Geologic Times

• Rocks ==record geological events and changing life forms== of the past.
• Earth is ==much older than anyone had previously imagined== and that ==its surface and interior have been changed by the same geological processes== that continue today.

{{Uniformitarianism - the forces and processes that we observe today have been at work for a very long time.{{

Relative Dating - Key Principles

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

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 i==t.

Principle of Original Horizontality - means that ==layers of sediment are generally deposited in a horizontal position.==

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==.

Inclusions

• Inclusions are ==rocks contained within other rocks.==
• Rocks containing inclusions are ==younger than the inclusions they contain.==

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.
• 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.

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.==

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==.

Altered Remains

• The remains of an organism are ==likely to be changed over time==.
• Fossils often become ==petrified== or turned to ==stone==.
• ^^Molds and casts^^ are another ==common type of fossil==.
• ^^Carbonization^^ is particularly ==effective in preserving leaves and delicate animals==. It occurs when an ==organism is buried under fine sediment==.

Indirect Evidence

• ^^Trace fossils^^ are indirect ==evidence of prehistoric life==.

Conditions favoring Preservation

• Two conditions are important for preservation: rapid burial and the possession of hard parts.

Fossils and Correlation

The principle of fossil succession - states that ==fossil organisms succeed one another== in a ==definite and== ==determinable order==. Therefore, ==any time period can be recognized by its fossil content==.

Index Fossils - are ==widespread geographically==, are ==limited to a short span of geologic time==, and ==occur in large numbers.==

Interpreting Environments - Fossils can also be ==used to interpret and describe ancient environments.==

Dating with Radioactivity

Basic Atomic Structures

◆ ==Orbiting the nucleus== are ^^electrons^^, which are ==negative electrical charges==.

◆ ^^Atomic number^^ is the ==number of protons== in the atom’s nucleus.

◆ ^^Mass number^^ is the ==number of protons plus the number of neutrons== in an atom’s nucleus.

Radioactivity - is the ==spontaneous decay of certain unstable atomic nuclei==.

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

- is the ==procedure of calculating the absolute ages of rocks and minerals== that ==contain radioactive isotopes==.

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

* As a radioactive isotope decays, ==atoms of the daughter product are formed== and accumulate.

* An ^^accurate radiometric date^^ can be ==obtained only if the mineral remained in a closed system== ==during the entire period since its formation.==

Dating with Carbon-14

Radiocarbon Dating - is the ==method for determining age== by ==comparing the amount== of ==carbon-14== to the amount of ==carbon-12== in a sample.

* When an organism dies, ==the amount of carbon-14 it contains gradually decreases as it decays==. By comparing the ratio of carbon-14 to carbon-12 in a sample, radiocarbon dates can be determined.

* Radiometric dating has supported the ideas of ==James Hutton==, ==Charles Darwin==, ==and others== who inferred that geologic time must be immense.

Evolution

{{Theme: Evolution drives diversity and unity of life{{

○ It refers to the ==cumulative change in a population of organism over time==

Principles of Geology ( Charles Lyell)

(1830-33)

- revived Hutton's principle of slow geological change, and presented a classic explanation of development over millions of years.

Natural Selection:

1. Each species show variation
2. There is competition within each species for food, living space, water, mates, etc.
3. The “better adapted” members of these species are more likely to survive “Survival of the fittest”
4. These survivors will pass on their better genes to their offspring who will also show this beneficial variation

Theory of Natural Selection:

• Descent w/ modification and adaption to the environment is the basis of natural selection
• Inherited better traits /adaptions, produce more offspring that reach reproductive age.

{{4 Main Points:{{

{{* Adaption{{

{{* Variation{{

{{* Overproduction{{

{{* Descent/ modification{{

The Geologic Time Scale

Structure of the Time Scale

Eons - represent the ==greatest expanses of time==. Eons are divided into ==eras==. Each era is subdivided into ==periods==. Finally, periods are divided into smaller units called ==epochs==.

◆ There are three eras within the ^^Phanerozoic eon^^: the ^^Paleozoic^^, which means ==“ancient life==,” the ^^Mesozoic^^, which means ==“middle life,”== and the ^^Cenozoic^^, which means ==“recent life.”==

◆ Each period within an era is characterized by ==somewhat less profound changes in life forms== as compared with the changes that occur during an era.

◆ The periods of the Cenozoic era are divided into still smaller units called ==epochs==, during which ==even less profound changes in life forms== occur.

Precambrian Time

- During Precambrian time, there were fewer life forms. ==These life forms are more difficult to identify== and the ==rocks have been disturbed== often.

