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? 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?” “ORIGINAL HORIZONTALITY” 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... 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). Click each Law for a link to an example. INTRUDED takes you to the definition.

Superposition- youngest to oldest Click anywhere in picture to skip back to LAWS.

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

Cross Cutting CROSS-CUTTING Click to skip back to LAWS.

Igneous Intrusion - Cross Cutting

Folds/Tilts

FOLDS - TILTS

Included Fragments

Included Fragments

What is the correct order of development from oldest to youngest?

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

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

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

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Sediments deposited, sea-level lowered, layers intruded, layers tilted, erosion and deposition under sea, sea-level lowered again. Click arrow to continue.

Practice: what happened here? Click to see arrows shoot in one at a time. #1= included fragments #2=intrusion, cross-cutting #3= angular unconformity #4=erosion. This is the outcome of the previous slide. Unconformities are usually shown in block diagrams with a wavy line to represent the buried eroded surface interface between layers

The trip route stays very close to the edge of the Catskill Mountains.

Taconic Unconformity Upper Silurian Carbonates Tilted Ordovician Shales and Sandstones We stopped along the side of the road where engineers had cut into the rock formations to make the road. The unconformity was clearly visible showing banded carbonate rock overlying tilted shales and sandstones. unconformity Taconic 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“ Fossils succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content

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1. Which layers are the same? 2. Which layer is older E or F? 3. What is correct sequence of rock layers from oldest to youngest? 4. An unconformity is represented by the interface between which 2 layers?

Which fossil might be found in Devonian rock layers?

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

Which letter would make a good index fossil? Location A Location B Location C Rock layer 1 W W Z Rock layer 2 W Z Y Z Rock layer 3 W 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