chapter+12+geologic+time
TRANSCRIPT
CHAPTER 12 GEOLOGIC TIME
Earth’s history is recorded in the rock record such as the rock layers of the Grand Canyon.
• Principle of Uniformitarianism – the forces and processes we observe on Earth today are the same forces and processes that occurred throughout Earth’s history.
• We study the present to understand the past.
• Relative Dating tells of the sequence of events, not how long ago the events occurred.
• The law of superposition – each bed of rock is older than the one above it. The oldest rock is at the bottom, the youngest rock at the top.
• The principle of horizontality – layers of sediment are deposited in horizontal layers or strata.
• The principle of cross-cutting relationships – a fault or intrusion that cuts the rock layer must be younger than the rock it cuts.
• Unconformity – a long time in which deposition stopped, erosion removed layers and then deposition began again
• Angular unconformity – indicates that a period of folding or tilting occurred (continental collisions, dune build-up, or at end of delta)
• Disconformity – sedimentary layers that a separated by an erosion
• Nonconformity – an erosional surface separates older metamorphic or igneous rock from younger sedimentary rock
• Fossils are any remains or traces or prehistoric life
• Conditions favoring fossilization: hard parts, buried quickly, and separated from the environment
• Unaltered remains – frozen in ice, shells
• Petrified – mineral-rich water fill the ore spaces. The minerals precipitate out and harden
• Mold – forms when a hard part is buried in sediment and then the hard part dissolves, accurately represents the shape and surface markings
• Casts – created when hollow spaces of a mold are later filled with mineral matter
• Carbonization – organism is buried in fine sediment, the liquid and gases are squeezed out and leaves of thin residue of carbon. Black shale shows the outlines of leaves
• Animal footprints made in soft sediment are compacted and cemented
• Burrows of animals in rock and wood later filled with mineral matter and preserved
• Coprolites – fossilized dung
• Gastroliths – fossilized polished stomach stones
• Principle of fossil succession – fossil organisms succeed each other in a definite order
• Age of Trilobites, Age of Fishes, Age of Coal Swamps, Age of Reptiles, Age of Mammals
• Index fossils – useful in dating other fossils found in the same rock layer. Index fossils are limited to a short time span, are geographically widespread, and occur in large numbers
• Fossils can also be used to interpret past environments. Shells, for example, would indicate shallow seas, and shorelines.
• Radioactivity – the spontaneous breakdown of an unstable nuclei into a different element
• Half-life – the time it takes for one-half of the nuclei to decay. The half-life ratio can be used to determine the age of an object
• Radiometric dating – using the rate of decay in the surrounding rock to determine the age
• Radiocarbon-14 dating – using the ratio of carbon-14 to carbon-12 to determine the age. The half-life of carbon-14 is 5730 years and is used to date objects up to 75,000 years ago.
• Geologic Time Scale • Eons are the greatest expanse of time.
Eons are divided into eras. Eras divided into periods and periods are divided into epochs
• Phanerozoic Eon – means “visible life”• Paleozoic Era – means “ancient life”• Mesozoic Era – means “middle life”• Cenozoic Era – means “recent life”• Precambrian – 88% of Earth’s history, few
life forms
In books onPage 365