Geologic HistoryVery Brief Version

charity mulig

Which is which?Uniformitarianism• Believes that the forces and

processes that we observe today have been at work for a very long time.

Catastrophism• Believes that the earth is

shaped by catastrophic events such as earthquakes, volcanic eruptions and meteorite collisions.

Relative dating tells us the sequence in which events occurred, not how long ago they occurred.

Relative Dating

Law of Superposition

states that in an undeformed sequence of sedimentary rocks, each bed is older than the one above it and younger than the one below it.

Key Principles

Principle of Original Horizontality

means that layers of sediment are generally deposited in a horizontal position.

Key Principles

Principle of Cross-Cutting

Relationships

states that when a fault cuts through rock layers, or when magma intrudes other rocks and crystallizes, we can assume that the fault or intrusion is younger than the rocks affected.

Key Principles

Inclusions

• rocks contained within other rocks

• Rocks containing inclusions are younger than the inclusions they contain.

Key Principles

Unconformities• An unconformity

represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumed.

• An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.

Key Principles

Unconformities• A nonconformity is when

the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks.

• A disconformity is when two sedimentary rock layers are separated by an erosional surface.

Key Principles

A Record of Uplift, Erosion, and Deposition

Correlation of Rock Layers

Correlation is

establishing the

equivalence of rocks of

similar age in different

areas.

Fossil Formation

• Fossils are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks.

• The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried.

Fossils

Types of Fossil

1. Unaltered Remains– Some remains of organisms—such as teeth, bones, and shells—may not have been altered, or may have changed hardly at all over time.

2. Altered Remains–Petrified, casts, molds

and carbonization.

Fossils

Fossil Formation

1. Indirect evident– trace fossils

2. Conditions Favoring Preservation– Hard parts and rapid

burial3. Interpreting

Environments

Fossils

Dating with

Radioactivity

RadioactivityRadioactivity is the spontaneous decay of certain unstable atomic nuclei.

Half-LifeA half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope.

Radiometric Dating

Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.

• Radiometric dating is the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes.

Geologic Time Scale

Precambrian

Lithosphere

Early Very hot, no permanent crust.

Middle Land cooled to temperature lower than the boiling point of water.

Late Formation of iron ores (hematite) which are usually found underwater (am indication that life is concentrated in the seas)

Atmosphere

Early No atmosphere. Air similar to gases released by volcanoes.

Middle No free oxygen. Reduced amount of vapor and CO2 in the atmosphere.

Late All free Fe have combined with 02. hence an increase in atmospheric O2.

Hydrosphere

Early No water on land.

Middle Formation of lakes and seas on low-lying areas.

Late Increased dissolved O2. Presence of aerobic organisms.

Biosphere

Early No life.

Middle Anaerobic life confined in the seas. Mostly algae and unicellular bacteria and other prokaryotes.

Late Presence of eukaryotes and photosynthetic plants.

Fossils

• Stromatolites and other trace fossils•Microfossils found in chert.

Rocks

Shields are deformed metamorphic rocks found at the heart of continents.

Paleozoic

Lithosphere

Early Gondwanaland in the south while Laurasia is yet unformed.Early N. America is barren lowland covered by shallow seas follwoed by mountain building and another period of inundation of shallow seas resulting to formation of evaporites

Late Collision of N. America and Africa (formation of Appalachian Mt)Convergence of other continents (W. Asia + Siberia = Ural Mt)Laurasia + Gonwanaland = Pangaea (lead to increase in elevation)

Atmosphere Late Drier climate.Hydrosphere Variable.

Biosphere Early Appearance of animals with hard part s(shells, exoskeletons, cartilage) and cephalopods (1st truly large animals).

Late Life diversified. Appearance of 1st terrestrial plants, insects , fishes , and swamps which became coal mines.

Fossils Trilobites, brachiopods.Lobe-finned fish and lung fish.

Mesozoic

Lithosphere

Early Most lands above sea level. Middl

eN. America covered by shallow seas (evidenced by continental sediment deposits). Much of SW America is desert.

Late N. America covered by swamps forming coal deposits.Start of break up of Pangaea. N America (formation of mountains in W America and intrusion in Midwest America). Formation of Rocky mountains.

Biosphere Survivors of Paleozoic extinctions diversified. Development of gymnosperms. Perfection of the reptilian egg. Domination of dinosaurs (adaptive radiation)

Fossils Example of gymnosperms (cycads, gingkoes, conifers) are found in Arizona’s Petrified forest National Park.Sarchosuchus imperator

Rocks Presence of iridium in the KT boundary suggest a meteoritic collision.Lava plateau suggests a volcanic eruption.

Cenozoic

Lithosphere Early E. America. Passive margin. Massive deposits caused down warpind and faulting. Faults created traps of oil and natural gas. Appalachians eroded.End of Rocky mt formation led to deposition in the great plains.Creation of Basin and Range province I Nevada and Mexico by crustal uplifting.

Late Raising of Applachians by isostatic movements lead to deposition in Atlantic and Gulf Coast plains.Uplifting of Rockies and formation of Grand Canyon and other gorges.Volcanic Activity lead to the cascade range and the formation of columbia plateau.

Biosphere Survivors of the KT extinction.Mammals replaced reptiles as dominant land animals.Angiosperm replaced gymnosperm as dominant plants. Rapid diversification of plants and birds.Development of grass, grazing animals and predators.

Fossils Foraminifera.Extinction of large mammals attributed to selective hunting by humans.

Rocks Folding and glaciations gave finishing touches.