Chapter 10 Flashcards
Compare numerical age and relative age.
Numerical Age: the age of a feature given in years
Relative Age: the age of one feature with respect to another.
Describe the Principles that allow us to determine the relative ages of geological events.
Physical Principles
The Principle of Uniformitarianism- Physical processes we observe operating today also operated in the past (at roughly comparable rates). (Physical processes do not occur at “exactly” the same time.)
Superposition- Sequence of sedimentary rock layers. (Layer below = older than one above.)
Original horizontality- When layers of sediment are originally deposited, they are fairly horizontal. (When we see folds & tilted beds, we are seeing the consequences of deformation that postdates deposition)
Original Continuity - Sediments generally accumulate in continuous sheets.
Cross-Cutting Relations -If one geologic feature cuts across another, the feature that has been cut is older.
Inclusions- If an igneous intrusion contains fragments of another rock, the fragments must be older than the intrusion. (The fragments in sedimentary layers are inclusions. The rock containing the inclusion must be younger than the inclusion.)
Baked or Chilled Contacts- An igneous intrusion “bakes” (metamorphoses) surrounding rocks. The rock that has been baked must be older than the intrusion.
How does the Principal of Fossil Succession allow us to determine the relative ages of Strata?
The predictability of fossil distribution which allows geologists to arrange fossil species in a progression from older at the bottom to younger at the top, (The sequence contains a definable succession of Fossils (A,B,C,D,E,F), that the range in which a particular species occurs may overlap with the range of other species, and once a species becomes extinct, it does not reappear higher in the sequence.
(With the Principle, we can define the relative ages of strata by looking at fossils.)
How does unconformity develop? Describe the differences in the three kinds of unconformities.
-Strata are not deposited continuously at a location. Unconformity is an interval of non-deposition and/ or erosion.
-An unconformity forms whenever the land surface does not receive & accumulate sediment.
Angular :-forms where rocks were either folded or tilted by faulting before being uplifted and eroded.
-(Cuts across the underlying layers; the layers below have a diff orientation from the layers above.)
Noncomformity: unconformity at which sedimentary rocks overlie intrusive igneous rocks or metamorphic rocks
The igneous metamorphic rocks must have cooled, been uplifted, and been exposed by erosion to form the substrate on which new sedimentary rocks were deposited
Discomformity: Layers of sediment are deposited,
Sea level drops and an erosion surface forms; (recently deposited beds become exposed for some time. During this time no new sediment accumulates & some preexisting sediments get eroded away.
Sea level rises and new sedimentary layers accumulate over the old.
-Even though the beds above & below the disconformity are parallel, the contact b/t them represents an interruption in deposition.
Describe two different methods of correlating rock units. How was correlation used to develop the geologic column? What is stratigraph formation?
Lithologic: geologists correlate formations between nearby regions based on similarities in rock type
Fossil: correlates units over broad areas, we must rely on fossils to define the relative ages of sedimentary units.
Geologic Columns: By correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a “composite” stratigraphic column. (Represents the entirety of Earth History)
Largest Subdivisions: Hadean, Archean, Proterozoic, & Phanerozoic Eons. (Zoic - means life)
Phanerozoic means “visible life” & Proterozoic means “earlier life”
Archean Eon: The earliest life, bacteria & archea, appeared.
Phanerozoic Eon: organisms w/ hard parts (shells & skeletons) became widespread, so there are abundant fossils from this Eon.
Precambrian time, only small organisms w/ no shells existed (hard to find)
Phanerozoic Eon is subdivided into eras
Oldest to Youngest: Paleozoic (ancient life) Mesozoic (middle life) Cenozoic (recent life)
Divides Further: era into periods into epochs
Geologists now give the age range of fossils species in terms of the periods & epochs of the geologic column, & can use fossils alone to determine the relative age of sedimentary rocks. How to determine Numerical Age
Radiometric Dating: (techniques developed to provide insight into the numerical ages of rocks) These techniques are based on the measurement of radioactive elements in rocks (the science of dating geological materials in years.)
Growth Rings: develop in trees, travertine deposits, and shelly organisms.
Rhythmic Layering: develops in sedimentary accumulations and glacier ice.
Counting rings in trees or layers in sediment
The growth rate of trees
The organic productivity of lakes and seas
The sediment supply carried by rivers
The growth rate of chemically precipitated sedimentary rocks
The growth rate of shell-secreting organisms
The layering in glaciers
What does the process of radioactive decay entail?
Radioactive isotopes are unstable: after a given time, they undergo a change (RAD) which converts them into a diff element.
In these reactions, the isotope that undergoes decay is the PARENT ISOTOPE, while the decay product is the DAUGHTER ISOTOPE.
To measure how long it takes for half of a group of isotopes to decay
The ratio of parent-to-daughter isotopes changes with the passage of each successive half-life.
How do geologists obtain an isotopic date? What are some of the pitfalls in obtaining a realistic one?
An isotope is an atom of a chemical element whose nucleus has the same atomic number but which has a different atomic weight. What this means is that an isotope can be thought of as a different version of a standard element on the periodic table. Due to its difference though, isotopes are atomically unstable, and prone to change into other, more stable elements. This process is referred to as radioactive decay. The rate at which radioactive decay occurs is that particular isotope’s half-life.
Explain the concept of uniformitarianism
all phenomena that happen now will have happened in the past and will happen in the future if the conditions are right for that phenomena occurring
Why can’t we date sedimentary rocks directly? Why don’t all periods on the geologic column last for the same amount of time?
Isotopic dates provide ages of minerals (grains or crystals) within rock. Igneous rocks form at the same time as their component minerals and are thus datable. Sedimentary rock grains, such as sand, mud, and gravel, derive from the physical and chemical weathering of preexisting rocks at relatively low temperatures. The mineral components within sedimentary rocks are generally much older than the rocks themselves (principle of inclusions).
Is there one place on Earth where we can see the complete geologic column?
No, all local sections are an incomplete record of geologic time
What is the age of the oldest rocks on Earth? What is the age of the oldest rocks known? Why is there a difference?
on earth: 4.03 Ga. known: 4.4 Ga. Difference between the temp of earth may have been too hot for the closure temp of minerals. or inner plants were bombarded so intensely by meteorites that almost all of the crust was completely destroyed
