Chapter 10: Geologic Time

Question 1

Geologic time provides a frame of reference for understanding:

Answer 1

• Rocks
• Fossils
• Geologic structures
• Landscapes
• Tectonic events
• Change

Question 2

Explain James Hutton’s principle of uniformitarianism

Answer 2

“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.

Question 3

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

Answer 3

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.

Question 4

What are the 6 physical principles of geologic time?

Answer 4

• 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

Question 5

Explain the principle of original horizontality

Answer 5

• 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.

Question 6

Explain the principle of superposition

Answer 6

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.

Question 7

Explain the principle of lateral continuity

Answer 7

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

Question 8

Explain the principle of cross-cutting relations

Answer 8

• 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

Question 9

Explain the principle of baked contacts

Answer 9

• An igneous intrusion cooks the invaded country rock.

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

Question 10

Explain the principle of inclusions

Answer 10

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.

Question 11

What do physical principles allow us?

Answer 11

Physical principles allow us to sort out relative age.

This is possible even in complex situations.

Question 12

What is the principle of fossil succession? Explain

Answer 12

• 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.

Question 13

What is conformable deposition?

Answer 13

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

Question 14

What is hiatus?

Answer 14

lapse of time recorded by an unconformity

Question 15

What are unconformities?

Answer 15

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

Question 16

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

Answer 16

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.

Question 17

What is a nonconformity?

Answer 17

Igneous/metamorphic rocks capped by sedimentary rocks

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

Question 18

What is a disconformity?

Answer 18

Parallel strata bounding nondeposition

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

Question 19

What is Stratigraphic Correlation?

Answer 19

• 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

Question 20

What is the geologic column?

Answer 20

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.

Question 21

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

Answer 21

• 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)

Question 22

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

Answer 22

• Cenozoic—“recent life”
• Mesozoic—“middle life”
• Paleozoic—“ancient life

Question 23

O2 from _______ built up in atmosphere by 2 Ga.

Answer 23

cyanobacteria

Question 24

Around 700 Ma, ______ evolved.

Answer 24

multicellular life

Question 25

Around 542 Ma marks the first appearance of _______.

Answer 25

invertebrates

Question 26

Life diversified rapidly as the “______".

Answer 26

Cambrian Explosion

Question 27

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 ```

Answer 27

C

Question 28

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 ```

Answer 28

A?

Question 29

What is numerical age and how are they determined?

Answer 29

- 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.

Question 30

Numerical age study is also called ______.

Answer 30

geochronology

Question 31

What are isotopes? What are the types? Explain

Answer 31

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)

Question 32

What are radioisotopes?

Answer 32

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)

Question 33

What is radioactive decay?

Answer 33

Radioactive decay progresses along a decay chain. - Decay creates new unstable elements that also decay. - Decay proceeds to a stable element endpoint.

Question 34

What is a parent isotope?

Answer 34

the isotope that undergoes decay

Question 35

What is a daughter isotope?

Answer 35

the product of the decay

Question 36

What is a half-life? Explain in detail

Answer 36

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

Question 37

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

Answer 37

- 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.

Question 38

What is radiometric dating?

Answer 38

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.

Question 39

What is an isotopic date?

Answer 39

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.

Question 40

How are numerical ages are possible without isotopes? Give examples

Answer 40

- Growth rings—annual layers from trees or shells | - Rhythmic layering—annual layers in sediments or ice

Question 41

How are sediments geologically dated?

Answer 41

- 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.

Question 42

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

Answer 42

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.

Question 43

When did isotopic dating begin?

Answer 43

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