Stratigraphic Principles and Earth’s Climate History
Stratigraphic Principles for Sedimentary Rocks
Principles of Stratigraphy
- Superposition: In undisturbed settings, the oldest sedimentary rocks are deposited first at the bottom of the sequence.
- Originally Horizontal and Lateral Continuity: Sedimentary beds are originally deposited horizontally and extend laterally in all directions.
- Cross-Cutting: Faults, igneous rocks, or unconformities that cut across sedimentary beds are younger than the beds they cut.
- Unconformity: An erosion surface representing a gap in time between the formation of two sections of rock.
Types of Unconformities
- Angular Unconformity: Older tilted rocks are covered by younger horizontal ones, showing erosion and a time gap.
- Nonconformity: Sedimentary rocks on top of older igneous or metamorphic rocks, indicating erosion and a change in environment.
- Disconformity: Break between parallel sedimentary layers, indicating a pause in deposition or erosion (sedimentary on bottom).
Fossils and Geologic Time
- Body Fossils: Fossil remains of part or all of a dead organism (shell/bone/casts).
- Trace Fossils: Remains of a living organism’s behavior (footprints/burrows/tracks).
Geologic Eras
- Cenozoic: “Recent life”, 65-0 Million Years
- Mesozoic: “Middle life”, 251-65 Million Years
- Paleozoic: “Ancient life”, 542-251 Million Years
Radioactive Decay and Dating Rocks
- Half-life: Time for half of the parent isotope to be converted to daughter isotopes. Half-lives for most commonly used isotopes last for millions or billions of years.
- Calculations: 1 HL = 1:1, 2 HL = 1:3, 3 HL = 1:7, 4 HL = 1:15, 5 HL = 1:31 (they work in fractions; 1:3 = 25% and 75%) (parent + daughter = the total).
- Radioactive decay determines the time of origin for igneous and metamorphic rocks only (not sedimentary!).
Earth’s Climate: Past, Present, and Future
Volcanic Eruptions and Climate
- Eruptions can produce dense clouds of debris (tephra) that block sunlight and produce a short-term decrease in temperatures (years).
- Large volcanoes release volcanic gases (water vapor, sulfur dioxide, carbon dioxide) that trap heat in the atmosphere to produce long-term temperature increases (millions of years). Nitrogen and oxygen make up 99% of dry air.
- The albedo of the Earth’s surface is highest at the poles (surface: snow).
- Large volcanic eruptions in the geological past produced huge volumes of basaltic lava called flood basalts.
- Flood basalts are formed by magma plumes rising through the mantle from the core/mantle boundary.
Greenhouse Effect and Solar Radiation
- Greenhouse Effect: Greenhouse gases absorb heat to maintain an average global temperature of 15 degrees C (59 degrees F).
- Distance from the sun DOES NOT contribute to temperature differences between winter and summer.
- The amount of solar energy reaching Earth’s surface depends on the angle at which the sun’s rays strike Earth.
- More heat is delivered by isolation where the Sun is directly overhead.
- Seasonal temperature contrasts are due to the tilt of the Earth’s axis and the angle of the Sun’s rays (Tilt = 23.5 degrees).
Orbital Cycles and Climate
- Eccentricity: The shape of Earth’s orbit changes its distance to the sun (100,000-year cycle).
- Precession: Direction of tilt of Earth’s axis changes (23,000-year cycle).
Glaciers and Ice Sheets
- Scientists interpret ice properties to determine temperature fluctuations from the last ice age to today (ice cores).
- Glacier: A long-lasting mass of ice that moves downslope under its own weight.
- If accumulation = ablation, the glacial end (toe) stays in the same place.
- If accumulation > ablation, the glacier advances.
- If accumulation < ablation, the glacier toe will retreat.
- Orbital cycles affect both intensity and distribution of solar energy (solar insolation).
- Ice Sheets: Move hundreds of meters/year (very thick and dense). Present in cold environments (Greenland, Antarctica).
- Alpine (mountain) glaciers: Move 5 to 50 m/yr (smaller and slower moving than ice sheets). Present in high US mountains (Alaska, Colorado, Washington). Average thickness 100-200 meters; short-lived → only decades old.
Glacial Deposits and Landforms
- Glacial deposits (till) are mostly dumped when ice melts and are so often unsorted with a mix of grain sizes.
- Features formed when unsorted glacial till form shaped mounds (drumlins) or ridges that mark where the glacier paused (moraines).
- Features with sorted sediment indicate an origin involving running water such as features deposited by streams under the glacier (eskers) or streams formed as the glacier melted (glacial outwash).
- Features formed from left-over ice blocks that melted (kettle lakes).
Greenhouse Gases and Climate Change
- Various greenhouse gases are carbon dioxide, methane, chlorofluorocarbons, ozone, nitrous oxide, and water vapor.
- What would happen if the tilt of our planet decreased to 21 degrees? Winters would get warmer, summers would get colder.
- What would happen if the tilt of our planet increased to 25 degrees? Summers would get warmer, winters would get colder.
- Climate Change: Any systematic change in the long-term statistics of climate elements.
- Positive forcing (results in more incoming energy or less outgoing energy) resulting in a warmer Earth; Example – melting of reflective sea ice, high wispy cirrus clouds.
- Negative forcing (results in more outgoing energy or less incoming energy) resulting in a cooler Earth; Example – volcanic eruptions where tephra creates dense clouds that block sunlight, thick cumulus clouds.
- Climate change may be due to:
- Natural external forcings → gradual changes in solar emission or changes in Earth’s orbit.
- Natural internal processes of the climate system (more/less ice cover, volcanic eruptions).
- Anthropogenic forcings caused by humans (more greenhouse gases, air pollution).
Studying Past Temperatures
- How do scientists figure out temperatures in the geological past?
- Recent weather records (100+ years)
- Historical records
- Growth rings of trees, corals, ice layers (middle three → 1000+ years)
- Pollen
- Oxygen isotopes (1,000,000+ years) The more O18, the cooler the period –> O16 releases as ice melts.
The Paleozoic Era
Check all the items below that occurred during the Paleozoic Era.
- Largest known extinction event in Earth’s history.
- Major diversification of life in the oceans.
- Evolution of amphibians and early reptiles.