Chapter 12

Eras within the Phanerozoic eon

1) Cenozoic- "Recent life" or "The Age of Mammals". 65.5 Ma-Present 2) Mesozoic- "Middle life" or "The Age of Dinosaurs". 251-65.5 Ma 3) Paleozoic- "Ancient life". 542-251 Ma

Ordered sequence of eons

1) Phanerozoic- "Visible life" or "Precambrian-Cambrian boundary explosion", marks the first appearance of hard shells; life diversified rapidly afterward. 542 Ma-Present 2) Protozeroic- "Earlier life", marks the development of tectonic plates like those of today, buildup of atmospheric oxygen, and multicellular life appears. 2.5 Ga-542 Ma 3) Archean- "Ancient", marks the birth of continents and formation of the oceans and secondary atmosphere. 4.0-2.5 Ga 4) Hadean- "Hell", marks internal differentiation and formation of the oceans and secondary atmosphere. 4.6-4.0 Ga

Most geologic periods were defined by relative dating.

ex: Fossil Succession- fossils preserved in lower layers of sedimentary rock are older than those in overlying layers. Specific fossils occurred at specific times in earth history, which allowed for the establishment of a geologic time scale.

Relative dating is performed by the application of 7 stratigraphic principles:

1) Principle of uniformitarianism- physical processes observed today operated in the same way in the geologic past, so modern processes help us understand ancient events. 2) Principle of superposition- in an undeformed sequence of layered rocks: each bed is older than the one above and younger than the one below, so the oldest layer lie at the bottom and the youngest layer lies at the top. 3) Principle of original horizontality- sedimentary rocks were horizontal when originally deposited. If tilted, it is due to a subsequent geologic event. 4) Principle of original lateral continuity- sedimentary beds are originally laterally continuous within their environment of deposition. Faulting, severe folding, and erosion may have separated the originally laterally continuous beds into what now appears to be separate units. 5) Principle of cross-cutting relations- younger features cut across older features, so faults, dikes, erosion, volcanoes etc. must be younger than the material that is faulted, intruded, or eroded. 6) Principle of inclusions- inclusions (a rock fragment within another rock) must be older than the material they are in. 7) Principle of baked contacts- thermal metamorphism occurs when country rock is invaded by a plutonic igneous intrusion. The baked rock must have been there first, so it is older.

How old is the earth?

4.57 Ga based on correlation with meteorites and moon rocks.

Unconformity

A time gap in the rock record. Three types of unconformities: 1) Disconformity- parallel strata bracketing nondeposition. This is due to an interruption in sedimentation. 2) Nonconformity- metamorphic or igneous rocks overlain by sedimentary strata because crystalline igneous/metamorphic rocks were exposed by erosion, and then covered by deposited sediment. 3) Angular unconformity- rocks below the unconformity were either folded or tilted.

Absolute dating

Ages (in years before present) calculated for geologic events. In general, means radiometric age dating. Can also date by tree rings, ice cores, some sediments, and magnetostratigraphy.

How are growth rings and ice cores useful in determining the ages of geologic events?

Because growth rings have annual layers from trees or shells, and ice cores have rhythmic layering, or annual layers in sediment or ice.

Why can't we date sedimentary rocks directly?

Because they are derived from grains.

Why is radiocarbon dating useful in archaeology, but useless for dating dinosaur fossils?

Carbon dating is not appropriate for use beyond several tens of thousands of years. Several other radionuclide dating methods serve for the multibillion year periods.

How do geologists obtain a radiometric date?

Depends on the radiometric decay of one element into another. It measures the amount of parent and daughter isotopes to determine the age. Half-life- the time it takes for half of the unstable parent atoms in a sample to decay to stable daughter atoms. After one half-life, half of the original parent remains.

Radiometric dating

Depends on the radiometric decay of one element into another. It measures the amount of parent and daughter isotopes to determine the age. Half-life- the time it takes for half of the unstable parent atoms in a sample to decay to stable daughter atoms. After one half-life, half of the original parent remains. ex: Uranium 238 --> Lead 206 with a half-life of 4.6 billion years

Relative dating

Geologic events placed in chronological order based on their position in the geologic record. "Chronostratigraphic"

Which rock is best for radiometric dating?

Igneous- best Metamorphic- possible, but can be troublesome because it will measure the time of metamorphism (recrystallization) instead of the time of original crystallization. Sedimentary- not possible

Describe two different methods of correlating rock units. How was correlation used to develop the geologic column? What is a stratigraphic formation?

Lithologic correlation is the use of physical and chemical characteristics of rocks to determine that spatially isolated strata were once continuous (through original lateral continuity). Fossil correlation uses fossils with known stratigraphic ranges to determine the approximate temporal equivalence of two bodies of rock (which may be of disparate lithologies). Because a single locality does not provide strata that span in age throughout Earth's history, fossil correlation was required to provide time equivalence for units that are geographically isolated. With successful correlation, rock sequences from all parts of the world could be brought together to form an accurate sequence of relative time: the geologic column. A stratigraphic formation is a recognizable layer of a specific (usually sedimentary) rock type or set of types that were deposited within a certain time interval and can be traced over a broad region.

Compare numerical age and relative age.

Relative age specifies whether one geologic feature is older or younger than another; numerical age provides the age of a geologic feature in years.

Describe the principles that allow us to determine the relative ages of geologic events.

The geologic history of a region can be determined by using principles such as uniformitarianism, superposition, original horizontality, original continuity, cross-cutting relations, inclusions, and baked contacts.

What is the age of the oldest rocks on Earth? What is the age of the oldest rocks known? Why is there a difference?

The oldest rocks on earth are: -Acasta Gneisses: 4.03 Ga -Isua Supracrustal Belt: 3.7-3.8 Ga -Single zircon crystals found in younger sedimentary rocks: 4.3 Ga

How does the principle of fossil succession allow us to determine the relative ages of strata?

The principle of fossil succession relies on two assumptions: no two species are exactly alike, and once a fossil species becomes extinct, it never reappears. Fossil Succession- fossils preserved in lower layers of sedimentary rock are older than those in overlying layers. Specific fossils occurred at specific times in earth history, which allowed for the establishment of a geologic time scale.

Geologic time scale (Geologic column)

Time blocks: 1) Eons- the largest subdivision of time 2) Eras- subdivisions of an eon 3) Periods- subdivisions of an era 4) Epochs- subdivisions of a period

How does an unconformity develop?

Unconformity is the boundary between two different rock sequences representing an interval of time during which new strata were not deposited and were eroded.