CEE 1030 1st Edition Lecture 7Review QuestionsA______ is characterized by the segregation of light- and dark-colored minerals into think layers or bandsGneisswhich of the following lists the rocks in order of increasing grain size and increasing grade of metamorphism?1. gneiss, phyllite, slate, schist2. slate, phyllite, schist, gneiss3. schist, slate, gneiss, phyllite4. gneiss, schist, phyllite, slate5. phyllite, slate, gneiss, schist Geologic time scale- “calendar” of Earth history - divides geologic history into units - originally created using relative ages  eon- greatest expanse of time Phanerozoic- (“visible life”): the rocks you find fossils in, the most recent eon began about 540 million years ago Proterozoic (2500-540 million years)Archean (3800- 2500 millions years)  Hadean- the oldest eon (4500- 3800 myrs)- structure of the geologic time scale:  era- subdivision of an eon eras of the Phanerozoic eon Cenozoic- (“recent life”) Mesozoic- (“middle life”) Paleozoic- (“old life”)eras are subdivided into – periodsperiods are subdivided into –epochs Geological history of outcrops- observations and logical reasoning- can apply a series of geological principles two ways to date geologic events relative dating- determine whether a rock is older or younger than surrounding rocksfield relationships, fossils absolute dating- determine actual number of years since rock was formed radioactive decayPrinciple of uniformitarianism  James Hutton (1785)“the present is the key to the past”- looking at modern processes allows us to interpret the geological past as preserved in the record of rocks Principle of superposition  Nicolaus Steno (1669) “in an undeformed sequence of layered rocks, the oldest rocks are on the bottom.” Principle of original horizontality- Nicolas Steno (1669) layered strata are generally deposited in a nearly horizontal position- If rock layers are flat, it means they have not been disturbed- Applies to both sedimentary and most volcanic rocks ex. rock strata in grand canyon still have their “original horizontality” Principle of cross- cutting relationships- younger features cut across older features: ex. igneous intrusions and faults \Principle of inclusions- a piece of rock that is enclosed within another rock inclusions- always older than the host rock - sedimentary rock inclusions in granite - granite pebbles in conglomerate sedimentary rock Unconformities- a break in the rock record produced by erosion and/or nondeposition of rock units three types of uncomformity: 1. angular unconformity: tilted rocks are overlain by flat- lying rocks2. disconformity: strata on either side of the unconformity are parallel 3. nonconformity: metamorphic or igneous rocks in contact with sedimentary strataGeological history of a region - principles of superposition, inclusions, cross-cutting relationships, and original horizontality fine for single location or outcrop of rock. But what about across a wide region?Principle of lateral continuity- sedimentary rock layers are often laterally continuousover 10’s of kmFossils and correlation-matching of rocks of similar ages in different regions is known as correlation-correlation often relies upon fossils -fossil= traces or remains of prehistoric life now preserved in sedimentary rocksPrinciple of fossil succession William Smith (1785) noted that sedimentary strata in widely separated areas could be identified and correlated by their distinctive fossil content - different types of fossils always appear and disappear in the same order within the geologic rock record any time period can be recognized by its fossil content  index fossil- geographically widespread fossil limited to a short span of geologic time Fossils and disconformities - fossils can be used to recognize a break in the sedimentary record