Monday, July 30, 2018

Spheroidally Weathered Diabase Dike – Garner, NC

Rock Type:  Diabase

Geologic terrane, element, or event:  Opening of the Atlantic Ocean

Age:  Jurassic – 200 million years old

Location:  Google Maps Link

USGS 7.5-minute Quadrangle:  Garner

Site Access:  This is an embankment next to an electrical substation.  Exercise caution, avoid the fenced-in enclosure and do not block access.

Here, a ridge was partially excavated to make room for the substation.  The excavation exposed part of a large diabase dike.

Diabase dikes
A dike is a sheet-like body of igneous rock intruded as molten magma that cuts across the older rocks.  In Wake County and surrounding areas, most diabase dikes are sheets that dip very steeply and trend close to north-south (Figure 1).  Diabase is a dark-colored igneous rock that is a variety of basalt, the rock type that makes up all the ocean floors of the earth.  Whereas basalt is volcanic, diabase is intrusive, meaning that the molten magma cooled below the earth's surface.  The age of the diabase in our region has been determined quite precisely; it is almost exactly 200 Ma (million years old).  In fact, our diabase was formed in response to the stretching and eventual breakup of the supercontinent Pangea, and the opening of the Atlantic Ocean.  Its age helps to date that event.  The dike exposed here runs through downtown Garner, roughly along Creech Road, passing along the west edge of Southeast Raleigh High School, and through Southgate Park.  It continues north at least as far as Millbrook Road.

Figure 1.  Example of geological map of a portion of northeastern Wake County, showing several diabase dikes.  They are the red lines labelled Jd that run N-S or NNW-SSE.  The solid red lines are dikes that have been confirmed by fieldwork and the dotted red lines are dikes that are inferred from their magnetic signatures.

Spheroidal weathering
Weathering is the gradual breakdown of hard rock at or very near the earth’s surface.  Chemical weathering transforms hard minerals like feldspar and hornblende into soft clay and iron oxide; it is key to the formation of soil.  At this site, the chemical process known as spheroidal weathering is displayed in spectacular fashion.  Imagine a body of rock, beneath the ground, that has a network of parallel horizontal and vertical cracks, intersecting at right angles (like a Rubik's Cube).  The cracks break the rock into many cubes or rectangular blocks.  Groundwater seeps into the cracks and the rock begins slowly to chemically decompose.  The process attacks the corners of the "blocks" most intensely because there are three rock surfaces in contact with the water.  Over time, this chemical weathering modifies these cubic blocks into rounded spheroids.  The “Rubik’s Cube” becomes a bunch of hard rounded rocks separated by soft decomposed material.  Erosion then can remove the soft material and only the rounded rocks remain (Figures 2 and 3).  Spheroidal weathering typically occurs in igneous rocks, including diabase, granite, and gabbro.

Figure 2.  Approach to the site, showing the electrical substation (left) and the embankment with diabase spheroids (right).

Figure 3.  Close-up of several diabase spheroids.  Such weathering commonly involves peeling off of successive layers, sometimes called “onion-skin” weathering.