Arizona State University
Geologic Maps
Importance of a Geologic Map
Most important type of geologic data for
Exploration for mineral and energy resources (certain mineral deposits, like copper, are associated with certain type of rocks)
Locating water
Geologic hazards (can evaluate potential for earthquakes by looking for the locations of recent faults that cut young rocks)
Suitability for building sites (shows flood plains versus bedrock, clay versus granite)
Understanding geologic history of area
Understanding the subsurface extent and geometry of rock units, such as an aquifer
What is a Geologic Map
Graphic representation (usually on paper) of the rock units and geologic features exposed on the surface
Map show outlines of areas where each type or age of rock is exposed
Contact (boundary) between two adjacent rock units is shown by a thick line if it is a fault and a thin line for any other type of contact (intrusive, conformable, unconformity, etc.)
Map also shows geologic features such as volcanic vents
Relative ages of rock units are shown on legend and usually can be determined from map
Can show a large area (North America) or a small area (such as Papago Park)
Made by geologist hiking/climbing through area and transferring what he/she sees onto a topographic base map or aerial photograph
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Geologists make geologic maps and reconstruct geologic history by making observations about
Characteristics of the rocks and how the rock was formed (red conglomerate with rounded cobbles of granite; deposited by river flowing to the west)
Contacts between rock units (whether a fault, intrusive, etc.)
Relations that show relative ages of rock units (the red conglomerate overlies and contains pieces of the granite)
Orientation (strike and dip) of bedding, layering
Position and direction of structures (faults and folds)
Age of rock by looking for fossils or by collecting samples for radiometric dating
Evidence for the kinds of geologic processes that may have occurred in the area
How geology is portrayed and interpreted depends on skills of geologist, how much time is available for the mapping project, and what geologic concepts the geologist can use to understand the area (as new geologic concepts are discovered, the interpretation of an area may change)
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