Amphibolite

Type: metamorphic

Age: 485 million – 600 million years old²

Amphibolite is an uncommon rock in Albemarle County, being found in narrow bands running north-south. It is considered to be a mafic rock, composed predominantly of dark-colored minerals like actinolite, epidote, and hornblende, alongside lighter colored ones such as quartz, albite, and mica. This gives it a “dark with light flecks” appearance. Amphibolite is rich in calcium, iron, and magnesium, all important nutrients for plant growth, so plant communities on soils weathered from it often have very high numbers of species. Thomas Jefferson chose to locate the University of Virginia’s gardens over this band of amphibolite that cuts through Charlottesville for this very reason! Some plants native to the Virginia Piedmont that are most common on mafic soils include fringetree (Chionanthus virginicus), redbud (Cercis canadensis), and bottlebrush grass (Elymus hystrix). Can you find any of them along the trail?

1 “Mafic Rock | Igneous Rock.” Encyclopedia Britannica, www.britannica.com/science/mafic-rock.

2 USGS. “Metagabbro (VACAZmd;0).” Usgs.gov, 2024, mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=VACAZmd. Accessed 12 Oct. 2024.

3 —. “What Are Metamorphic Rocks? | U.S. Geological Survey.” Www.usgs.gov, www.usgs.gov/faqs/what-are-metamorphic-rocks.

Definitions:

Metamorphic: a rock which was originally a different type, but was transformed by natural processes like pressure or extreme heat.³

Mafic: A rock containing abundant dark minerals like amphibole and pyroxene. Soils weathered from these rocks are usually rich in iron, magnesium, and calcium.¹