greenspun.com : LUSENET : Introductory Geology, Oswego State : One Thread

what is the definition of conchidal and subconchoidal fracture. Also what is a conch and what are their significance. More one ? with the igneous rock composition do both the fine and the coarse rocks contain the same type ingredients. For example do both granite and rhyolite contain quartz, feldspar, and they have fewer mafics.

-- Tanya (flowers@oswego.edu), February 15, 2001


Subconchoial is a type of fractue with a curved breakage surface. Conchoidal fractures have a curved breakage surface with faint concentric striations on it. Concentric striations look like curved lines. A conch is those big shells that you can "hear the ocean" with, that is where the word conchoidal comes from. The only difference between granite and rhyolite is that granite is an phaneritic igneous rock and rhyolite is a aphanitic igneous rock. In phaneritic igenous rocks, the magma cool slowly, so the crystals have time to grow and thats why these rocks are coarse grained. In aphanitic igenous rocks the magma cools very fast, so the crystals do not have time to grow. That is why they are fine grained.

-- Hannah VanDam (petunia130@hotmail.com), February 16, 2001.

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