geologic age of Earth - about 4.6 billion years

Geologic Time

Geologic Time geologic age of Earth - about 4.6 billion years

Geologic Time very difficult to appreciate from our human perspective necessary to understand history of Earth two basic ways to make sense of geologic time: Relative Ages Absolute Ages

Geologic Time Relative Dating placing geologic events in sequential order as determined by their position in geologic record has resulted in Geologic Time Scale

Geologic Time Absolute Dating gives specific dates for events and materials expressed in years before present radiometric dating most common method used to obtain absolute dates

Early Estimates of Earth s Age Archbishop James Ussher (1664) through genealogies and history recorded in Bible, determined date of Earth creation at 4004 BC required the Earth and all its features to be no more than about 6,000 years old ideas dominated Western thinking about Earth history before 18th century

Early Estimates of Earth s Age Georges Louis de Buffon (mid-1700s) assumed Earth originally molten and used a rapid cooling rate and Earth's present temperature calculated age to be at least 75,000 years

Early Estimates of Earth s Age John Joly (1800s) calculated age from current salinity of ocean assumed originally pure water and salt derived from erosion of continents erosion rate is not constant, loss and recycling of salt not considered, and salt not only obtained from continents got an age of 90 million years

Early Estimates of Earth s Age Rate of Sedimentation attempts to calculate age from deposition rate of sediments and thickness of sediments in Earth's crust deposition rate is not uniform; sediment removed by erosion, modified by compaction, or not deposited complete record of sedimentation does not exist gives ages from 3 million to 1.5 billion years

Early Estimates of Earth s Age Lord Kelvin (1866) most influential physicist assumed originally molten Earth and conventional heat sources for Earth and Sun calculations indicated Earth could be no more than 100 million years or younger than 20 million years old

Relative Dating Methods Principle of Uniformitarianism Present-dav processes have operated throughout geologic time, and with enough time, small changes can have tremendous effects. proposed by James Hutton (mid-1700s) and popularized by Charles Lyell's book, Principles of Geology (1830)

Uniformitarianism

Relative Dating Methods

Relative Dating Methods chronological sequence of rock units determined using six fundamental principles: Principle (Law) of Superposition Principle of Original Horizontality Principle of Lateral Continuity Law of Cross-Cutting Relationships Principle of Inclusion Principle of Faunal Succession

Relative Dating Methods Principle (Law) of Superposition In an undeformed sequence of sedimentary rocks, the youngest beds are at the top and the oldest beds are at the bottom (also applies to volcanic rocks). no place on earth where entire history of sedimentation preserved

Superposition

Relative Dating Methods Principle of Original Horizontality Sediment particles deposited from water under the influence of gravity form essentially horizontal layers.

Original Horizontality

Relative Dating Methods Principle of Lateral Continuity Most rock layers (sediments) extend laterally in all directions until they thin, pinch out, or terminate against the edge of the depositional basin. applicable to sedimentary but not volcanic rocks

Lateral Continuity

Relative Dating Methods Law of Cross-Cutting Relationships An intrusion or fault that cuts through another rock is younger than the rock it cuts.

Cross-Cutting Relationships B A C A C A B C B

Cross-Cutting Relationships

Cross-Cutting Relationships sills vs buried lava flow - must look for heat effects

Relative Dating Methods Principle of Inclusion Inclusions are older than the rock that contains them. applies to both inclusions in igneous rocks and rock clasts in sedimentary rocks

Inclusion

Relative Dating Methods Principle of Faunal Succession Fossil organisms succeed one another in a definite and determinable order, so any time period can be recognized by its fossil content. general pattern of development is from simple to complex organisms

Faunal Succession

studies using these principals have demonstrated that some rock sequences may not represent continuous depositions, but rather are characterized by distinct breaks in the geologic record Unconformities

Unconformities surfaces of non-deposition or erosion that separate younger strata from older rocks

Unconformities time gap in rock record, known as hiatus, result in incomplete rock records

Unconformities three types of unconformities: disconformity angular unconformity nonconformity

Disconformities

Disconformities surface of nondeposition or erosion between parallel layers of older and younger rocks

Disconformities

Disconformities may look like bedding plane

Disconformities fossils must be used to determine length of break in deposition

Angular Unconformities

Angular Unconformities surface of erosion between nonparallel layers of older and younger rocks older rocks have been tilted (folded or faulted) and eroded prior to deposition of younger strata

Angular Unconformities

Nonconformities

Nonconformities surface of erosion between older igneous or metamorphic and younger sedimentary rocks

