Download Interpreting Earth`s History

The Geologist as Detective
 THE
PRINCIPLES USED
IN DETERMINING THE
ORDER IN WHICH
ROCKS WERE FORMED
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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
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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
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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
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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
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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
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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
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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
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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
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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
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NYS HAS FOSSILS OF CORAL AND WARMWATER 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