Exam 11: Geologic Time

There is geologic evidence that the earth has experienced many episodes of mountain building and erosion.

Which of the following geologic observations would not bear directly on working out the sequence of geologic events in an area?

superposition correlation original horizontality inclusions

A geologist observes abundant fossils of a distinctive trilobite species in a shale,but this trilobite disappears as she walks through the shale.In younger rocks,just above the horizon where the fossil disappears another species of trilobite becomes the prominent fossil in similar shales.Which conclusion is most logical for this observation?

The half-life of carbon-14 is about 6000 years.Assume that a sample of charcoal formed by burning of living wood 15,000 years ago.How much of the original carbon-14 would remain today?

What is the name of a Phanerozoic era that means ancient life?

You go for a hike across an outcropping of sandstone and you see large footprints in the sandstone.You have just found ________.

Faunal succession recognized in the sequence of fossils in stratified sedimentary rocks was a key concept that led to ________.

Oysters are an invertebrate the lives today in brackish waters along coastal zones.When a geologist working in stratified rocks observes a limestone bed with oyster fossils overlain by beds with clam and echinoderm fossils indicative of marine conditions,the fossils probably indicate ________.

An important tool in understanding the actual numerical dates of past geologic events is ________.

What is the age of the Earth accepted by most scientists today?

Modern geochronology studies recognize that most radiometric dates record a time when a mineral cools below a temperature where the mineral closes to gain or loss of parent and daughter isotopes.This "closure temperature" varies between different minerals and different isotopic systems.This has allowed the absolute dating of sedimentary rocks through dating of single crystals.For example,if a rock is heated above the closure temperature of the rocks constituent minerals,the date of that mineral provides a minimum age.In contrast,if the rock contains a mineral with a closure temperature higher than the maximum temperature reached by the rock,that mineral will retain the age from its source.The mineral zircon dated by U-Pb methods is an example. So,you look at a rock that has Cretaceous fossils in it,but your paleontologist friend looks at them and cannot determine if they are Early or Late Cretaceous in age.The rock contains the mineral apatite,which accumulates He through the decay or U and Th,but closes to He loss at temperature of around 75°C.You know from the stratigraphic section that the rocks were originally buried at least 3km.The local geothermal gradient is about 35°C/km.You obtain an apatite He date of 105 +/- 2 million years.You also date 100 grains of zircon from the rock.The zircons yield dates that range in age from 2.1 billion years to 125 million years.Three different zircon grains yield ages between 125 and 127 million years.You google "cretaceous time scale" and learn that the Cretaceous begins at about 144 million years and ends at about 65 million years ago.The Early Cretaceous ends about 100 million years ago.How old are these sedimentary rocks?

A geologist is working in an area with exposed sedimentary rocks and finds a well exposed sequence of stratified rocks.As he works through a thick sequence of limestone,he sees abundant large clam fossils that produce an interlocking texture suggesting a reef,but within the limestone section these clams disappear and the overlying limestone is made up primarily of fragments of corals.He now moves 50 km away and finds limestones interbedded with shales,and the limestones contain the same clam fossils he had seen previously 50 km away.As he continues through this section,the rocks become entirely limestone,and the clams disappear again with coral fossils above forming a reef structure.He concludes ________.

What kind of unconformity is produced when tilted,older strata are eroded and buried by younger strata?

A disconformity is an erosional unconformity with parallel beds or strata above and below.

The percentage of radioactive atoms that decay during one half-life is always the same.

The term Paleozoic describes the era of ancient life forms.

Assume that man's recorded history can be stretched back to 4600 years before the present.This is approximately what fraction of geologic time?

In an undeformed sequence of sedimentary rocks,each bed is older than the one above and younger than the one below.The preceding is a statement of the law of ________.

About 88 percent of geologic time is represented by the time span called the ________ era.