GEOLOGICAL TIME GEOLOGY NEEDS A TIME SCALE An

GEOLOGICAL TIME

GEOLOGY NEEDS A TIME SCALE • An investigation of the history of Earth • Understanding how features of landscape developed and formed • Reconstructing the history of Earth

UNIFORMITARIANISM (HUTTON (LATE 1700 S)) • Processes that formed past landscapes are the same processes that exist on Earth today • These processes are extremely slow • “The present is the key to the past” • In order for uniformitarianism to occur, Earth must be at least millions of years old

RELATIVE AGE DATING • Describes if layers are older or younger than one another • • Not an exact age • • A way of reconstructing the geological history of an area

LAWS OF RELATIVE AGE DATING • 1. Superposition • 2. Original Horizontality • 3. Cross-cutting relationships • 4. Inclusions

LAW OF SUPERPOSITION • sedimentary layers on the bottom are the oldest • sedimentary layers on top are the youngest • occurs in undisturbed sequence of rock layers only

2. ORIGINAL HORIZONTALITY • Sedimentary layers are deposited horizontally • If tilted, or non-horizontal layers are observed, some type of deformation has occurred to tilt the layers

3. CROSS-CUTTING RELATIONSHIPS • intrusions (dikes, sills, batholiths) & faults are younger than the layers they cut through • Folded gneiss intruded by felsic dikes

FAULTING • If faulting occurs after layers are deposited, the fault is younger than the layers it displaces.

THE PRINCIPLE OF CROSS CUTTING RELATIONSHIPS

4. INCLUSIONS • an inclusion is a broken-off piece of rock found within another rock. • The included rock is surrounded by younger material. • The rock that the inclusions come from is older.

SEQUENCING EVENTS IN ROCKS: TERMS TO KNOW AND USE • Unconformity – a break or gap in the geological rock record. • emergence (uplift) • submergence (sinking) • deposition • tilting • faulting • intrusion/extrusion • melting/solidification

IN CONCLUSION • Keep the Laws of Relative Age Dating in mind when deciding the order of events in rock formation!!!

LET’S TRY IT!!!!!

UNCONFORMITIES CONFORMABLE STRATA ARE LAYERS OF ROCK (STRATA) THAT SHOW A COMPLETE GEOLOGICAL HISTORY

UNCONFORMITIES • Gap in the rock record • Missing layers • Cause is either non-deposition or erosion 3 Types: 1. Non-conformity 2. Angular Unconformity 3. Disconformity

1. NON-CONFORMITY • Boundary between sandstone and granite layers • Igneous or metamorphic rock layer overlain by sedimentary rocks • Represents an unknown period of time during which the granite eroded.

2. ANGULAR UNCONFORMITY • Boundary between tilted and horizontal layers • Tilted (folded) layers overlain by flat layers

3. DISCONFORMITY • Noticed as a change in sedimentary environment. • Gap between layers of sedimentary rock

CORRELATION What does ‘To Correlate’ mean? • Connect in a systematic way, match-up How do we correlate rocks? • Walking the outcrop • Looking for time markers – volcanic ash • Matching up index fossils

PRINCIPLE OF FOSSIL SUCCESSION (STATES…) • “Fossil organisms succeed each other in a definite determinable order” • Any time period can be recognized by its fossil content fish imprint dinosaur footprint recrystallized shells

WHAT IS AN INDEX FOSSIL? • Widespread geographically • Limited to a short span of geologic time • Index fossils can be found in ESRT pages 8 & 9 letters A to Z.

BIAS OF THE FOSSIL RECORD: • The fossil record typically shows records of organisms with: a. Hard parts b. most of the fossil record is made up of marine organisms • Ex. Limestone with clam shells = region of shallow seas