Geologic Time Flashcards
Uniformitarianism
The physical, chemical and biological laws that operate today have also operated in the geologic past. So forces and processes that shape planet have been occurring for very long time. Put forth in late 1700s by James Hutton
Relative dating
Placing rocks and events in a relative sequence
Principles of relative dating
Law of superposition
Principle of original horizontality
Principle of cross-cutting relationships
Inclusions
Principle of superposition
In an informed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.
Oldest rocks on bottom
Principle of original horizontality
Sediments are deposited horizontally by gravity
Principle of cross-cutting relationships
When a fault cuts through other rocks, or when magma intrudes and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected
Inclusions
Fragments of one rock contained within another (rock containing inclusions is younger)
Conformable layers of rocks
Deposited essentially without interruption
Uncomformity
Long period of time during which deposition ceased, erosions removed previously formed rocks, and then deposition resumed. A break in the rock record.
Angular unconformity
Consists of tilted or folded sedimentary rocks overlain by younger flat lying strata
Indicates that period of deformation and erosion occurred
Disconformity
Strata on either side are parallel or conformable
Nonconformity
Older metamorphic or intrusive igneous rocks below younger sedimentary rocks
Correlation of rock layers
Matching rocks of similar age in different regions.
Often relies upon fossils (across continents)
Lithologies (noting position of layers of rock in a sequence of strata)
Fossils
Remains or traces of prehistoric life in sediment and sedimentary rocks
Principle of fossil/faunal succession
Fossil organisms succeed one another in a definite and determinable order and therefore any time period can be recognized by its fossil content
Index fossils
Wide spread geographically and are limited to a short span of geologic time so their presence provides an important method of matching rocks of the same age
Catastrophic
Earths varied landscapes formed primarily by great catastrophes
Believed during 1600s and 1700s b/c people thought (James ussher) that earth was only a few thousand years old
Radioactivity
Spontaneous breaking apart (decay) of unstable atomic nuclei.
Alpha particles
May be emitted from nucleus
Consists of 2 protons and 2 neutrons
Emission means: mass number of isotope is reduced by 4 and atomic number decreased by 2
Beta particles
Or electron is given off from nucleus
Mass number remains unchanged because electrons have basically no mass
Emissions causes 1 more proton so atomic number up by 1
Electron capture
More rare, upper atmosphere
Sometimes electron captured by nucleus
Electron combines with proton and forms another neutron
Mass number unchanged but atomic number decreased by 1
Parent isotope
Unstable radioactive isotope
Daughter products
Isotopes resulting from decay of the parent
Radiometric dating
Using radioactivity as a reliable method for calculating the ages of rocks and minerals that contain particular radioactive isotopes
Half-life
The time required for one half of the nuclei in a sample to decay
Radiocarbon dating
Dating recent events with the radioactive isotope of carbon which is carbon-14
Geologic timescale
Divides geologic history into units. Originally created using relative dating
Eon
Greatest expanse of time
4 eons: Phanerozoic, Proterozoic, Archean, Hadean
Era
Eons divided into eras
Three eras in Phanerozoic are Paleozoic, Mesozoic, Cenozoic
Periods
Each era in Phanerozoic eon divided into units known as periods
Paleozoic has 7
Mesozoic and Cenozoic have 3
Epochs
Each period divided into epochs
Seven epochs for the periods of Cenozoic era
Epochs of other periods not usually specific names but terms early, middle and late
Precambrian
4bn years prior to Cambrian divided into two eons, Archean and Proterozoic, which are divided into four eras
