CHAPTER 9 Flashcards
sediments
sedimentary rocks form from small organic or inorganic particles called sediments that accumulate and are cemented together.
Principle of Superposition
The tendency for rock layers to be chronologically stacked is called the Principle of Superposition.
where do igneous rocks form from?
Igneous rocks that form from volcanic activity may vertically cut through horizontally arranged layers of rocks, and mountain building events may tilt, fold, and even flip rock layers.
Stratigraphy
Stratigraphy is the science of using the arrangement and composition of rock layers to interpret geological history.
formation
A large uninterrupted sequence of rock that is made of multiple layers that all share similar properties (such as mineral composition and average sediment grain size) and that all formed under similar conditions is termed a formation
When a sequence of rock changes from one formation to another, what does it indicate?
it indicates that a large scale change occurred in the environment where the rocks were being deposited.
The Principle of Superposition allows a stratigrapher to
to infer the relative age of rock layers (that is, how old one layer is relative to another), but it does not determine the absolute age (that is, how old in years the layers are).
To age rocks in absolute terms, a technique called _______ _____ is used.
radiometric dating
isotope
a variant of a chemical element that has an unusual number of neutrons
decay products
Some isotopes are unstable and will undergo radioactive decay, whereby energy is released and a new atom (or atoms) with a different composition of particles results. These resulting atoms with different particle compositions are called the decay products.
When a new rock forms, it has a …
ratio of isotopes and decay products that matches that of the environment.
As the rock ages…
the isotopes decay and the ratio of isotopes to decay products decreases.
mass spectrometer
Using a special machine called a mass spectrometer, it is possible to measure the isotope ratio of a rock, and this ratio can tell you how long ago the rock formed.
why can sedimentary rock usually not be radiometrically dated
Because they are made of sediments that had already formed and that were already potentially undergoing radioactive decay
why can igneous rocks can be radiometrically dated
igneous rocks are formed anew and can usually be radiometrically dated.
if fossils are usually found in sedimentary rocks and not in igneous rocks, how can we ever tell how old a fossil and its rock layer is?
By combining radiometric dating and the principle of superposition. If sedimentary rocks that contain fossils are found between two horizontally deposited layers of igneous rocks, then dating the igneous rocks above the sedimentary layer will tell us what age the fossils must be older than, and dating the igneous rocks below the sedimentary layer will tell us what age the fossils must be younger than. So, we can confidently bracket the age of the fossils.
___________ are a key tool in fossil dating.
Volcanic ash deposits
how are volcanic ash deposits used for dating fossils.
we can be certain that particular particles of igneous rocks that compose the sedimentary rocks that the fossils are buried in were incorporated into the sediment at nearly the same time that they were formed. For instance, fossils may be buried by, or be buried near, deposits of volcanic ash. Volcanic ash forms at the moment of an eruption, and the time between when an eruption occurs and when its ash falls to the surface is inconsequently short.
________ and ________ dating are combined to piece together the history of the earth and to create the geologic time scale.
Stratigraphy and radiometric
The largest units in the time scale are
Eons - Eras - Periods - Epochs
THE HADEAN EON – 4.6 TO 4 BILLION YEARS AGO
- by the beginning of this eon, the rest of the universe was already over nine billion years old
- surface of the earth partially molten and with volcanic activity widespread
- By the end of the Hadean, the earth had cooled and large oceans covered much of its surface. Complex organic molecules are thought to have formed in these early oceans and possibly the earliest true life forms.
how and when was the moon created
in the Hadean Eon and was formed when the earth was at roughly 4.5 billion years ago, the young earth collided with a smaller planetoid. This collision ejected a large mass of debris, which was held in orbit by the earth’s gravity and eventually formed the moon.
THE ARCHEAN EON – 4 TO 2.5 BILLION YEARS AGO
- The oldest known fossils come from the Archean Eon. These fossils are of simple single- celled organisms
- More advanced forms later evolved in the Archean, including cyanobacteria
cyanobacteria
The cyanobacteria were photosynthetic and eventually produced large amounts of oxygen gas, which became concentrated in the earth’s atmosphere. Some cyanobacteria formed structures called stromatolites
stromatolites
some of the best records of early life. Stromatolites look like lumpy stones, but when you cut them in half you can see the layers that were created as the cyanobacteria secreted sticky films that trapped particles of sediment.
THE PROTEROZOIC EON – 2.5 BILLION TO 541 MILLION YEARS AGO
- 1.7 billion years ago, the first multicellular organisms evolved.
- Within the Proterozoic, the time span from 630 to 542 million years ago is known as the Ediacaran Period.
