This is a quick Earthcache which can be found along the R617 between Bulwer and Howick. It is most easily accessed travelling from Bulwer, but a u-turn into the picnic area is possible if the road is quiet. Please be careful if you cross the road for a closer look; the corner to the north is not entirely blind, but it may hide one from oncoming traffic. The questions are all answerable without needing to cross the road.
Background
Intrusive rocks and dolerite
Intrusive rocks are those which form from molten rock (magma), and cool underground, before reaching the surface. They are one of the two major divisions of igneous rocks, with the others cooling at surface and being termed extrusive. The rock around an intrusion is termed country rock.
Dolerite is a particular type of intrusive rock, commonly found in South Africa. It is a medium-grained rock dominated by plagioclase and pyroxene. Olivine and quartz are also common. A wide variety of accessory minerals, including hornblende and biotite are also found within dolerite. This composition is the same as basalt, which is the extrusive equivalent. Because extrusive rocks cool much faster, individual crystals are not visible in basalt, but are in dolerite.
Note for geology types from the USA: You call this rock diabase. That term is used only for altered dolerite in most Commonwealth countries.
Intrusion geometries
Magma can force its way into country rock in a variety of ways. These are governed by weaknesses in the existing rock, the volume of magma, the physical properties of the magma (such as viscosity and temperature), and more. Common geometries can be seen in the following sketch:
Dykes and sills are essentially planar features, which are either (sub-)vertical and (sub-)horizontal respectively. They can be thought of a large sheets which cut across (dykes) or are aligned with (sills) the layers in the country rock. Laccoliths push between sedimentary rocks and cause the layers above the intrusion to dome upwards. Batholiths are large intrusions that can be thought of as relatively homogenous blobs of magma.
What you can see
The roadcut opposite the picnic site has two major rock types. There are more or less horizontal sedimentary rocks, which are light in colour, and a darker dolerite intrusion. The dolerite has typical onion-skin weathering, where blocks become rounded as weathering continues.
Take note of the way the dolerite has pushed into the sedimentary rocks, and then have a go at logging this cache.
To log this cache:
Please send a message with the answers to the following questions. Logs without answers will be deleted.
- How can the intrusion be identified from the country rock?
- What intrusion geometry is present here? (Batholith, dyke, laccolith, sill?)
- Estimate the thickness of the intrusion.
References
Imperial College Rock Library Glossary: Dolerite https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?gID=00000000060
The Geological Society - Igneous Intrusions http://www.geolsoc.org.uk/ks3/gsl/education/resources/rockcycle/page3598.html