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Earth History - Table of Contents. RELATIVE vs. ABSOLUTE LAWS UNCONFORMITIES GEOLOGIC SECTIONS. Which graph shows the relative duration of geologic time for the Precambrian, Paleozoic, Mesozoic, and Cenozoic time intervals?. Who’s got the TIME?.

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earth history table of contents
Earth History- Table of Contents
  • RELATIVE vs. ABSOLUTE
  • LAWS
  • UNCONFORMITIES
  • GEOLOGIC SECTIONS
slide4

Which graph shows the relative duration of geologic time for the Precambrian, Paleozoic, Mesozoic, and Cenozoic time intervals?

who s got the time
Who’s got the TIME?
  • RELATIVE: order/sequence known, but not the actual date of occurrence. “Time Line”
  • ABSOLUTE: actual date determined by radioactive decay

“Clocks in Rocks”

how d that get there
“How’d that get there?”
  • In the 17th C., Nicolas Steno made an important observation:

"Sediments are usually deposited in horizontal layers."

He called this

“ORIGINAL HORIZONTALITY”

finding relative time the law of
Finding Relative TimeThe LAW of...
  • SUPERPOSITION:a sedimentary sequence will be OLDEST on BOTTOM (if undisturbed – flat and level).
  • CROSS-CUTTING:

a. Igneous intrusion is younger than rock it has intruded(cut across). Pre-existing rock will undergo CONTACT METAMORPHISM

b. Faults – younger than rock displaced

  • FOLDS/TILTS: younger than rocks themselves
  • INCLUDED FRAGMENTS:pieces of rock found IN another rock must be OLDER (formed first).
slide9

GRAND CANYON- LAW OF SUPERPOSITION

YOUNGEST ON TOP

OLDEST ON BOTTOM

slide10

What is the geologic age of layer B?

(1) Cambrian (3) Devonian

(2) Ordovician (4) Permian

unconformities buried eroded surfaces
Unconformities – Buried Eroded Surfaces
  • Sometimes layers of rock are missing
  • There is a break or gap of geologic time not represented by the layers in an area. The gap represents an unknown length of time
  • No way of knowing exactly what happened but we do know UPLIFT exposed rocks to weathering and erosion.
  • Rocks above unconformity are younger – rocks below older
slide23

Bedrock outcrops A and B are located at two different locations along the Genesee River in western New York State. Rock layers 1, 2, and 4 are the same in both outcrops.

Which statement best explains why rock layer 3 is missing from outcrop B?

(1) A fault exists between outcrops A and B.

(2) Erosion created an unconformity between rock layers 2 and 4 in outcrop B.

(3) A volcanic eruption destroyed rock layer 3 in outcrop B.

(4) Metamorphism of outcrop A created rock layer 3.

4 steps produce an unconformity
4 steps produce an unconformity
  • Uplift – area of crust uplifted above sea level (deposition – under water)
  • Erosion – some time after
  • Submergence (subsidence) below sea level
  • Deposition – new sediments deposited on top of the buried eroded surface
slide25

Layers formed according to superposition

  • Something happens to uplift area
  • Erosion wears away the uppermost layers
  • Area submerges and deposition begins again
  • AT THE INTERFACE BETWEEN STEPS 3 AND 4 THERE IS A BURIED EROSIONAL SURFACE AKA AN UNCONFORMITY
practice what happened here
Practice: what happened here?

Unconformities are usually shown in block diagrams with a wavy line to represent the buried eroded surface interface between layers

slide31

Upper SilurianCarbonates

Tilted Ordovician Shales and Sandstones

unconformity

Taconic Unconformity

applying principles of relative dating to determine geologic history of an area
Applying Principles of Relative Dating to Determine Geologic History of an Area
  • The process of matching rocks or geologic events occurring at different locations of the same age is called

CORRELATION

correlation of rock layers often relies upon fossils
Correlation of rock layers often relies upon fossils
  • William Smith (late l700’s) noted that rock layers in widely separated areas could be identified and correlated by their distinctive fossil content
  • This led to the "principle of fossil succession“
  • Fossils succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content
slide43
Eurypterus

NY State Fossil

Silurian index

fossil

index fossil s page 8 9 esrt
Indexfossils – Page 8-9 ESRT

Index fossils - any animal or plant that is characteristic of a particular span of geologic time or environment.

2 criteria must be met

  • Life form lived over a wide geographic area – horizontal distribution
  • Life form existed for a short period of time – short vertical distribution
other methods of correlation
OTHER METHODS OF CORRELATION
  • Layers of bedrock exposed (outcrops) on either sides of river valleys/excavations

“walking the outcrop”

  • Volcanic ash – large eruption – widely distributed – represents a small time interval

LIKE AN INDEX FOSSIL

slide47

VOLCANIC ASH LAYER AT THE SAME TIME AS THE DINOSAURS BECAME EXTINCT LEAD SCIENTISTS TO LOOK FOR METEORITE CRATER Chicxulub Crater 65 mya 170 km. Wide