Geologic Time: Concepts and Principles Introduction · Geologic Time: Concepts and Principles Introduction ... James Hutton’s Contributions to Geology (1726-1797)!1 - Father of

Geologic Time: Concepts and Principles

Introduction

- An appreciation for the immensity of geologic time is essential for understanding the history of our planet - Geologists use two references for time

- Relative time: places events in chronological order - does not tell us how long ago events were

- Absolute time: results in specific dates for rocks or geologic events

- calculated from natural rates of decay

Concept of Geologic Time and Earth’s Age

- Scriptural references and analyses led to the belief of a very young Earth

- Archbishop Usher and John Lightfoot used the Book of Genesis in the Old Testament to get the age of earth - sum of the lifespans of Adam and Eve and their descendants, added five days - October 26, 4004 BC at 9:00am

- de Buffon assumed that Earth cooled at the same rate as molten iron balls and arrived at an age of 75,000 years

- Deposition rates have been used, but don’t take into account varying rates or erosion so they yield wide estimates

- Ocean salinity and the rate of dissolved ion transport from streams to the ocean yielded an age of 90 million years

James Hutton’s Contributions to Geology (1726-1797) ! 1

- Father of modern geology: because of careful observations of outcrops

- tried to deduce how much time was required for sandstone to form - longer than 6000 years!

- Established the principle of uniformitarianism

- The same chemical and physical process that we see today have operated the same throughout earth’s history. “The present is the key to the past.”

- Charles Lyell published Principles of Geology in 1830, and the idea that slow changes over many years could have tremendous cumulative effects gained favor among geologists

Relative-Dating Methods

- Six fundamental principles are used to place geologic features and events in sequential order

- Superposition: oldest units on the bottom - Original horizontality: sedimentary rocks deposited as flat layers, any other attitude(position) implies a deformation event after deposition - Lateral continuity: strata extends laterally in all directions until it thins, pinches out, or reaches the edge of the depositional basin

These rock layers of the Grand Canyon show the above principles

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(Superposition, Original horizontality, Lateral continuity)

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- Cross-cutting relationships: a feature or unit that cuts another is younger

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- Inclusions: fragments of one rock contained within a layer of another are older than the layer containing them

A. Granite is younger than the sandstone because it contains inclusions and the sandstone is baked B. Sandstone is younger than the granite because it contains inclusions. Granite is the source for the sediment that formed the sandstone.

(A) (B)

- Fossil succession: fossil assemblages succeed one another through time in a regular and predictable order

- Depends on three criteria

1. Life has varied through time 2. Fossils are visually different from each other 3. Relative age can be determined

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! (Fossils can be used to identify strata of the same age in different areas)

Unconformities

Unconformities: surfaces of nondeposition or erosion representing missing time in the rock record (hiatus)

- Three types of unconformities

- disconformity: an erosional surface between parallel beds - may resemble a sequence of ordinary strata - many must be identified on the basis of fossil

assemblages

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In (b) there was a one million year period of erosion that eroded two million years of rock. This results in a 3 million year gap that is represented in (c). Deposition resumes

three million years ago to the present. Unconformity at 3/6 million year boundary.

–angular unconformity - an erosional surface on top of tilted strata, over which younger rocks have been deposited

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–nonconformity - an erosional surface on top of igneous or metamorphic rock

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uplifting

nonconformity

Applying the Principles of Relative Dating

- The geologic history of an area can be deciphered by using the principles outlined previously

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! How Do Geologists Correlate Rock Units?

- Correlation: is the demonstration of time equivalency of rock units in different areas

- lateral continuity: no location has a record of all geologic events that have occurred, therefore, must correlate from one area to next.

- key beds: distinctive layers that helps with identification different areas

- guide fossils (index fossils): widespread geographically and existed for short periods of time

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Subsurface Correlation

How do we correlate when we can’t see the rock layers?

