The Virtual Rock Kit -

Rock 4: Metadacite lava

This rock was formed from the cooling of hot magma that was extruded onto the earth’s surface as lava.   The rock is fine-grained since the magma cooled quickly.  A close examination of the surface of the rock indicates the presence of white-colored crystals called phenocrysts of the mineral plagioclase (a type of feldspar mineral).  Dacite is a felsic igneous rock that contains abundant minerals rich in the elements silicon and aluminum.

Individual minerals are not easily visible in this rock. The magma that formed this rock cooled quickly and created a fine-grained rock.

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

metadacite lava sample

When the surface of the rock is wet, white-colored specks can be easily seen in a fine-grained (aphanitic) matrix. The white specks are the mineral plagioclase feldspar.

The light colored surface of the rock ( left side of the sample) is called a weathering rind. 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.

metadacite lava sample wet

If you look closely at the surface of the rock, individual crystals of the mineral plagioclase felsdar (the white specks) are present. When mineral crystals are visible in a fine-grained matrix, the crystals are called phenocrysts.

metadacite annotated

Example of a dacite dome composed of dacitic lava in the crater of Mt. St. Helens in Washington State. Note people for scale indicated by red circle.

Photograph from USGS taken from the south side of the dome on May 26, 1983.

USGS Dacite Dome

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.