Geologic Time and Climates

Flashcards covering key concepts from the Geologic Time and Climates module, including definitions of geologic dating methods and key terms related to radioactive decay.

Geologic Dating Methods

Methods used to determine the age of geologic materials and events.

Relative Dating

Sequence of events that places rock units and geologic events in order, from oldest to youngest.

Absolute Dating

Assigning a numeric age or range to rock units or events.

Half-Life

The time required for half of the parent isotopes in a sample to decay to daughter isotopes.

Parent Isotope

The unstable isotope that undergoes decay.

Daughter Isotope

The stable isotope that is produced from the decay of the parent isotope.

Principle of Original Horizontality

Layers of sediment are originally deposited horizontally under the action of gravity.

Law of Superposition

In an undeformed sequence of sedimentary rocks, the oldest layers are at the bottom.

Cross-Cutting Relationships

If a geologic feature cuts through another, the feature that is cut is older.

Radioactive Decay

The process by which an unstable atomic nucleus loses energy by emitting radiation.

Occurs when an isotope loses or gains protons and/or neutrons
(results in changes of atomic number and/or mass number)

Isotopes

Atoms of the same element with different numbers of neutrons.

Can be stable or unstable

Alpha Decay

A type of radioactive decay where an atom loses 2 protons and 2 neutrons.

Beta Decay

A type of radioactive decay where a neutron is converted into a proton and an electron.

Uranium-238

A parent isotope used in radiometric dating with a half-life of 4.5 billion years.

Carbon-14

A radioactive isotope used for dating organic materials, with a half-life of 5730 years.

Most commonly used isotopes

Half-lives for these last for millions or billions of years!

Parent Isotope Daughter Half-life

Uranium 238 —→ Lead 206 —→ 4.5 billion years

Uranium 235 —→ Lead 207 —→ 704 million years

Potassium 40 —→ Argon 40 —→ 1.25 billion years

Carbon 14 —→ Nitrogen 14 —→ 5730 years

Calculating the age of rocks

Age of rock = Number of half-lives a radioactive isotope has experienced x Length of one half-life

Mass extinction

A Widespread and Rapid Decrease in Biodiversity
• The rate of Extinction outpaces Speciation
• Identified using the fossil record of multicellular organisms

Albedo

Reflectivity= % of light reflected from a surface

Albation Zone

where glaciers lose mass from melting, calving icebergs, evaporation = darker color
Bare Ice albedo ~ 40%
Snow albedo ~ 80-90%

How does albedo relate to climate change?

High albedo means higher reflectivity so that the Earth’s surface will warm less

What are the two most common gases in the Earth’s atmosphere?

Oxygen and Nitrogen

Approximately what proportion of the Earth’s atmosphere is composed of carbon dioxide?

0.04%

Ice Age

period of long-term reduction in Earth’s temperature Presence and
expansion of continental and polar ice sheets. Last for millions of years.

cold glacials (100,000 years long, 5-10^oC colder than today)

warm interglacials (10-20,000 years long = modern temps)

Glacial periods

Occur within ice ages. Cycles of cold glacials and
warm interglacials. We are presently in a warm interglacial of the
current ice age.

Two oxygen isotopes present in oceans

• ¹⁶O, lighter – evaporates more easily
• ¹⁸O, heavier – remains in oceans

Long-term Proxy Climate Records

• Comparing ratios (¹⁸O/¹⁶O) of isotopes to modern environments
allows reconstruction of climate

• Colder climates – ¹⁶O is trapped in ice, increased
proportion of ¹⁸O in ocean, fossils (ratio = >0.002)

• Warmer climates – ¹⁶O released as ice melts, increased proportion of ¹⁶O
in ocean, fossils (ratio = <0.002)

Why is Earth warm enough to support life?

Infrared radiation from Earth’s surface is absorbed by gases in the lower
atmosphere

Composition of the Earth’s atmosphere

mostly oxygen and nitrogen with trace greenhouse gases

Glacial periods vs Nonglacial periods

Nonglacial period
• ¹⁶O leaves ocean by evaporation but returns by streamflow
• ¹⁸O/¹⁶O ratio stays the same

Glacial period
• ¹⁶O leaves ocean by evaporation and is trapped in glaciers
• ¹⁸O/¹⁶O ratio in ocean increases (less ¹⁶O)