- Well cuttings: rock chips that are examined during well drilling to determine rock types and position in sequence - can also tell us porosity, permeability, fossils - Well logs: are developed based on the electrical or radioactive properties of subsurface rocks

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Bad guide fossil

Good guide fossils

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- Seismic profiling: indicates rock density and contacts and is especially useful in offshore and remote locations

- explosions create vibrations that have different vibrations through different material

Absolute-Dating Methods and Their Importance

- Revisit atoms - Nucleus: Protons and Neutrons: found in center of atom - Energy orbits: Electrons: encircle atom - Isotopes: elements that have varying number(s) of neutrons - Atomic number: number of protons - Atomic mass: mass of protons and neutrons

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- Radioactive isotopes: spontaneously decay to more stable isotopes, releasing energy in the process

ex. Carbon

! ! ! Radioactive Isotopes

- Decay rate is constant: this gives geologists a ‘clock’ to accurately date rocks, as well as a mechanism to explain Earth’s high residual heat

- The decay rate of isotopes is measured to determine the absolute age

- Three types of radioactive decay 1) Alpha decay: 2 protons and 2 neutrons are lost

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Atom

- loss of 2 atomic numbers and 4 mass numbers

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2) Beta decay: loss of an electron from a neutron - increase of one atomic number. (1 proton)

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3) Electron capture: proton captures an electron - decrease one atomic number

- proton to neutron

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- Radioactive Decay and Half-Lives

- Half-life: the time it takes for one-half of the unstable parent element (radioactive isotope) to decay to atoms of a more stable daughter element - Half-lives for given elements are constant and have a wide range and can be accurately measured - Some elements undergo one decay step in their conversion, others require multiple decay steps - Decay is geometric, not linear

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! example

How many radioactive isotopes and daughter elements will you have after 4 half-lives if you start out with 80 parent elements?

answer 1st half-life: 80 x 50% = 40 40 parent 40 daughter

2nd half-life 40 x 50% = 20 20 parent 60 daughter

3rd half-life 20 x 50% 10 parent 70 daughter

4th half-life 10 x 50% = 5 5 parent 75 daughter

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example: If a fossil has only one fourth the original number of parent elements, and the half-life of the parent is 4000 years, how old is the fossil?

answer: - Determine the number of half-lives. - The sample has undergone 2 half-lives - 2 x 4000 years = 8000 years old

example: If the ratio of parent/daughter elements is 1:8, how many half-lives have taken place

answer: - 1st half-life: 1:2 ratio - 2nd half-life: 1:4 ratio - 3rd half-life: 1:8 ratio

Sources of Uncertainty ! 16

•The most accurate radiometric dates are obtained from igneous rocks, and date the time of their crystallization •Sedimentary rocks can’t be reliably dated •A closed system must exist - parent or daughter atoms can’t have been added or removed since crystallization •Metamorphism may drive parent or daughter atoms from the mineral •Two different pairs may be used to cross-check

Long-Lived Radioactive Isotope Pairs •Half-lives in the range of millions or billions of years •Uranium-Lead and Thorium-Lead are the most commonly used for igneous intrusions, lunar samples, and some meteorites •Rubidium-Strontium used for very old samples •Potassium-Argon used for whole-rock analysis of fine grained volcanics

Fission Track Dating •Crystal structure damage during decay leaves tracks that are visible after etching with acid •The number of tracks corresponds to the age of the sample: greater number of tracks = older sample

Radiocarbon and Tree-Ring Dating Methods •Carbon 14 dating is based on the ratio of C 12 to C 14 and is used to date once-living material •Tree-ring dating is used for recent geologic events; cross dating makes this technique useful up to 14,000 years

How Was the Geologic Time Scale Developed? •4.6 billion-year history of Earth is divided into time units •Evolved from the efforts of many people using relative dating techniques, fossil assemblages, and eventually absolute dates

Stratigraphy and Stratigraphic Terminology •The study of layered rocks required that standard terminology be developed to differentiate between units recognized by lithology only, rock units of a particular age, and units recognized by age only

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