Dolerite Dyke Intruding into Granite Rock Wall

Here we are at our first stop at The Strand, across the road from Tobruk swimming pool, and down to the right of where the ice-cream and fish and chip stores are. What we're looking at here is a large outcropping of the igneous rock granite, the same material that Castle Hill is made of. If you have a look around, you will also see a large 'streak' of a darker igneous rock in this outcrop, which appears to be slicing through the orangey, pinky granite. 

Here's a question: which rock type do you think is older?

The answer is: the granite!

We call these features where one igneous rock type cuts across or through another rock type 'intrusions' and in the vast majority of cases, the 'intruding' material, in this case, the dark dolerite, is younger than the surrounding material, which is the granite in this case.

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. And the end result is what you see here. 

Sills are exactly the same thing as dykes, except they intrude horizontally, rather then vertically.

Dykes are a fairly common igneous feature, and can range in size from just a few centimetres across to up to 10 metres across. So keep an eye out for these igneous intrusions throughout the rest of the field trip, and see how many you can spot!