Chapter 10: Deep Time – How old is old? Reading Quizzes
order of formation of disconformity (form from transgression-regression sequence)
1. Sediments were deposited in a marine environment.
2. Sea level fell, exposing marine deposits subaerially.
3. Erosion takes place.
4. Sea level rose, covering the erosional surface, which was buried by marine sediments.
Sediments are deposited in a marine environment. When sea level falls, exposing the marine sediments, erosion takes place. When sea level rises again, the erosional surface is buried by more marine sediments, creating a disconformity.
order of formation of angular disconformity
1. Sediments are deposited and lithified.
2. Regional mountain building takes place, and sedimentary layers are folded into anticlines and synclines.
3. Erosion takes place.
4. Sediments are deposited on top of erosional surface.
Sedimentary rocks are emplaced and then folded. Erosion takes place, followed by more deposition.
uncomformity between folded or tilted rock layers below and horizontal sedimentary rock layers
– uncomformity between two sedimentary rock sequences
– surface between parallel sedimentary beds of significantly different ages
order of formation of nonconformity
1. Sedimentary layers are deposited and lithified.
2. A granitic pluton intrudes sedimentary rock layers.
3. Erosion takes place, exposing the pluton at the surface.
4. Sedimentary deposition occurs again, covering the erosional surface.
Sedimentary rock layers are deposited and then intruded by a pluton. Erosion exposes the pluton at the surface. Sediments are once again deposited, covering the erosional surface.
uncomformity between nonsedimentary rock layers (ie. igneous and metamorphic) and sediementary rocks
strata at the base
in place before the strata above could be deposited
– oldest rocks are at the bottom of the sequence and the youngest are at the top
– sequence of sedimentary beds, the youngest is on top
principle of inclusions
– states that a rock containing an inclusion (fragment of another rock) must be younger than the _______
– Rock that occurs as an inclusion in another rock must be the older of the two
– “the rock containing the inclusion” is a single layer of a specific type of rock
– “__________” is a small sample of another rock (such as pebbles of igneous rock included in a layer of sedimentary rock or fragments of sedimentary rock included in an igneous layer)
– inclusions are older than the surrounding rock
– dike cuts across the sedimentary beds, so the dike is younger
– feature doing the cutting is younger than the feature it cuts
pluton baked the adjacent rock, so the adjacent rock is older
develop when new sedimentary layers accumulate atop old, eroded layers, resulting in a geologic hiatus
– principle that geologists use to determine the age of rock
– look at the rock surrounding fossils with unique characteristics, such as a geologically short lifespan and easily identifiable features, and use this information to estimate the age of a rock layer in other areas that contain the same type of fossil or group of fossils
only one that contains both fossils C and E
– illustrated by scientists’ seeing pillow lava form only underwater
– then theorizing that pillow lava found high in the mountains today did nevertheless form underwater
relative age (dating)
Specifying the age of one feature with respect to another
must be younger than the sedimentary rock layers that it cuts across because the rock layers need to be there before they can be cut by the intrusion
basic principles for determining relative ages
1. The oldest sedimentary layer is the bottom layer.
2. The sedimentary layers started out lying flat.
3. The igneous intrusion “baked” (metamorphosed) the sedimentary rock it touched.
intrusion (batholith G)
You should be able to number the events in the accompanying figure in the proper order of occurrence. If the first (oldest) event is number 1, and the last (most recent) is number 8, which occurred fifth in the sequence?
sedimentary layers began as continuous expanses of sediment
type of unconformity forms when sedimentary rocks overlie either igneous or metamorphic rocks
occurs when flat lying sedimentary layers overlie tilted layers
formed when igneous intrusions create contact metamorphism (a baked zone) around the intruding magma
C and E
– On the diagram shown, several surfaces between rock layers are identified by letters. Choose the answer below that correctly identifies the unconformities.
– C = angular unconformity
– E = disconformity
— the entire Mesozoic is missing
– boundary surface between two stratigraphic formations
key bed (or marker bed)
unique bed that helps in correlation of strata
can be a type of contact, but not all contacts are considered this
Cenozoic, Mesozoic, Precambrian
Choose the proper listing of names to fit the following three descriptions: Age of Mammals, Age of Dinosaurs, and longest geologic time period.
eon, era, period, epoch
Which of the following accurately ranks the subdivisions of geologic time in order from largest to smallest?
method to determine numerical age
1. analyzing growth ring patterns in trees
2. radioactive decay of uranium to lead
3. carbon-14 dating
– metamorphic rock tells when the high temperatures of metamorphism cooled below the closure temperatures of the minerals involved.
– can begin only when the isotopes cool enough to lock into the crystal lattice.
– sedimentary rock dates the time of crystallization of the sedimentary minerals, not the time of sedimentary rock formation.
FALSE: can be used only if there is some uranium present in the rock.
– method of determining the age of materials is a radiometric dating technique
– method of determining the numerical age of a fossil due to the radioactive decay of carbon
used to date organisms, not minerals
starts when a radioactive mineral cools below its closure temperature
relies on the parent-daughter ratio
T/F: Scientists can date any radioactive material back only three half-lives; after this there’s not enough parent material left to measure.
After three half-lives, there’s still one-eighth of the radioactive parent material left, which is easily detected. Carbon-14 can be detected for about 12 half-life periods (about 70,000 years).
The age could logically be 5,300 years.
A corpse with flesh intact, found in the Alps in 1991, was dated by the carbon-14 method and showed a parent-daughter isotope ratio of approximately 1:1, with slightly more parent material than daughter material. The half-life of carbon-14 is 5,730 years.
At the end of one half-life, 1:1; two half-lives, 1:3; three half-lives, 1:7.
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?
The skull should be dated by the carbon-14 method, since this is the method used for organic material older than 1 million years.
A hominin (human family) skull was found in a shale layer between two fine-grained igneous rock layers. There were no baked zones on the rocks above the igneous layers. The skull had been 100% fossilized and changed to stone; there was no original skull material left. On the basis of this information and the diagram, decide which of the following statements is FALSE.
A hominin (human family) skull was found in a shale layer between two fine-grained igneous rock layers. There were no baked zones on the rocks above the igneous layers. The skull had been 100% fossilized and changed to stone; there was no original skull material left.
– The igneous layers must be lava flows, because they are fine-grained rock and there are no baked zones on the rock above them.
– The igneous layers can be dated by radiometric methods; the shale layers cannot be dated radiometrically.
– The skull itself cannot be dated, because it is no longer organic material and it is not igneous material. The age of the skull is determined by its position between the igneous layers.
false; Equal amounts of parent and daughter material exist only after one half-life.
T/F: there are equal amounts of parent and daughter material present after the passage of two half-lives.
Varieties of an element that differ only in the number of neutrons
4.54 billion years
name of a rock layer identified by such factors as rock type and approximate geologic age
An unconformity is a break in the rock record that indicates the area was underwater for millions of years.
the last hour before midnight on New Year’s Eve.
If you equate all Earth history to one calendar year, the history of our species (Homo sapiens) would occupy
Human history (the history of our human species, Homo sapiens) represents an incredibly small portion of Earth history, equivalent to the last hour before midnight if all of Earth history were equated to one calendar year.