Week 5 Flashcards
Relative Time
based on the relative timing of geological events (i.e. the order in which events occurred)
Absolute Time
the determination of age in years before present, based on the decay of radioactive isotopes.
Law of Original Horizontally
Sediments and some bodies of extrusive igneous rocks are deposited as horizontal layers (strata).
States that strata are laid down horizontally.
If strata are not horizontal, it means that they were disturbed later by tectonic forces.
Law of Superposition
In any sequence of layered rocks, a given stratum must be older than any stratum on top of it.
Law of Fossil Succession
Fossils occur in a consistent vertical order in sedimentary rocks all over the world.
Thus the relative age of “slices” of time represented in rocks can be determined from the fossil content of rocks.
Law of Cross-cutting relationships
a fault or intrusive igneous body must be younger than the rock through which it has cut
Inclusions
Dislodged fragments of rock that are incorporated into younger rocks
Law of Inclusions
if a rock body (Rock B) contains fragments of another rock body (Rock A), it must be younger than the fragments of rock it contains; Rock A must have been there first to provide the fragments.
Inclusions in igneous rock bodies
dislodged from walls of magma chamber
Inclusions in sedimentary rocks
fragments eroded from underlying rocks
Unconformities
The rate of sediment deposition is not uniform through time
Nonconformities
An unconformity that separates underlying metamorphic or igneous rocks from overlying horizontal sedimentary strata.
Angular Unconformity
Separates underlying tilted rocks from overlying horizontal sedimentary rocks.
Disconformity
Separates underlying horizontal sedimentary rocks from from overlying horizontal sedimentary rocks.
Isotopes
Atoms of the same element with differing atomic weights
Radioactive Decay
Certain isotopes (parent isotopes) will lose/gain particles in its nucleus to form isotopes of a new elements (daughter isotopes), releasing energy in the process
Radiometric Dating
The higher the proportion of daughter atoms to parent atoms, the older the sample
Half Life
the time required for half of the atoms in a sample to decay to daughter atoms
Importance of the Geological Time Scale
1) Provides a framework for the timing of geological events
2) Makes scientific communication more efficient
3) Comprises a hierarchy of time units so that time can be expressed in both large and small units
The Rock Cycle
The cycle of rock recycling
Athenosphere
not quite a liquid, but flows like a liquid over long periods of time
Mohorovicic discontinuity
The boundary between the crust and mantle
Called “Moho” for short
Convection Model
convection currents in the asthenosphere drag the lithospheric plates along
Ridge-Push Model
plates are actively pushed from sites of spreading as magma is injected and plates slide down flanks of mid-ocean ridges under the influence with gravity
Slab-Pull Model (most likely)
Slab could be pulled by the sinking, leading edge of the plate
Divergent Boundaries
represented by mid-ocean ridges
Convergent Boundaries
represented by trenches
Erosion
the physical removal of material by mobile agents such as water, wind, ice, or gravity.
Frost-Wedging
Occurs when liquid water penetrates cracks within a rock and freezes
Water expands when it converted to ice, widening the cracks
Freeze-thaw weathering
With repeated cycles of water penetration and freezing, cracks get increasingly large, ultimately resulting in the breakup of the material
Root wedging
Occurs when roots of plants (e.g. trees) penetrate into small cracks. As the root increases in size, it increases the size of the crack until pieces of the rock break apart.
Unloading
When a plutonic igneous rock body expands as overlying rock is removed by erosion.
Outer layers of the intrusion expand more than the rock below and separate like layers of an onion (exfoliation).
Sheets of igneous rock “pop” off the surface as the body continues to expand.
Dissolution
The process in which a material is dissolved in a liquid
Oxidation
Occurs when oxygen combines with another element to form an oxide that can be subsequently removed by erosion.
EX. Rust (iron forming hematite)
Hydrolysis
when minerals react with water to form other products.
Why is liquid water one of Earth’s most potent agents of erosion?
1) It can transport dissolved substances released by chemical weathering.
2) Flowing water can loosen and pluck particles.
3) These particles, in turn, can impact underlying rock and abrade it.
Erosion by wind (deflation)
Flowing air, much like flowing water, also plucks grains that are loosened by weathering, and can produce similar effects of erosion.
Erosion by ice
Like water and wind flowing glacial ice can pluck loose sediment grains from the ground. Plucked fragments of rock under the ice can then abrade rock as they are dragged across the bed of the glacier.
Erosion by Gravity
Material weakened by weathering can fail and be transported downslope. The rapid downslope movement of materials due to gravity is called mass-wasting
