Welcome to Townsville! This Earthcache thankfully doesn't involve scaling Castle Hill and is wheelchair friendly. Who would of thought you could get an Earthcache on The Strand. I did, and geology of the rock cutting here is really clear.
What is a intrusion?
Where one igneous rock type cuts across or through another rock type, we call these 'intrusions' and in the vast majority of cases, the 'intruding' material, is younger than the surrounding rock.
There are two main kinds of igneous intrusions: dykes and sills. Dykes form when magmatic material rises up from deep in the Earth and flows through the cracks and fissures in the pre-existing rocks along the way. As the material rises it cools and eventually forms a solid rock too.
Dykes are a fairly common igneous feature, and can range in size from just a few centimetres across to up to 10 metres across.
In the picture below is an example of dolerite dyke cross cutting horizontal limestone beds in Arizona.
Mafic versus Felsic
Mafic is a term to describe rocks that are lower in silica and higher in iron and magnesium and the minerals are darker in colour. The dolerite dyke in the image above is mafic. They can be grey, green and black and many shades of these colours.
Felsic is used for light coloured rocks, high in silica and low in iron. Examples of a felsic rocks are granite, a very common rock type. Pink, beige, white and pale grey are typical felsic rock colours. The Castle Hill granite is orange pink in colour and is a felsic rock.
Orientation
As dykes are tabular or sheet like then the orientation can be determined. The strike of a planar feature is the way it runs along the ground. The best way to measure this orientation is to stand on the feature and point your arms in the direction the rock is going. e.g. north-south, east-west.
Dip is the angle of the plane off the horizontal. For example, steep dips are close to 60 to 75 degrees off the horizontal. The image above could be classified as steeply dipping to almost vertical
Earthcache description and questions
Stand on the path and look at the exposed rock cutting to the southwest. There are two different coloured rocks. You will also see a large 'streak', an igneous intrusion in this outcrop.
- What is the dominant rock type in the cutting? Is this rock mafic or felsic?
- The large streak slicing through the cutting is an intrusion called a dyke. Is the dyke mafic or felsic?
- Which rock type is older and why?
- Is the dyke horizontal, vertical or dipping (sloping towards) the east or west?
- Please post a photo of the you, your GPS or a special object overlooking The Strand. (Please no spoiler photos).
You are welcome to log your find straight away though logs without answers will be discreetly deleted within 14 days.
I hope you enjoyed The Strand EC. Go get yourself a coffee and head to the beach.!