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6-Understanding Planet Earth > Chapter 10: Geologic Time > Flashcards

Chapter 10: Geologic Time Flashcards

1
Q

Geologic time provides a frame of reference for understanding:

A
  • Rocks
  • Fossils
  • Geologic structures
  • Landscapes
  • Tectonic events
  • Change
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2
Q

Explain James Hutton’s principle of uniformitarianism

A

“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.
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3
Q

What are the two ways of dating geological materials? Explain them.

A

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.

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4
Q

What are the 6 physical principles of geologic time?

A
  • 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
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5
Q

Explain the principle of original horizontality

A
  • 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.
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6
Q

Explain the principle of superposition

A

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.
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7
Q

Explain the principle of lateral continuity

A
  • Strata often form laterally extensive horizontal sheets.
  • Subsequent erosion dissects once-continuous layers.
  • Flat-lying rock layers are unlikely to have been disturbed.
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8
Q

Explain the principle of cross-cutting relations

A
  • 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
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9
Q

Explain the principle of baked contacts

A
  • An igneous intrusion cooks the invaded country rock.

- The baked rock must have been there first (it is older).

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10
Q

Explain the principle of inclusions

A

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.
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11
Q

What do physical principles allow us?

A

Physical principles allow us to sort out relative age.

This is possible even in complex situations.

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12
Q

What is the principle of fossil succession? Explain

A
  • 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.
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13
Q

What is conformable deposition?

A

Sediments in the ocean generally accumulate without interruption for millions or tens of millions of years - conformable

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14
Q

What is hiatus?

A

lapse of time recorded by an unconformity

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15
Q

What are unconformities?

A

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

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16
Q

What is an angular unconformity? How is it formed? Explain

A

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.
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17
Q

What is a nonconformity?

A

Igneous/metamorphic rocks capped by sedimentary rocks

  • Igneous or metamorphic rocks were exposed by erosion.
  • Sediment was deposited on this eroded surface.
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18
Q

What is a disconformity?

A

Parallel strata bounding nondeposition

  • Due to an interruption in sedimentation:
  • Pause in deposition
  • Sea level falls, then rises
  • Erosion
  • Often hard to recognize
19
Q

What is Stratigraphic Correlation?

A
  • 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
20
Q

What is the geologic column?

A

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.

21
Q

What are the names of eons on the geologic time scale?

A
  • 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)
22
Q

What are the names of eras on the geologic time scale?

A
  • Cenozoic—“recent life”
  • Mesozoic—“middle life”
  • Paleozoic—“ancient life
23
Q

O2 from _______ built up in atmosphere by 2 Ga.

A

cyanobacteria

24
Q

Around 700 Ma, ______ evolved.

A

multicellular life

25
Q

Around 542 Ma marks the first appearance of _______.

A

invertebrates

26
Q

Life diversified rapidly as the “______”.

A

Cambrian Explosion

27
Q

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
A

C

28
Q

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

A?

29
Q

What is numerical age and how are they determined?

A
  • 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.
30
Q

Numerical age study is also called ______.

A

geochronology

31
Q

What are isotopes? What are the types? Explain

A

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)

32
Q

What are radioisotopes?

A

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)

33
Q

What is radioactive decay?

A

Radioactive decay progresses along a decay chain.

  • Decay creates new unstable elements that also decay.
  • Decay proceeds to a stable element endpoint.
34
Q

What is a parent isotope?

A

the isotope that undergoes decay

35
Q

What is a daughter isotope?

A

the product of the decay

36
Q

What is a half-life? Explain in detail

A

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

37
Q

What is Uranium-Lead dating? How is it done?

A
  • 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.
38
Q

What is radiometric dating?

A

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.

39
Q

What is an isotopic date?

A

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.

40
Q

How are numerical ages are possible without isotopes? Give examples

A
  • Growth rings—annual layers from trees or shells

- Rhythmic layering—annual layers in sediments or ice

41
Q

How are sediments geologically dated?

A
  • 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.
42
Q

How did they dated the age of the Earth before radiometric dating? What were the theories?

A

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.
43
Q

When did isotopic dating begin?

A
  • Radioactivity discovered in 1896 by Henri Becquerel. Led to isotopic dating beginning in 1950s.

6-Understanding Planet Earth (18 decks)

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