Geological Time Notes

Geological Time

Introduction

• Geologists study Earth's history, spanning geologic time.
• Earth is approximately $4.56$ billion years old.
• Humans (genus Homo) have existed for about $2$ million years, a small fraction of Earth's history.
• This understanding is a major contribution to human knowledge.
• Comparable to the astronomical finding that Earth is a small part of the Milky Way galaxy.
• Evidence matters in science; scientific observation takes precedence over personal beliefs.
• Our perspective should increase our sense of responsibility as custodians of Earth.
• It should also increase our appreciation of nature's complexity and the human mind's capacity for discovery.

Fossils

• Fossils are remnants or indications of prehistoric life preserved in rock and are vital to understanding geological time.
• Paleontology, the study of fossils, is of interest to both biologists and geologists.

Types of Fossil Preservation:

Original Remains

• Insects preserved in tree resin (amber), found globally but mainly in eastern Europe and the Dominican Republic.
• Entire bodies of woolly mammoths frozen in the permafrost of Alaska and Siberia.
• Hard parts such as bones, shark teeth, and shells of marine organisms.

Replaced Remains

• Hard parts replaced by minerals like silica $(SiO<em>2)$, calcite $(CaCO</em>3)$, and pyrite $(FeS_2)$ in groundwater.
• This results in a copy of the original material, such as petrified wood.

Carbonization

• Plant remains decompose under anaerobic conditions.
• Hydrogen, nitrogen, and oxygen are removed, leaving a carbon residue.
• This process forms coal, retaining features of the original plant.

Molds and Casts

• Molds: Hollow depressions left when embedded shells or bones dissolve.
• Casts: New mineral material fills the mold and hardens.
• Molds and casts only show the original shape.

Trace Fossils

• Fossil imprints made by animal movement.
• Examples include tracks, borings, and burrows, including dinosaur tracks.

Relative Geologic Time

• Relative geologic time places rocks and geologic events in chronological order without actual dates.
• It establishes the sequence of events (e.g., graduation, military service, marriage).
• Absolute time, or numerical time, refers to knowing the actual dates of events.

Principles for Determining Relative Ages of Rocks

Principle of Original Horizontality

• Sediments and lava flows are deposited as horizontal layers.
• Non-horizontal layers were originally horizontal and later altered by other forces.

Principle of Superposition

• In an undisturbed sequence, each layer is younger than the one beneath it and older than the one above it.
• Layers can be disturbed by folding or faulting.
• Geologists must account for such disturbances.

Principle of Cross-Cutting Relationships

• Igneous rock is younger than the rocks it has intruded.
• Faults are younger than the rocks they affect.

Radiometric Dating

• The establishment of the relative geologic time scale was a significant achievement.
• Geologists needed to know the order and absolute ages of geologic events.
• Absolute (numerical) geologic time became important in 1785 with James Hutton’s uniformitarianism concept.
• It also became crucial in 1859 with Charles Darwin’s theory of organic evolution.
• Radiometric dating determines age using radioactivity and is geology’s best tool for establishing absolute geologic time.
• Radioactive atomic nuclei decay spontaneously.
• The decay product (daughter nucleus) may be stable; if not, it undergoes further decay.
• Radioactive series can be long and complex (e.g., uranium-238 to lead-206).

Half-Life

• The rate of decay of a radioactive isotope is expressed in terms of half-life.
• Half-life is the time required for half of the parent nuclei to decay.
• Decay rate is constant, unaffected by temperature, pressure, or chemical environment.
• Radioactivity serves as a “clock” to measure geologic time.
• The older the rock, the smaller the ratio of radioactive parent to stable daughter.
• For uranium-238 (half-life of $4.46$ billion years) decaying to lead-206, every $1000$ atoms of uranium-238 present $4.46$ billion years ago, only $500$ atoms of uranium-238 remain, and $500$ atoms of lead-206 have been formed.
  $Ratio: 50\%/50\% = 1$
• $4.46$ billion years in the future, only $250$ atoms of uranium-238 will remain, with $750$ atoms of lead-206 formed.

Ideal Conditions for Dating

1. No addition or subtraction of parent or daughter elements, except by radioactive decay.
2. The rock's age should be close to the half-life of the parent isotope.
3. No daughter element was present when the rock formed, or if present, its quantity is known.

• Satisfying condition 1 is usually not difficult.
• Condition 2: If the rock is much older than the half-life, too little parent isotope remains to be measured.
• If the rock is much younger, too little daughter nuclei may have formed to be measured accurately.
• Many rocks lack appropriate isotopes for radiometric dating.
• Condition 3 can be problematic; lead from radioactive decay is radiogenic lead, and lead not from decay is primordial lead.

