The Virtual Rock Kit -

Rock 3: Metabasalt 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 black-colored crystals called phenocrysts of the mineral pyroxene (now altered to a type of amphibole mineral).  Basalt is a mafic igneous rock that contains abundant amounts of minerals rich in the elements calcium, iron and magnesium.

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.

metabasalt sample

When the surface of the rock is wet, a very fine-grained grainy texture is apparent. The individual minerals are difficult to see with the naked eye.

metabasalt sample wet

If you look closely at the surface of the rock, individual crystals of the mineral pyroxene are present. When mineral crystals are visible in a fine-grained matrix, the crystals are called phenocrysts.

metabasalt sample annotated image

Example of basaltic lava erupting from a vent on Kilauea Volcano in Hawaii.

Photograph from USGS taken by C. Heliker, September 12, 2003 .

USGS basalt photograph

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.