It is estimated that the earth formed along with the solar system 4.6 billion years ago. (4600 MYA)
Geologic time scale
A summary of the major events in Earth's History.
-Eon- largest segment of geologic time.
--Era
---Period
----Epoch- smallest segment of geologic time.
Reading the Rock Record Relative Time- Places events Identifies the actual in a sequence dates of geologic events. But does not identify their actual date of occurrence. Absolute Time- Identifies the actual dates of geologic events.
Finding the Age of Rocks with Relative Time
Law of superposition- in undisturbed sedimentary rocks the oldest rock layers are at the bottom and the youngest are at the top.
Law of cross-cutting relationships- an igneous intrusion is younger than the rock it has intruded into. (Look at the whiskers!)
Law of included fragments- if fragments of one type of rock are found in another rock layer, the rock fragments myst be older then the rock layer in which they are found.
Faulted and folded layers- layers of rock that have been faulted or folded must have been present before the actions of faulting or folding took place.
Rock Correlation- Matching of rock layers that can be seen at the earth's surface, over a large area.
An OUTCROP is exposed rock layers at the earth's surface.
Replaced Remains- the soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material (petrified wood).
Molds and casts- Fossil shells or bones are dissolved completely out of the rock leaving a hollow depression in the rock. New mineral material fills the mold it forms a cast of the original fossil.
Trace fossils- evidence of life other then remains, which included any impressions left in the rock. (Trails, footprints, tracks, burrows.)
RADIOACTIVE DATING
Used to date far back in time. Certain rocks contain radioactive isotopes.
-----Radioactive Isotopes are atoms of elements that give off radiation from their nuclei.
-----Radioactive Decay is the process by which a radioactive isotope changes into a new stable element.
HALF-LIFE
-----The rate at which a radioactive element decays. It is the time it takes for half of the atoms of the radioactive elements to decay to a stable end product. (SEE PAGE 1 OF THE ESRT)
-----At the end of each half-life, half of the radioactive material remains.
Radiocarbon Dating- Uses the radioactive isotope carbon-14 found in all living things. Because carbon-14 is continually absorbed by food and water it stays constant in living things. When the living thing died the percentage of carbon-14 decreases at the rate of its half-life. Can be used to date back about 100,000 years.
Uranium lead method- is useful to date rocks older than 10 millin years. Can be used only in igneous rocks that contain the right kind of uranium.
Rubidium-Strontium Method- can also be used to date older rocks because of its long half-life. It is also very commonly found in igneous rocks.
Potassium-Argon Method- is very useful since potassium 40 can be found in metamorphic, sedimentary, and igneous rocks. It can date older rocks but may also date rocks as young as 50,000 years.
Geologic time scale
A summary of the major events in Earth's History.
-Eon- largest segment of geologic time.
--Era
---Period
----Epoch- smallest segment of geologic time.
Reading the Rock Record
Relative Time- Places events Identifies the actual in a sequence dates of geologic events. But does not identify their actual date of occurrence.
Absolute Time- Identifies the actual dates of geologic events.
Finding the Age of Rocks with Relative Time
Law of superposition- in undisturbed sedimentary rocks the oldest rock layers are at the bottom and the youngest are at the top.
Law of cross-cutting relationships- an igneous intrusion is younger than the rock it has intruded into. (Look at the whiskers!)
Law of included fragments- if fragments of one type of rock are found in another rock layer, the rock fragments myst be older then the rock layer in which they are found.
Faulted and folded layers- layers of rock that have been faulted or folded must have been present before the actions of faulting or folding took place.
Rock Correlation- Matching of rock layers that can be seen at the earth's surface, over a large area.
An OUTCROP is exposed rock layers at the earth's surface.
A KEY BED is a thin, widespread layer, usually of volcanic ash, that can be used to correlate an exact point of time.
FOSSILS
A fossil is any evidence of earlier life preserved in the rock.
Original Remains (rare)- The actual unchanged remains of the planet or animal are preserved.
Replaced Remains- the soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material (petrified wood).
Molds and casts- Fossil shells or bones are dissolved completely out of the rock leaving a hollow depression in the rock. New mineral material fills the mold it forms a cast of the original fossil.
Trace fossils- evidence of life other then remains, which included any impressions left in the rock. (Trails, footprints, tracks, burrows.)
INDEX FOSSIL
-Easily identifiable
--Short lived
---Widespread occurrence
MEASURING ABSOLUTE TIME
Tree Rings- Each ring represents a single year (spring/fall) the width of the ring depends upon the temperature and rainfall.
Varves- Glacial lake deposit. A think light colored layer in the summer and a thin dark layer in the winter.
RADIOACTIVE DATING
Used to date far back in time. Certain rocks contain radioactive isotopes.
-----Radioactive Isotopes are atoms of elements that give off radiation from their nuclei.
-----Radioactive Decay is the process by which a radioactive isotope changes into a new stable element.
HALF-LIFE
-----The rate at which a radioactive element decays. It is the time it takes for half of the atoms of the radioactive elements to decay to a stable end product. (SEE PAGE 1 OF THE ESRT)
-----At the end of each half-life, half of the radioactive material remains.
Radiocarbon Dating- Uses the radioactive isotope carbon-14 found in all living things. Because carbon-14 is continually absorbed by food and water it stays constant in living things. When the living thing died the percentage of carbon-14 decreases at the rate of its half-life. Can be used to date back about 100,000 years.
Uranium lead method- is useful to date rocks older than 10 millin years. Can be used only in igneous rocks that contain the right kind of uranium.
Rubidium-Strontium Method- can also be used to date older rocks because of its long half-life. It is also very commonly found in igneous rocks.
Potassium-Argon Method- is very useful since potassium 40 can be found in metamorphic, sedimentary, and igneous rocks. It can date older rocks but may also date rocks as young as 50,000 years.