Carbon Dating

• Developed in 1950 by Willard Libby, used for dating ancient, once-living remains.
• Carbon-14 ($^{14}C$) is a radioactive isotope with a half-life of $5730$ years.
• Carbon-14 dating compares the amount of $^{14}C$ in an ancient sample to that in present-day organic matter.
• $^{14}C$ is produced in the atmosphere by neutrons reacting with atmospheric nitrogen.
• Newly formed $^{14}C$ reacts with oxygen to form radioactive carbon dioxide ($^{14}CO_2$).
• Plants use $^{14}CO_2$ in photosynthesis.
• About one in every trillion ($10^{12}$) carbon atoms in plants is $^{14}C$.
• Animals incorporate radioactive $^{14}C$ into their cells.
• All living matter has a similar level of radioactivity due to $^{14}C$, about $16$ counts per minute per gram of total carbon.
• Once an organism dies, it stops taking in $^{14}C$, and the $^{14}C$ decays.
• The longer the organism has been dead, the lower the radioactivity.
• The newest method counts both $^{14}C$ and $^{12}C$ atoms.
• By comparing the ratio of isotopes, the time since death can be calculated.
• This method uses tiny samples and can date specimens up to $75,000$ years old.
• Beyond that, only about $0.02\%$ of the original $^{14}C$ remains, which is too little to measure.

The Age of the Earth

• In the mid-to-late 1800s, Lord Kelvin attempted to determine Earth’s age from its rate of heat loss.
• Kelvin assumed Earth began as a molten body and cooled over time.
• He calculated that Earth solidified $20$ to $40$ million years ago.
• This was at odds with geologists and biologists, who suggested a greater age based on geologic processes and evolution

Discovery of Radioactivity

• After the discovery of radioactivity in 1896, Kelvin’s calculation was found to be incorrect.
• Kelvin's assumption that all heat was residual was wrong. Most heat comes from radioactive decay, a continuous process.
• Geologists estimate the Earth's age to be about $4.56$ billion years.

Evidence Supporting Earth's Age

1. Age of rocks: Radiometric dating of igneous and metamorphic rocks; zircon crystals in Western Australia dated as $4.4$ billion years old.
2. Age of meteorites: Meteorites from the asteroid belt date to about $4.56$ billion years based on uranium-lead and rubidium-strontium methods; Martian meteorites also show this age.
3. Age of Moon rocks: Lunar highlands rocks date to $4.45$ billion years.

• The planets, moons, and asteroids of the solar system formed about $4.56$ billion years ago.
• Earth's surface was likely molten for hundreds of millions of years destroying many old rocks.
• Weathering and subduction have also destroyed ancient rocks.

The Disappearance of the Dinosaurs

• About $70\%$ of plant and animal species, including dinosaurs, became extinct about $66$ million years ago.
• A team from the University of California at Berkeley investigated a clay layer marking the Mesozoic-Cenozoic boundary in the late 1970s.

K-Pg Boundary

• The Cretaceous (K) period (Mesozoic era) is followed by the Paleogene (Pg) period (Cenozoic era).
• The boundary between these periods is the K-Pg boundary.
• The K-Pg extinction event marks this division.
• The team, led by Walter Alvarez and Luis Alvarez, studied the clay layer in Gubbio, Italy.

Findings

1. The clay layer had a high concentration of iridium (Ir), akin to that in meteorites.
2. It contained glassy beads (spherules) associated with meteorite impacts.
3. Shocked quartz grains were present, which are associated with meteorite impacts.
4. Soot particles were found, evidence of extensive fires.

• The Alvarez team hypothesized that a $10$ km meteorite struck Earth $66$ million years ago causing fires and injecting about $100$ trillion tons of dust into the stratosphere.
• The cloud blocked sunlight and disrupted food chains, causing acid rain.
• Iridium, spherules, shocked quartz, and soot have been found at $95$ K-Pg boundary layer locations.

Chicxulub Crater

• In 1990, a $180$ km diameter crater was found in the Yucatan peninsula of Mexico (Chicxulub).
• The rock strata contain sulfate rock, which would have caused deadly acid rain.
  Cores drilled near the site show a fossil-rich layer topped with an iron-rich band and glass spherules confirming a meteorite impact.
• Evidence confirms that dinosaurs and other species may have become extinct because of a visitor from space.