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The Geologist as Detective
THE PRINCIPLES USED IN DETERMINING THE ORDER IN WHICH ROCKS WERE FORMED
The idea that processes that shape the Earth, such as volcanoes, earthquakes, deposition and erosion, are the same processes that occurred in the geologic past
THE PRINCIPLE IS BASED ON THE FACT THAT SEDIMENT IS USUALLY DEPOSITED IN HORIZONTAL LAYERS
IF SEDIMENTARY ROCKS LIE AT AN ANGLE THEN SOME TECTONIC FORCES MUST HAVE ACTED ON THE ROCK
THAT SEDIMENTARY ROCKS ARE OLDEST ON THE BOTTOM AND YOUNGEST ON TOP
THIS IS TRUE AS LONG AS NO TECTONIC FORCES HAVE TURNED THE LAYERS UPSIDE DOWN OR ON THEIR SIDE
ROCK MUST FIRST EXIST BEFORE ANYTHING CAN HAPPEN TO IT
THEREFORE AN INTRUSION IS YOUNGER THAN THE ROCK IT HAS ENTERED
INTRUSION: WHEN MAGMA SQUEEZES INTO OR BETWEEN LAYERS OF PRE-EXISTING ROCK
EXTRUSION: WHEN MOLTEN ROCK FLOWS ONTO THE EARTH’S SURFACE, THEN COOLS
INCLUSION: OLDER ROCKS SURROUNDED BY MOLTEN MAGMA
ONE PIECE OF ROCK FOUND IN ANOTHER ROCK MUST BE OLDER THAN THE ROCK IT IS FOUND IN
A PEBBLE FOUND IN CONGLOMERATE IS OLDER THAN THE CONGLOMERATE
WHERE LAYERS OF ROCK ARE MISSING IF THE SURFACE OF THE ROCK WAS
ERODED SOME TIME IN ITS HISTORY UNCONFORMITIES ARE GAPS IN
GEOLOGICAL TIME
FOLDS: BENDS IN ROCK LAYERS PRODUCED BY CRUSTAL PLATE MOVEMENTS
FAULTS: BREAKS IN ROCK WHERE MOVEMENT HAS OCCURRED
FOLDING OR FAULTING CAN LEAD TO EXCEPTIONS TO THE LAW OF SUPERPOSITION
FOSSILS: PRESEVED REMAINS OR TRACES OF LIVING THINGS
RELATIVE AGE: COMPARATIVE AGE; AGE EXPRESSED AS BEFORE OR AFTER OTHER EVENTS WITHOUT SPECIFYING THE AGE IN UNITS
ABSOLUTE AGE:AGE OF A ROCK UNIT, FOSSIL OR EVENT EXPRESSED IN UNITS OF TIME, SUCH AS YEARS
Casts and Molds- shapes/outlines left behind; these are COPIES of originals.
Petrifaction- mineral replacement of organic material
Amber and Ice- organisms “frozen” in time
Trace Fossils- footprints, fecal pellets, burrows
Impressions- 2-Dimensional leftovers
NYS HAS FOSSILS OF CORAL AND WARM-WATER MARINE ORGANISMS, INDICATING THAT THE AREA WAS ONCE A TROPICAL SEA
IT ALSO HAS MORE RECENT FOSSILS OF WOOLY MAMMOTHS AND MASTODONTS, REVEALING THE COLD, SNOWY CLIMATE OF THE LAST ICE AGE
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