Nonconformities may look like intrusive contact, but no heat effects inclusions may be useful in distinguishing nonconformity

Applications... so how do we make use of these principles and relationships to to interpret rock sequences

Applications stratigraphy - study of layered sequences

Applications seek to establish equivalency of rock units in different areas in order to interpret the Earth s history interpretations involve making correlations between rock exposures

Correlation of Strata correlation - demonstration of time equivalency

Correlation of Strata correlation by: Rock Type or Mineralogy - distinctive rocks may allow correlation (e.g. volcanic ash layer etc...) Position in Sequence - distinct sequences of lithologic changes can be correlated Geophysical Responses - magnetic character, conductivity, etc... from surface or wells

Correlation of Strata tracing beds (key beds or marker horizons) - good for small areas that are well-exposed

Correlation of Strata

Magnetic Correlations

Fossil Correlation correlations based on fossils - better resolution of time equivalence over widespread areas

Fossil Correlation guide or index fossils - remains of organisms that were geographically widespread, but lived for a short time forminifera fusilinids

Fossil Correlation

Fossil Correlation assemblage zones - overlapping ranges of fossils

Fossil Correlation assemblage zones - can produce better temporal resolution than one fossil

Fossil Correlation

Correlation of Strata application of these principals and methods allows geologists to recognized and construct the history of geological development of an area

The Grand Canyon such an approach can be applied to understanding the rock sequence within the Grand Canyon of the Colorado River in Arizona

The Grand Canyon Early Precambrian

The Grand Canyon

The Grand Canyon Early Precambrian

The Grand Canyon Late Precambrian

The Grand Canyon

The Grand Canyon Cambrian

The Grand Canyon

The Grand Canyon Permian- Mississippian

The Grand Canyon

The Grand Canyon Mesozoic- Holocene

The Grand Canyon

The Grand Canyon Mesozoic- Holocene

The Grand Canyon

Absolute Dating Methods

Absolute Dating Methods specific dates for events and materials expressed in years before present based on radiometric methods

Absolute Dating Methods atoms small particles comprised of a nucleus [protons and neutrons] surrounded by electrons elements atoms have variable numbers of protons [atomic or Z number] and an equal number of electrons [when neutral] each stable configuration is an element

Absolute Dating Methods isotopes not all atoms of an element have same number of neutrons in the nucleus atoms with the same Z, but different numbers of neutrons are isotopes

Radioactive Decay process by which an atomic nucleus spontaneously decays into a nucleus of a different element

Radioactive Decay parent -the unstable isotope daughter - the product of decay of an unstable isotope

Radioactive Decay decay can result from: alpha decay beta emission electron capture

Radioactive Decay alpha decay: Z = -2, AMU = -4

Radioactive Decay beta emission: Z = +1, AMU = 0

Radioactive Decay electron capture: Z = -1, AMU = 0

Radioactive Decay the amount of time required for 1/2 of the parent atoms to decay half-life

Some Radioactive Isotopes parent daughter half-life Uranium 238 Lead 206 4.5 b.y

Some Radioactive Isotopes

Some Radioactive Isotopes parent daughter half-life. Uranium 238 Lead 206 4.5 b.y Uranium 235 Lead 207 0.7 b.y Thorium 232 Lead 208 14 b.y Samarium 147 Neodymium 143 106 b.y. Rubidium 87 Strontium 87 48.8 b.y Potassium 40 Argon 40 1.3 b.y.

Applications sedimentary rock radiometric dates generally meaningless because minerals making up rock are parts of other, preexisting rocks metamorphism can affect the parent/daughter ratio most accurate dates obtained from igneous rocks

Absolute Dating Methods

Applying Absolute Dating other geochronologic methods

Radiocarbon Dating Carbon 14 radioactive C 14 produced from N 14 in atmosphere by interaction of cosmic radiation C 14 has a half life of 5,730 y useful for dating materials less than 70,000 years old

Radiocarbon Dating Carbon 14 plants and animals absorb C 14 while living after death, ratio of C isotopes change due to decay of C 14

Radiocarbon Dating

Fission Track Dating Uranium 238 spontaneously decays by fission particles from nucleus make tracks in minerals counted and tied to number of years has largest useful age range of any radiometric method (40,000 to 1 million years)

Radiometric Dating has allowed dates to be placed on geologic events and ages to be placed on formation of geologic materials oldest evidence for life about 3.6 billion years oldest rocks found on Earth (Australia) are 3.96 billion years old

Radiometric Dating meteorites date from 4.5 to 4.8 billion years old

Radiometric Dating Earth about 4.6 billion years old