CEE 1030 1st Edition Lecture 7 Review Questions A is characterized by the segregation of light and dark colored minerals into think layers or bands Gneiss which of the following lists the rocks in order of increasing grain size and increasing grade of metamorphism 1 gneiss phyllite slate schist 2 slate phyllite schist gneiss 3 schist slate gneiss phyllite 4 gneiss schist phyllite slate 5 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 decay Principle 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 rocks 2 disconformity strata on either side of the unconformity are parallel 3 nonconformity metamorphic or igneous rocks in contact with sedimentary strata Geological 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 continuous over 10 s of km Fossils 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 rocks Principle 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 disconformity