MadSci Network: Earth Sciences

Re: how do you differenciate between Sedimentary, Metamorphic, and Igneous Rock

Date: Mon Nov 22 16:45:31 1999
Posted By: Dave Clark, Staff, Chemical and Environmental Technologies, Battelle
Area of science: Earth Sciences
ID: 942867734.Es

Tough question that really does not have a simple answer.  In real life, 
experience and training are the best tools for recognizing what kind of 
rock you have.  After working with different kinds of rocks, you just get 
to "know" what kind they are; after seeing and handling many different 
kinds of limestone, for example, you get to be able to recognize another 
piece of limestone when you see it.  Some information comes from the 
provinence of the rock, that is, where did it come from.  If a rock came 
from the side of a volcano, it would probably be igneous, for example.

Sometimes, there are internal clues.  For example, fossils indicate 
sedimentary rocks (or sometimes metamorphic rocks that come from 
sedimentary rocks) because igneous processes are so hot they burn up the 
material to be fossilized.  Usually, rocks with parellel planes of rock are 

Sometimes microscopic examination of a rock can tell something about its 
origins, for example, a rock made of fine crystals could be igneous 
(granite) or metamorphic (gneiss) or sedimentary (conglomerate or 
sandstone).  The crystals in granite would be jagged at the edges, the 
crystals in gneiss would be squished and deformed by pressure and heat, and 
the crystals in sandstone would likely be rounded on the edges by erosion 
before the sand was deposited.

Rocks are composed of minerals, of course.  Trained rock-hounds (or 
petrologists) learn to recognized the important minerals that make up 
rocks.  Then, by knowing where and how different minerals are made, they 
can often tell much about the rock that contains them.

There are other, more complicated chemical tests that can be done but they 
are not usually available when you first look at a rock.

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