Earth History- Table of Contents RELATIVE vs. ABSOLUTE LAWS UNCONFORMITIES GEOLOGIC SECTIONS

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”

First Things First…or… “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).SUPERPOSITION CROSS-CUTTING: an igneous intrusion is younger than rock it has intruded (cut across).CROSS-CUTTINGintruded INCLUDED FRAGMENTS: pieces of rock found IN another rock must be OLDER (formed first).INCLUDED FRAGMENTS FOLDS/TILTS: younger than rocks themselves

Superposition- Superposition- youngest to oldest

Grand Canyon

IGNEOUS INTRUSION: Occurs when magma squeezes into or between layers of pre- existing rock. Surrounding pre-existing rocks will undergo contact metamorphism

Cross Cutting

Igneous Intrusion - Cross Cutting

Included Fragments

Folds/Tilts

Folds/Tilted

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

Upper Silurian Carbonates Tilted Ordovician Shales and Sandstones Taconic Unconformity unconformity

Mr. Orgonik pointing out the Taconic Unconformity

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

Practice: what happened here?

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

Index fossils 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

Eurypterus NY State Fossil Silurian index fossil

OTHER METHODS OF CORRELATION Layers of bedrock exposed (outcrops) on either sides of river valleys/excavations “walking the outcrop” Rock similarities Volcanic ash – large eruption – widely distributed – represents a small time interval