The Rocks of Bolin Creek




Metamorphic Rocks - Introduction


The majority of the rocks in the Piedmont upland of the Chapel Hill - Hillsborough area have been metamorphosed.  Metamorphism is the changing of a rock due to heat and pressure.  Mountain-building events - typically from the collision of islands with other islands, islands with continents or continents with continents - cause rocks to be buried deep in the Earth.  Temperatures and pressures are much higher deep in the crust than at the surface of the Earth.  The depth to which a rock has been buried (and the resulting temperature and pressure) determines the degree of metamorphism.  A rock buried deep in the Earth generally will have a higher degree of metamorphism than a rock that remained closer to the surface.  Metamorphism can be extreme (like extensive plastic surgery) or slight (like someone receiving a suntan at the beach).  Extreme metamorphism (called high-grade metamorphism by geologists) involves the re-crystallization of the rock in which new minerals replace the original ones.  This causes the high-grade metamorphic rock to look nothing like the original rock.  Slight metamorphism (called low-grade metamorphism) is sometimes so slight that the rocks barely look changed except to the trained eye of a geologist.  The igneous rocks associated with the Carolina terrane have been subjected to low-grade metamorphism; subsequently, the prefix “meta” is commonly added to the rock name.  The metamorphosed igneous rocks should technically be called meta-intrusive rocks (e.g., metagranodiorite, metagranite) and metavolcanic rocks (e.g., metatuff and metabasalt).  Since many of the rocks exposed along the Chapel Hill - Hillsborough area retain the original intrusive and volcanic textures present in the rock at formation, geologists often leave off the “meta” prefix.


Hydrothermal alteration – a form of metamorphism

Rocks altered by hydrothermal fluids are common in the Piedmont upland portion of the Chapel Hill - Hillsborough area.  Hydrothermal alteration occurs when water, heated by magma, permeates through rocks or deposits and changes their composition by adding, removing or redistributing chemical elements.  Hydrothermal alteration can be considered a low temperature and very low-pressure type of metamorphism.

Most hydrothermally-altered rocks in the Chapel Hill - Hillsborough area are associated with the edges of small intrusive bodies.  As these intrusive bodies of hot magma ascended upward in the Earth’s crust, they intruded pre-existing volcanic deposits.  The intrusive bodies heated the local groundwater and circulated it through the volcanic deposits.  When the heated water and fluids rose to the surface, they formed hot springs, geysers and fumaroles.  Hydrothermal alteration drastically changes the appearance of rocks, often forming rocks that are whitish in color (Figures 13 and 14).  In the Chapel Hill - Hillsborough area, hydrothermally-altered rocks are often identified by their white coloration with red and yellow mottling (Figures 13).  Hydrothermally-altered rocks are sometimes very resistant to erosion and weathering when the alteration has been via the process of silicification.  Silicification occurs when silica-rich hydrothermal fluids replace virtually all minerals in the rock with quartz, which is a very resistant mineral.  Other types of hydrothermally-altered rocks may be easily eroded (e.g., sericite and pyrophyllite phyllites).

Altered rock example
Pyrophyllite phyllite from Occonechee Mtn

Figure 13:  - Example of a hydrothermally altered rock composted of sericite and quartz with red and yellow mottling.

Figure 14:  - Example of a hydrothermally altered rock composed of the mineral pyrophyllite. Sample from Occonechee Mountain near Hillsborough.

Ancient Hydrothermal Activity and the Altered Rock of Occoneechee Mountain Near Hillsborough, NC

A prominent topographic feature in the Chapel Hill - Hillsborough area that is composed almost entirely of altered rock is Occoneechee Mountain. Occoneechee Mountain, located south of the Town of Hillsborough, is home to an active mine and an abandoned quarry that contains a white rock with one of the softest minerals known in geology – pyrophyllite.  Ironically, the western peak of Occoneechee Mountain is the highest point in Orange County.  So, why does a mountain that is partly composed of one of the softest minerals form the highest peak in Orange County? Click here to find out.