Download Chapter 15 Notes

Chapter 15 Notes
15-1
James Hutton’s observations lead to the foundations of Modern Geology. 1788 he wrote
“Theory of the Earth”. The process we observe today erosion and deposition do not change
over time. They are the same today as they were millions of years ago (Uniformitarianism).
Uniformitarianism really states the same processes have been shaping the Earth much the
same as throughout all history, in other words very gradually. Catastrophism is the principle
that states all geologic change occur suddenly. Most scientist in Hutton’s time believed in
Catastrophism versus uniformitarianism. Charles Lydell 1830-1833 reintroduced Hutton’s
theories on uniformitarianism and finally put down Catastrophism. Steven Gould theory that
both uniformitarianism and catastrophism both are at work shaping the Earth geologic history.
Paleontology- scientist who study fossils and ancient life. Paleobotanist studies plant fossils.
Vertebrate Paleontologist studies animal fossils with backbones and Invertebrate Paleontologist
studies animal fossils without backbones.
15-2
A history of the Earth can be compiled from the layers of soil and it reads like a book. Geologist
rely on rocks and fossils to help them in studying what geologic events in the Earth’s history
occur and in what order they occurred. Relative Dating is determining whether an object or
event is older or younger than other objects or events in the surrounding area. Law of
Superposition states that in a series of sedimentary rock the younger rocks and sediment
normally lie on top of the older rock.
Geologic Column- an arrangement of rock layers in which the oldest rocks are on the bottom.
The column may be made from many rock sequences from layers of rock from around the
world. Nowhere in the world do all these rock sequences in the geologic column actually occur
as one column.
Four types of disturbed rocks layers
1. Fault- rock layers slide down, or up, or side to side due to Earth’s natural forces
2. Intrusions- molten rocks squeezes into cracks in the layers of rock
3. Folding- rock layers bend and buckle from the Earth’s internal forces of heat and
pressure.
4. Tilting- Internal Earth forces slant rock layers relative to each other
Faults
Intrusions
Folding
Tilting
Faults and intrusions cut across rock layers therefore they must be younger than the rock
layers. The rock layers had to be there first for the fault or intrusion to occur. Geologist
assume that layers of rock and sediment are laid down in horizontal layers and if the rock layers
are not horizontal something must have disturbed them after they were laid down. Folding and
tilting always are younger than the rock layers they are found in.
A missing rock layer(s) in a sequence of rocks is called an Unconformity. When geologist find an
unconformity they must decide whether the “missing layers ever existed or were they
somehow removed. Unconformities are generally created by erosion or non-deposition. There
are three major types of unconformities:
1. Disconformity- formed where part of a sequence of parallel rock layers is missing.
represents thousands to millions of years of missing time.
2. Nonconformity- horizontal sedimentary layers lie on top of eroded metamorphic or
igneous rock. Represents millions of years of missing time.
3. Angular Unconformity- found between horizontal layers of rocks that have been tilted
or folded. Represents millions of years of missing time.
Angular Unconformity
Index fossil- found in many locations on earth. Must come from the same time period and
lived for a short period of time. This helps determine other time periods accurately.
Absolute Dating- is the process of establishing the age of a object by determining the number of
years it has existed (place a numerical date on it). Radioactive dating is the most common
method of absolute dating. Isotopes are atoms of the same element that have the same
number of protons but different number of neutrons. Some isotopes are unstable or
radioactive and these try to break down to more stable elements and this is called radioactive
decay. Radioactive decay occurs at a very stable rate and can be used to predict the age of
objects. As the unstable isotope (parent material) breaks down or decays a stable isotope or
element is produced (daughter material). So scientist can compare the amount of parent
material to daughter material and determine the age of the object. The older the age the more
daughter material there will be. Radiometric dating is a ratio of parent material to daughter
material. Half life is the time that it takes for ½ of the radioactive sample to decay (parent
material becomes daughter material). Examples:
1 half life = 50% parent material and 50% daughter material.
2 half lives= 25% parent material and 75% daughter material.
3 half lives= 12.5% parent material and 87.5% daughter material.
4 half lives=6.25% parent material and 93.75% daughter material
There are 4 different radiometric dating methods; Potassium-Argon, Uranium-Lead, RubidiumStrontium, and Carbon-14.
Potassium-Argon- Potassium 40 has a half life of 1.3 billion years as it decays to Argon and
Calcium. Rocks must be at least 100, 000 years old to use this method effectively.
Uranium-Lead- Uranium 238 decays to Lead in a series of steps and has a half life of 4.5 billion
years. Rocks must be at least 10 million years old to use this method effectively.
Rubidium-Strontium- has a half life of 49 billion years. Rocks must be at least 10 million years
old for this method to be used effectively.
Carbon-14- Begins to decrease as soon as the plant or animal dies. Its half life is 5730 years. It
is used to date things within the past 50,000 years to be effective.
Fossils- the remains or physical evidence of an organism preserved by geological processes.
Found most often in sedimentary rock. Bones and shells most often form fossils and the
soft tissue normally does not form fossils. Six types of fossils are listed below:
1. Amber- hardened tree sap- mostly associated having insect fossils trapped in the sap.
2. Petrification- process in which minerals replace an organism’s tissues. Per
mineralization is the process where pores of an organism hard tissue (bones or wood
tissue) are filled up with minerals.
