An example of angular unconformity
An example of disconformity
What does the term unconformity mean?
a missing rock layer in a sequence that represents a period of erosion or nondeposition
What characteristic most directly DISTINGUISHES an angular unconformity from a nonconformity?
Angular unconformities separate rock layers along nonparallel surfaces.
Law of superposition
In sedimentary rocks, the rock bed on the bottom must be older than the rock bed on the top.
Principle of original horizontality
Sedimentary rocks were originally deposited as flat-lying, horizontal layers.
Principle of cross-cutting relationships
Any rock or feature, cutting through another rock or feature, must be younger than the material through which it cuts.
Inclusions
Any rock fragments included within another rock must be older than the rock in which they are included.
A sandstone contains inclusions of metamorphic rock. An igneous dike cuts both the sandstone and inclusions. List the rocks from youngest to oldest.
igneous dike, sandstone, metamorphic rock
If a sequence of sedimentary units is cut by a fault, what does the principle of cross-cutting relationships tell a geologist?
All of the sedimentary units must have been deposited and lithified before being cut by the fault.
An undeformed sequence of sedimentary rocks is exposed in a large river canyon. Which two principles would be demonstrated by the rocks?
principles of superposition and lateral continuity
An igneous dike cuts through limestone, but not through the overlying sandstone. How did this happen?
First, the limestone was laid down, then intruded by the igneous dike, and lastly the sandstone was deposited.
Following the deposition of a sequence of sedimentary rocks, which event is the first to occur to produce an angular unconformity?
deformation
A __________ exhibits sedimentary layers that are parallel to each other above and below an erosive surface.
disconformity
An accurate description of a nonconformity?
younger sedimentary strata overlie uplifted and weathered igneous or metamorphic rocks
The sedimentary layers above an erosive surface are __________ relative to all rocks present below the surface.
younger
A common feature that all unconformities exhibit?
erosive surface
What is the category name for the largest division of time used on the geologic time scale?
Eon
Why is the geologic time scale more detailed in the Phanerozoic than in previous eons?
The Phanerozoic Eon is more detailed because of the presence of organisms with hard parts and the rapid increase in biodiversity.
The Jurassic Period lies in the _____ Era, which was dominated by large terrestrial vertebrates, or _____.
Mesozoic; dinosaurs
Using a calendar year as an analogy to the geologic time scale, when do the oldest anatomically modern humans appear on Earth?
The oldest human fossils show up at about eleven minutes to midnight on December 31st.
Name the epoch, period, era and eon in which we currently live.
We live in the Holocene Epoch of the Quaternary Period, which is part of the Cenzoic Era and Phanerozoic Eon.
Definition of fossil range
The period of time when an organism first and last appears in the sedimentary record.
A characteristic most desirable for constraining the relative age of rocks?
organisms with overlapping ranges and geologically short lifespans
If a rock contains ten different kinds of fossils, which of the following must be true?
All of the organisms had to be alive when the rock was deposited.
How is the geologic time scale related to the fossil record?
The periods of the time scale were constructed based on ranges of fossil organisms.
The fundamental principle of geology put forth by James Hutton is called _______.
the principle of uniformitarianism
What hypothesis promoted the view that Earth has been shaped largely by sudden, drastic events over a very short period of time?
catastrophism
Which principles are key to relative dating?
-principle of cross-cutting relationships
-law of superposition
-principle of original horizontality
3 types of unconformities
-disconformity
-angular unconformity
-nonconformity
Which of the following is an example of a trace fossil?
-COPROLITES
-skull
-teeth
-shells
-claws
What is the difference between a mold and a cast?
Molds reflect the shape of an organism; casts fill in the molds.
What two special conditions are necessary to preserve organisms as fossils?
rapid burial and hard parts
A radioactive isotope used to date very recent events
carbon-14
What subdivision of the geologic time scale represents the greatest expanse of time?
Eon
What is the best method to determine a numerical age for a sedimentary rock?
Date surrounding rock layers, such as igneous rocks.
What is the primary problem in assigning numerical dates to rocks?
Not all rocks can be dated by radiometric dating methods.