November 5, 2024
Picture of the month prepared by Andrew Stone, Hydrogeologist, (andrewstone @ gmail.com)
THREE ROCK TYPES IN ONE PHOTOGRAPH
1. Dolerite (igneous – intrusive volcanic)
2. Mudstone (sedimentary)
3. Where the dolerite as molten magma “cooked” the sedimentary rock, (metamorphic)
Photo Credit: Andrew Stone
The sedimentary layers in the photograph are in the Beaufort Group of the Karoo Sedimentary Basin in South Africa and were deposited about 250 million years ago. The igneous dolerite dike (1.) resulted from volcanic activity about 180 million years ago. Dikes can serve as barriers or as conduits for groundwater movement. Geophysical techniques are typically used to help characterize the geometry of subsurface geology in areas where dykes and sills occur.
Sill or Dike?
A sill of igneous rock does not cut across preexisting rock beds and is described as concordant. An igneous dike (as shown in the photograph) is described as discordant and does cut across the layers of older rocks.
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