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Chapter 19

Chapter 19.doc

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Environmental Science
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Nick Eyles

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Chapter 19 Time and Geology  James Hutton is regarded as the father of modern geology  He has a theory called uniformitarianism – states that geologic processes operating at present are the same processes that operated in the past o “the present is the key to the past” o this term is a bit unfortunate, because it suggests that changes take place at a uniform rate o in some countries, actualism is used instead of uniformitarianism  this term comes closer to conveying Hutton’s principle that the same processes and natural laws that operated in the past are those that we can actually observe or infer from observation as operating at present  it is based on the assumption that physical laws are independent of time and location  dating based on radioactivity allows us to determine a rock’s numerical age, also called absolute age  relative time is the sequence in which events took place Principles Used to Determine Relative Age  contacts are the surfaces separating two different rock types or rocks of different ages  formations are bodies of rocks of considerable thickness with recognizable characteristics that make each distinguishable from adjacent rock units o named after local geographic features  to determine the relationship of geologic events to one another, four principles are applied: o original horizontality  states that beds of sediment deposited in water formed as horizontal or nearly horizontal layers o superposition  states that within a sequence of undisturbed sedimentary or volcanic rocks the layers get younger going from bottom to top  if sedimentary rock is formed by sediment settling onto the sea floor, then the first layer must be there before the next layer can be deposited on top of it o lateral continuity  states that an original sedimentary layer extends laterally until it tapers or thins at its edges  this is what we expect that the edges of a depositional environment, or where one type of sediment interfingers laterally with another type of sediment as environments change o cross-cutting relationships  states that a disrupted pattern is older than the cause of disruption The Grand Canyon  regional metamorphism took place resulting in the schists of the lower part of the canyon  erosion followed and levelled the land surface  sedimentation followed  these sedimentary layers were subsequently tilted  once again, erosion took place  the lowermost of the currently horizontal layers of sedimentary rock was deposited  each of the layers progressively higher up the sequence formed  finally, the stream eroded its way through the rock, carving the Grand Canyon The principle of inclusion states that fragments included in a host rock are older than the host rock Unconformities  a surface that represents a gap in the geologic record, with the rock unit immediately above the contact being considerably younger than the rock beneath  most unconformities are buried erosion surfaces  classified into three types: o disconformity  the contact representing missing rock strata separates beds that are parallel to one another  the hardest type of unconformity to detect in the field  usually the disconformity can be detected only by studying fossils from the beds in a sequence of sedimentary rocks o angular unconformity  a contact in which younger strata overlie an erosion surface on tilted or folded layered rock  it implies the sequence of events from oldest to youngest o nonconformity  a contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock  generally indicated deep or long-continued erosion before subsequent burial, because metamorphic or plutonic rocks form at considerable depths in the earth’s crust How can rock units be traced from one area to another?  correlation usually means determining time equivalency of rock units  physical continuity – being able to trace physically the course of a rock unit o one way to correlate rocks between two different places  the Grand Canyon is an ideal location for correlating rock units by physical continuity  correlation between two regions can be made by assuming that similar rock types in two regions formed at the same time  cross-bedding indicates that both types of rocks were once a series of sand dunes  correlation by similarity of rock types is more reliable if a very unusual sequence of rocks is involved  fossils are common in sedimentary rock, and their presence is important for correlation  principle of faunal succession: fossil species succeed one another in a definite and recognizable order  historical geology is concerned with the study of life and changes in life forms throughout geologic time as well as with the physical evolution of the earth o palaeontologists are specialists in the study of fossils  they have identified thousands of species of fossils and determined which plant and animal species lived during which subdivision of geologic time  they have also been able to describe the physical environment in which the fossil species lived  index fossil: a fossil from a very short-lived geographically widespread species known to exist during a specific period of geologic time
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