evolution)
- A time period is recognized by its fossil content.
What is conformable deposition?
Sediments in the ocean generally accumulate without interruption for millions or tens of millions of years - conformable
What is hiatus?
lapse of time recorded by an unconformity
What are unconformities?
In contrast, on the continents, sedimentation is disrupted periodically by environmental changes (e.g., uplift, transgression/regression, glaciations) that lead to intervals of erosion or non-deposition - unconformities
What is an angular unconformity? How is it formed? Explain
An angular unconformity represents a huge gulf in time.
- Horizontal marine sediments deformed by orogenesis
- Mountains eroded completely away
- Renewed marine invasion
- New sediments deposited.
What is a nonconformity?
Igneous/metamorphic rocks capped by sedimentary rocks
- Igneous or metamorphic rocks were exposed by erosion.
- Sediment was deposited on this eroded surface.
What is a disconformity?
Parallel strata bounding nondeposition
- Due to an interruption in sedimentation:
- Pause in deposition
- Sea level falls, then rises
- Erosion
- Often hard to recognize
What is Stratigraphic Correlation?
- determination of equivalence, in geological age and position, of the succession of stata found in two or more different areas
- To develop a geological calendar that is applicable to the whole Earth, rocks of similar age in different regions must be matched up
What is the geologic column?
Intervals are distinguished according to the types of life forms of the time. The succession of fossils preserved in strata defines the course of life’s evolution through Earth’s history.
What are the names of eons on the geologic time scale?
- Phanerozoic—”visible life” (542 to 0 Ma)
- Proterozoic—“before life” (2.5 to 0.542 Ga)
- Archean—“ancient” (3.8 to 2.5 Ga)
- Hadean—“hell” (4.6 to 3.8 Ga)
What are the names of eras on the geologic time scale?
- Cenozoic—“recent life”
- Mesozoic—“middle life”
- Paleozoic—“ancient life
O2 from _______ built up in atmosphere by 2 Ga.
cyanobacteria
Around 700 Ma, ______ evolved.
multicellular life
Around 542 Ma marks the first appearance of _______.
invertebrates
Life diversified rapidly as the “______”.
Cambrian Explosion
The “present is the key to the past” best describes the principle of ________.
A.Original horizontality B.superposition C.uniformitarianism D.Lateral continuity E.inclusions
C
An eroded granite covered by flat-lying sedimentary rocks is an example of a(n):
A.nonconformity B.disconformity C.inclusion D.Angular unconformity E.Marker bed
A?
What is numerical age and how are they determined?
- Numerical ages give age of rocks in years.
- Based on radioactive decay of atoms in minerals.
- Radioactive decay proceeds at a known, fixed rate.
- Radioactive elements act as internal clocks.
Numerical age study is also called ______.
geochronology
What are isotopes? What are the types? Explain
Elements that have varying numbers of neutrons
Isotopes have similar but different mass numbers.
- Stable—isotopes that never change (i.e., 13C)
- Radioactive—isotopes that spontaneously decay (i.e., 14C)
What are radioisotopes?
have an unstable combination of protons and neutrons in the nucleus, in which case the parent radio-isotope undergoes radioactive decay to form an isotope of the same element or of a different element (a daughter)
What is radioactive decay?
Radioactive decay progresses along a decay chain.
- Decay creates new unstable elements that also decay.
- Decay proceeds to a stable element endpoint.
What is a parent isotope?
the isotope that undergoes decay
What is a daughter isotope?
the product of the decay
What is a half-life? Explain in detail
The half-life is the time it takes for half of an unstable nuclei to decay. The half-life is a unique characteristic of each isotope. As a parent disappears, the daughter “grows in.
After one half-life, one-half of the original parent remains. After three half-lives, one eighth of the original parent remains
What is Uranium-Lead dating? How is it done?
- The most trusted decay system for dating ancient rocks and minerals is the U-Pb sequence.
- 235U decays with a half-life of 713Ma; 238U with 4.5Ga.
- The daughter products are 207Pb and 206Pb.
- Chemically, in rocks and in minerals, both U parents and Pb daughters behave almost identically and are assembled into mineral (zircon being the preferred) similarly.
- We have, then, two clocks ticking away within the zircon and one checks the other.
- By forming ratios 207Pb/ 235U and 206Pb/ 238U according to the amounts of each isotope measured by mass-spectroscopy and plotting one ratio against the other, the Concordia diagram gives us age of the zircon.
What is radiometric dating?
The age of a mineral is determined by measuring the ratio of parent to daughter isotopes. The age can be calculated from a knowledge of the parent half-life. Geochronology requires analytical precision.
What is an isotopic date?
Isotopic dating give the time a mineral began to preserve all atoms of parent and daughter isotopes, which requires cooling below a “closure temperature.” If rock is reheated, the radiometric clock could be reset. Igneous and metamorphic rocks are best for geochronologic study; sedimentary rocks cannot be directly dated.
How are numerical ages are possible without isotopes? Give examples
- Growth rings—annual layers from trees or shells
- Rhythmic layering—annual layers in sediments or ice
How are sediments geologically dated?
- Geochronology is less useful for sedimentary deposits.
- Sediment ages can be bracketed by numerical ages.
- Date adjacent igneous and megamorphic rocks.
- Apply principle of cross-cutting relationships.
- Age ranges narrow as data accumulate.
- Geologic time scale dated in this way.
How did they dated the age of the Earth before radiometric dating? What were the theories?
Before radiometric dating, age estimates varied widely.
- Lord Kelvin estimated Earth cooling at –20 Ma.
- Uniformitarianism and evolution indicated an Earth much older than ~100 Ma.
When did isotopic dating begin?
- Radioactivity discovered in 1896 by Henri Becquerel. Led to isotopic dating beginning in 1950s.