3. Asphalt- sticky tar like material that preserves an organism. La Brea tar pits in L.A., Cal.
4. Frozen- cold slows down decay (1999 Wooly Mammoth). Subfreezing climate contain
almost no decomposing bacteria.
5. Sedimentary rocks- fossils are mostly found in sedimentary rocks because igneous and
metamorphic rocks have too much heat or pressure for fossils to survive the fossil
making process.
6. Carbonized fossils- forms when organisms or parts like leaves , stems, flowers, fish are
pressed between layers of soft mud or clay that hardens squeezing almost all the
decaying organism away leaving the carbon imprint in the rock.
Trace Fossils- Are any naturally preserved evidence of animal activity, ie. footprint, burrows,
coprolite (fossilized animal dung) Mold- a cavity in rock where a plant or animal was buried
creating an indention. Cast- an object created when sediment fills in a mold and becomes rock.
Our fossil record is incomplete because:
1. Most organisms never became a fossil.
2. Many fossils are yet to be discovered.
3. Not all dead organisms become fossils.
4. Only a small fraction of the organisms in Earth’s history became fossils.
When we find fossils on mountain tops that are ocean or marine animals it suggests that uplift
(upward pushing up of the Earth’s surface because of volcanic or plate tectonic movement) has
occurred for these fossils to appear so high up. Tropical fossils in Antarctica suggests that
continental drift has occurred due to finding tropical fossils in the freezing cold of Antarctica.
Index Fossils- are fossils of organisms that lived a relatively short well defined geologic time
period. Must be found in rock layers throughout the world to also be considered an Index
fossil. Trilobites called Phacops are an index fossil that date back to 400 million years ago.
Trilobites are recognized to have lived since 540 milion years ago to the Permianian mass
extinction of 248 million years ago. Many different species of trilobites. Any time you see
Trilobites you are looking at really old life forms. Ammonites called Tropites is an index fossil
that lived 230 to 208 million years ago. Index fossils because of their well known life time dates
are used to determine relative ages of the rock or soil layers around them.
Mass Extinctions- many time periods are ended with a mass extinction of life. Extinctionmeans the death of every member of a species. There are five major mass extinctions most
paleontologists agree on. The one at the end of the Permian period was the worst of all time
(96%) and the one at the end of the Cretaceous period was the one that made dinosaurs go
extinct (caused by an asteroid in the Gulf of Mexico).
Geologic Time Scale- The earth is 4.6 billion years old. We can see about 2 billion years of time
in the walls of the Grand Canyon. Nearly ½ of the Earth’s history. The largest time period of
the geologic time scale is called Eons. There are four eons; Hadean, Archean, Proterozoic , and
Phanerozoic. The first three eons are known as the Precambrian Time where the Earth was
forming its surface, oceans, and atmosphere (7/8 of earth’s history). The Phanerozoic eon is
also known as the eon of “Visible Life” and we are still living in it today. The Phanerozoic Eon
begins with the Cambrian period. The next unit of geologic time is the Era. There are three
eras in geologic time; Paleozoic, Mesozoic, and Cenozoic. Paleozoic era is also known as “Old
Life’, the Mesozoic is also known as “The Age of Reptiles”, and the Cenozoic era is also known
as “The Age of Mammals”. There are twelve Periods in geologic time. They are smaller than an
era and larger than an Epoch. Cambrian period is the first period of the Paleozoic era and was
the first time life really became abundant on Earth. This time was called the “Cambrian
Explosion “ because so many types of life began to thrive. Most of our coal and oil comes from
the Carboniferous periods called Mississippian and Pennsylvanian times. The Permian period
was the last period of the Paleozoic Era and was the time of the largest mass extinction on
Earth. The Mesozoic era had the Triassic, Jurassic, and Cretaceous period. The Jurassic period
is known as the “Golden Age of Dinosaurs” and the Mesozoic ended with a mass extinction
killing off the dinosaurs by an asteroid in the Gulf of Mexico. The Cenozoic era begins with the
Tertiary period where mammals really began to expand on Earth. We are currently living in the
Quaternary period. Modern man appeared about 10,000 years ago. Epochs are the smallest
unit of geologic time. So man lives in the Phanerozoic eon, Cenozoic era, Quaternary period,
and the Holocene epoch. Man and dinosaurs never lived during the same time periods.
Kingdom, Phyla, Class, Order, Family, Genera, and Species are the units of Taxonomy in naming
different forms of life.
Ice cores are sections of ice that have been drilled out of ice formations that allow scientist to
see what types of gases were present at different times of history. Ice cores are used to find
out about atmospheric gases.
Reasons for global cooling are
1. ash from volcanic activity blocking out the sunlight
2. asteroid impact throwing dust and dirt into the atmosphere blocking the sunlight
3. Ice age where glaciation of the water and atmospheric water vapor cooled causing the
Earth’s surface to become covered with ice sheets.
Pangaea is the single large continent of 260 million years ago. Panthalassa is the ingle large
ocean that surrounded Pangaea.
Fossils are found in sedimentary rocks formed in layers. Metamorphic is heat and pressure
formed. Igneous is formed from magma. Intrusive stays underground and extrusive forms
above the surface