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 1700S)) 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!!!!! 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 imprintdinosaur footprintrecrystallized 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