ch23 introduction to earth science

• If it is horizontal, we can use this to study organisms that lived on the Earth during the same periods of time... – Shifting sites of erosion and deposition • As sedimentary rock is e

•Geological Time

• The Painted Desert, Arizona This landscape has a story to tell, and each individual rock and even the colors mean something about the past.

• Fossils

• Early Ideas about Fossils

– Herodotus

• realized that fossil shells found far from oceans were remnants of an ancient sea.

– Aristotle

• Believed in Spontaneous Generation

• Thought that fossils grew in place in rocks.

– Leonardo da Vinci

• Argued that fossils were remnants of living organisms from the Earths past history.

– William “Strata” Smith

• Discovered a relationship among the strata from different areas.

• Found that each strata has a distinct group of fossils associated with it and that these fossils were different from the fossils found in other strata.

• Types of Fossilization

– Preservation or alteration of hard parts

• This is the actual preservation of the hard parts themselves

• Could include shells, bones, exoskeletons, etc…

– Preservation of the shape

• The fossil record of the hard parts is beautifully preserved, along with a carbon film, showing a detailed outline of the fish and some of its internal structure.

• These logs of petrified wood are in the Petrified Forest National Park, Arizona.

• Reading Rocks

• Arranging Events in Order

– Principle of uniformity

• Stated that the present is the key to the past

• Look at geological features of today’s Earth as being shaped by its past

• States that the processes that are going on today to shape the Earth are the same processes that have always been going on

– Principal of original horizontality

• Based on the premise that sedimentary rock is

deposited in flat-lying layers

• This means that if a sedimentary layer is not

horizontal, it has been subjected to forces that have moved or changed it

• If it is horizontal, we can use this to study organisms that lived on the Earth during the same periods of

time

• The principle of

original

horizontality (A) Sediments tend

to be deposited in horizontal layers (B) Even where

the sediments are draped over an

irregular surface, they tend toward the horizontal

– Principal of superposition

• As sedimentary layers are laid down they are

compacted and cemented together, which keeps the organisms in that layer together

• The principle of superposition In an undisturbed

sedimentary sequence, the rocks on the bottom were

deposited first, and the depositional ages decrease as you progress to the top of the pile

• The Grand Canyon, Arizona, provides a majestic cross

section of horizontal sedimentary rocks According to the principle of superposition, traveling deeper and deeper into the Grand Canyon means that you are moving into older and older rocks

– Principal of crosscutting relationships

• Any igneous or metamorphic rock that cuts across a sedimentary strata must be younger than the layer it crosscuts

• This is helpful in tracking events in the Earth’s

history

– Shifting sites of erosion and deposition

• As sedimentary rock is eroded and moved to another place, there will always be a time lapse between the shifting and the compacting and cementing

• A granite intrusion cutting across older rocks.

– Unconformity

• States that as there is a time lapse between

sedimentation and compacting and cementing there

will be a surface break within the sedimentary

• Angular unconformity Development involved some deformation and erosion before

sedimentation is

resumed.

• A time break in the rock record in the Grand Canyon,

Arizona The horizontal sedimentary rock layers overlie

almost vertically foliated metamorphic rocks Metamorphic rocks form deep in the earth, so they must have been

uplifted, and the overlying

• Correlation

– Principal of faunal succession

• Recognizes that life has changed throughout time

• As one group of organisms disappears from the fossil record it is replaced with new ones

• The same form is never exactly duplicated

• This allows fossils to be put in chronological order based on their appearance in the strata and the fossil record

– Index Fossils

• A fossil that occurred widely, but for a very brief time period

– Correlation

• The use of index fossils and the other methods of

reading fossils to make it possible to compare rocks that have been exposed in two locations

– Relative dating

• Fossil dating using these methods is called relative

dating

• It is a way of dating fossils and geological events

relative to other fossils and geological events

• Deciphering a complex rock

sequence The limestones must be oldest (law of superposition),

followed by the shales The

granite and basalt must both be younger than the limestone they crosscut (note the metamorphosed zone around the granite) It is not possible to tell whether the

igneous rocks predate or postdate the shales or to determine

whether the sedimentary rocks

were tilted before or after the

igneous rocks were emplaced

After the limestones and shales were tilted, they were eroded, and then the sandstones were

deposited on top Finally, the lava flow covered the entire sequence.

• Similarity of fossils suggests similarity of ages, even

in different rocks widely separated in space.

• This dinosaur footprint is in shale near Tuba City, Arizona

It tells you something about the relative age of the shale,

since it must have been soft mud when the dinosaur stepped here

• (A)Fossil

trilobites from Cambrian

• Geological Time

• Early Attempts at Earth Dating

– Introduction

generations of people mentioned in the Bible and decided that the Earth was created at 9:00 AM on Tuesday, October 26 4004 BC

that the Earth must be much older

• People began to look for a way of dating the Earth using some sort of Geological Clock

– Geological Clocks

• Process must have been present since the Earth was first created

• The process must occur at a uniform rate

• The process must be able to be measured

– Clocks considered

• Rate of salt addition to the ocean

– Resulted in an age for the Earth of 100 million years

• Rate of sediment deposition

– Resulted in an age for the Earth of 20 to 1,500 million Years

• Rate of cooling of the Earth

– Resulted in an age for the Earth of 20 to 40 million

– Geomagnetic time scale

• Looks at the duration and number of magnetic field reversals occurring during the past 6 million years

• Used in combination with Radiometric dating

• The Geological Time Scale

– Time of middle life

– Some fossils resemble modern day living organisms

– Some are different from any living organism on the Earth today

• Paleozoic

– Time of ancient life

– Fossils are different from anything found on the Earth today

• Precambrian

– Time before the time of ancient life

– Very few fossils are found in this time period

• Geologic history is divided into four main eras The

Precambrian era was first, lasting the first 4 billion years, or about 85 percent of the total 4.6 billion years of geologic

time The Paleozoic lasted about 10 percent of geologic

time, the Mesozoic about 4 percent, and the Cenozoic only about 1.5 percent of all geologic time

• The divisions of the geologic

time scale.

• The periods of the

Paleozoic era, which refers to the time of ancient life Ancient life means that the

fossils for this time period are very