The Virtual Rock Kit -

Rock 5: Lithic Metatuff

This rock is composed of clay- to sand-sized particles of volcanic ash and angular pieces of pebble-sized rocks.  A rock composed of ash and pieces of other rock is called a lithic tuff.  This rock has been metamorphosed and is called a lithic metatuff.  The black colored pieces are small pieces of rock identical to rock sample #4 (the metadacite lava).  A close examination of the black rock pieces indicate that they have small white specks that are plagioclase phenocrysts (crystals of the mineral plagioclase).  The angular nature of the black rock pieces within the rock indicate that the original lava flow was violently blasted into millions of pieces of rock and ash by a volcanic eruption.

Lithic metatuff sample with angular pieces of rock called lithic clasts. The lithic clasts are small pieces of rock identical to rock sample #4 (metadacite lava).

The rock is an extrusive igneous rock that has been metamorphosed.

Lithic metatuff sample

Same rock as above wet. Black-colored rock fragments are easy to recognize when the rock is wet.

Similar to rock sample #4, this rock also has a light colored weathering rind (left side of sample). A weathering rind forms as the surface of the rock chemically weathers. The color of the weathered surface of a rock hints at its chemical compostion. Felsic rocks (rocks with abundant silicon and aluminum minerals) typically have light-colored weathering rinds.

Lithic metatuff sample wet

If you look closely at the black-colored rock fragments, individual crystals of the mineral plagioclase feldspar (the white specks) can be seen.

lithic metatuff slab

Please remember that rock and mineral collecting is STRICTLY PROHIBITTED on protected land!

 

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Want to learn more about the geology of the Eno River area?

You may be interested in A Geologic Adventure Along the Eno River - A richly illustrated publication from the North Carolina Geological Survey.

link to rock sample #4