Difficulties with the Geologic Time Scale

◆ A ^^sedimentary rock^^ ==may contain particles that contain radioactive isotopes==, but these particles are not the same age as the rock in which they occur.

◆ The age of a particular mineral in a ^^metamorphic rock^^ ==does not necessarily represent the time when the rock was first formed==. Instead, the ==date may indicate when the rock was metamorphosed==.

Precambrian History

- The Precambrian ==encompasses immense geological time==, from Earth’s distant beginnings 4.56 billion years ago until the start of the Cambrian period, over 4 billion years later.

Precambrian Rocks:

• ^^Shields^^ are large, relatively flat expanses of ancient metamorphic rock within the stable continental interior. • Much of what we know about ==Precambrian rocks comes from ores mined from shields==.

Earth’s Atmosphere Evolves

• Earth’s original atmosphere was made up of gases similar to those released in volcanic eruptions today—water vapor, carbon dioxide, nitrogen, and several trace gases, but no oxygen.

• Later, primary plants evolved that used photosynthesis and released oxygen.

• Oxygen began to accumulate in the atmosphere about 2.5 billion years ago.

Precambrian Fossils

Stromatolites - The most common fossils. They are ==distinctively layered mounds or columns of calcium carbonate==. They are ==not the remains of actual organisms== but are the ==material deposited by algae==.

* Many of these ancient fossils are preserved in ^^chert^^—==a hard dense chemical sedimentary rock==.

Early Paleozoic

Following the long Precambrian, the most recent ==540 million years== of Earth’s history are divided into three eras: ==Paleozoic, Mesozoic, and Cenozoic.==

Early Paleozoic History

• During the Cambrian, Ordovician, and Silurian periods, the vast southern continent of ^^Gondwana^^ encompassed five continents (South America, Africa, Australia, Antarctica, and part of Asia).

• Life in early Paleozoic time was restricted to the seas.

Late Paleozoic History

Laurasia - is the ==continental mass== that formed the northern portion of Pangaea, consisting of present-day North America and Eurasia.

* By the end of the Paleozoic, all the continents had fused into the supercontinent of ==Pangaea==.

• Some ==400 million years ago,== plants that had adapted to survive at the water’s edge began to move inland, becoming ==land plants==.

• The ^^amphibians^^ rapidly diversified because they had minimal competition from other land dwellers.

The Great Paleozoic Extinction

◆ The world’s ==climate became very seasonal==, probably ==causing the dramatic extinction of many species==.

◆ The late Paleozoic extinction was the ==greatest of at least five mass extinctions to occur over the past 500 million years.==

Mesozoic Era

◆ Dinosaurs were land-dwelling reptiles that thrived during the Mesozoic era.

Mesozoic History

• A major event of the Mesozoic era was the breakup of Pangaea.

Mesozoic Life

• ^^Gymnosperms^^ are ==seed-bearing plants that do not depend on free-standing water== for fertilization.

• The gymnosperms quickly became the ==dominant plants== of the Mesozoic era.

The Shelled Egg

• Unlike amphibians, ==reptiles have shell-covered eggs== that can be laid on the land.

• The ==elimination of a water-dwelling stage== (like the tadpole stage in frogs) was an i==mportant evolutionary step.==

Reptiles Dominate

• With the perfection of the shelled egg, ==reptiles quickly became the dominant land animals==.

• ==At the end of the Mesozoic era==, many ==reptile groups became extinct==.

Cenozoic North America

◆ The ^^Cenozoic era^^ is divided into two periods of very unequal duration, the ==Tertiary period== and the ==Quaternary period==.

◆ ^^Plate interactions^^ during the Cenozoic era caused many events of ==mountain building==, ==volcanism==, and ==earthquakes== in the West.

Cenozoic Life

Mammals - ==animals that bear live young== and maintain a steady body temperature— ==replaced reptiles as the dominant land animals== in the Cenozoic era.

Angiosperms - ==flowering plants with covered seeds==—replaced gymnosperms as the dominant land plants.

Mammals Replace Reptiles

• Adaptations like being warm blooded, developing insulating body hair, and having more efficient heart and lungs allow mammals to lead more active lives than reptiles.

Large Mammals and Extinction

• In North America, the mastodon and mammoth, both huge relatives of the elephant, ==became extinct==. In addition, saber-toothed cats, giant beavers, large ground sloths, horses, camels, giant bison, and others died out on the North American continent.

• The reason for this recent wave of extinctions puzzles scientists.