Which graph shows the relative duration of geologic time for the Precambrian, Paleozoic, Mesozoic, and Cenozoic time intervals?
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?” In the 17 th 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... SUPERPOSITION: a sedimentary sequence will be OLDEST on BOTTOM (if undisturbed – flat and level).SUPERPOSITION CROSS-CUTTING:CROSS-CUTTING a. Igneous intrusion is younger than rock it has intruded (cut across). Pre-existing rock will undergo CONTACT METAMORPHISM intruded 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).INCLUDED FRAGMENTS
Superposition- Superposition- youngest to oldest
GRAND CANYON- LAW OF SUPERPOSITION YOUNGEST ON TOP OLDEST ON BOTTOM
What is the geologic age of layer B? (1) Cambrian (3) Devonian (2) Ordovician (4) Permian
By the way… How can you tell if folding has gone so far as to flip the layers over?
Sequence 4: Igneous Intrusion 1.Limestone deposited 2.Sandstone deposited 3.Shale deposited 4.Igneous Intrusion Note: contact metamorphism is the same event as the intrusion.
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
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 1.Uplift – area of crust uplifted above sea level (deposition – under water) 2.Erosion – some time after 3.Submergence (subsidence) below sea level 4.Deposition – new sediments deposited on top of the buried eroded surface
1.Layers formed according to superposition 2.Something happens to uplift area 3.Erosion wears away the uppermost layers 4.Area submerges and deposition begins again AT THE INTERFACE BETWEEN STEPS 3 AND 4 THERE IS A BURIED EROSIONAL SURFACE AKA AN UNCONFORMITY
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 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“succession Fossils succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content
Eurypterus NY State Fossil Silurian index fossil
Index fossils – 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
Location ALocation BLocation C Rock layer 1WWW Z Rock layer 2W Z Y Z Rock layer 3W X X X Z Which letter would make a good index fossil?
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
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