Rock Type: Diabase
Geologic terrane, element, or event: Opening of the Atlantic Ocean
Age: Jurassic – 200 million years old
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.
Introduction
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.
