8 - Geologic Time
Modern humans (Homo sapiens) appeared about 0.2 million years ago. What percentage of the geologic time scale includes the "human occupation of the Earth"?
0.004%
What fossil is this?
ammonite
What is the geologic contact of tilted sedimentary formations overlain with a younger sedimentary formation called?
angular unconformity
What fossil is this?
brachiopod
Anthropogenic means:
caused by the action of Homo sapiens
What are the subdivisions of geologic time in order from largest to smallest?
eon, era, period, epoch
In 1815, William Smith correlated strata from many locations and plotted it on paper to show the spatial distribution of rock units on Earth's surface. This document was the first modern:
geologic map
An unconformity develops when:
new strata are deposited atop eroded strata.
What is the geologic contact of a metamorphic or igneous formation overlain with a younger sedimentary formation called?
nonconformity
If a rock is inside another rock, which is older?
the rock inside
What fossil is this?
trilobite
The first fossils of animals with hard parts appeared about 545 million years ago. Assuming Earth is 4,600 million years old, what percentage of geologic time does the fossil record represent?
11.85%
Which of the following is NOT a method to determine numerical age? 1. analyzing growth ring patterns in trees 2. cross-cutting relations 3. carbon-14 dating 4. radioactive decay of uranium to lead
2. cross-cutting relations
A radioactive isotope of the element potassium decays to produce argon. If the ratio of argon to potassium is found to be 7:1, how many half-lives have occurred?
3 half-lives
Mammal fossils most likely correlate with what Era?
Cenozoic
T or F? Low-energy environments are not suitable for preserving fossils.
F
T or F? Scientists can date any radioactive material back only three half-lives; after this there's not enough parent material left to measure.
F
T or F? The dike is older than the sedimentary rock layers it cuts across.
F
T or F? The principle of inclusions says rock containing inclusions is older than the inclusions.
F
T or F? The slower an organism is buried with sediment after death, the more likely it is to become a fossil.
F
T or F? Unconformities represent periods of active deposition in the rock record.
F
T or F? A dead organism will most likely become a fossil if there is abundant oxygen in the depositional environment.
F
T or F? Organisms form fossils when they are preserved in recrystallized limestone called marble.
F
Why do you think geologists are able to divide up the Tertiary and Quaternary periods into several geologically short subdivisions (epochs) while the Precambrian is only divided into 3 very long Eons?
I think geologists are able to do this for a few reasons. First, geologic time is divided by significant events and for almost 90% of Earth's geologic time (Precambrian eon), no fossil records were found. There weren't any hard parts until the Paleozoic era. Also, dividing periods into epochs can only be done for the most recent part of the geologic time scale because many older rocks are unable to be interpreted due to deep burial and deformation. Because the more recent rocks and fossils are able to be interpreted and dated, this information is shown in our scale and I think we would have more epochs if we could precisely date more rocks from a very long time ago.
Dinosaur fossils most likely correlate with what Era?
Mesozoic
Trilobite fossils most likely correlate with what Era?
Paleozoic
What are the four time divisions listed from oldest to youngest?
Precambrian, Paleozoic, Mesozoic, Cenozoic
Why are geologists not able to date sedimentary rocks directly?
Sedimentary rocks are younger than the minerals that they are made up of.
T or F? Carbon-14 dating is used to date organisms, not minerals.
T
T or F? Isotopic dating relies on the parent-daughter ratio.
T
T or F? The "radiometric clock" starts when a radioactive mineral cools below its closure temperature.
T
T or F? The oldest sedimentary layer is the bottom layer.
T
T or F? The principle of original continuity says sedimentary layers began as continuous expanses of sediment.
T
T or F? The principle of superposition says in a sequence of sedimentary beds, the youngest is on top.
T
T or F? There are three major types of unconformities
T
T or F? Unconformities represent periods of erosion in the rock record.
T
T or F? Unconformities represent periods of nondeposition in the rock record.
T
T or F? Organisms with hard parts made of shell or bone are more likely to become fossilized.
T
T or F? The principle of cross-cutting relations says the feature doing the cutting is younger than the feature it cuts.
T
Explain the Cambrian Explosion.
The Cambrian Explosion was a time when a burst of different organisms with skeletal remains and most of the major body plans known today came to be in the fossil record. They were very diverse. It happened during the Cambrian Period, in the early-Paleozoic Era and went on for millions of years.
Explain what the principle of cross cutting relationships means.
The principle of cross cutting relationships means that in a sequence of rocks, whatever rock is cutting across the others is the younger rock, while what is being cut through is older.
Explain what the principle of inclusions means.
The principle of inclusions means that whatever rock is inside another body of rock (the inclusion) is older than that with which it is inside. This is because it had to be formed first to then be surrounded by another body of rock.
Explain what the principle of lateral continuity means.
The principle of lateral continuity means that beds of rock start out as continuous layers that extend in every direction, but as they extend they eventually thin out into a different sediment type.
Explain what the principle of original horizontality means.
The principle of original horizontality means that layers of sediment are generally deposited in a horizontal way before they are possibly disrupted.
Explain what the principle of superposition means.
The principle of superposition means that within an arrangement of layered rocks that has not been disrupted or overturned (by faulting), that the oldest rocks will be at the bottom and the youngest at the top.
Explain how, when, and why there are banded iron deposits.
There are banded iron deposits because about 3.5 million years ago, photosynthesizing bacteria began to release free oxygen into the Earth's oceans. This free oxygen first bonded with dissolved iron in the ocean. This is what created banded iron deposits called banded iron formations.
Explain why there are no Cambrian or Silurian coal deposits.
There are no Cambrian or Silurian coal deposits because there weren't any swamps, and that is the creation environment of coal. During the Cambrian and Silurian, there were not ideal conditions to form coal. As forests spread and plants grew adjacent to water, more and more coal was able to form.
