One of the reasons I fell in love with geology was finding evidence of igneous activity in the form of a dike through the sandstones where I grew up. You see, I grew up in an area where you could find nothing but sandstone for many miles in every direction. While sandstone can be beautiful, its slow steady formation by rivers or the ocean just could not fire up my imagination like a rapid intrusion and cooling of molten rock deep in the Earth…or the explosive eruption of a volcano.
So I owe a lot to that dike. It made me yearn to learn more about our dynamic planet – how did molten material form that dike?
So what is a dike?
A dike is a long narrow, mostly plane-like intrusion of molten material that cuts through older rocks. The molten rock cools to form a fine grained igneous rock. When exposed at the surface the material in the dyke can be more resistant to weathering and therefore ‘stand up’ in the landscape or be less resistant to weathering and erode away forming ‘channels’ in the landscape.
Eroded channel of a dike – this one at EarthCache I (http://coord.info/GCHFT2) in Australia. In fact, this dike, and the one above are exactly the same – but on different sides of the same headland.
When the molten material in a dike is intruded into the older rocks, often the hot material cools quickly on the edges due to the cold ‘country rock, forming a very fine chilled margin of the dike. The heat of the material can also cook the country rock to form a small zone of metamorphic rock close to the dikes edges.
Also sometimes the center of the dike cools slowly enough for larger crystals to form and so you can find a zone of courser crystalline materials right in the center of the dike.
Dikes can cut through older sedimentary, metamorphic or igneous rocks. This amazing set of dikes at Binge Binge Point, NSW has a set of light colored dikes being cut by a dark colored dike…and all cutting through older igneous rocks.
So the next time you are driving past road cuttings or walking around a rocky coastline, look for amazing dikes and see if you can see some of the features I have mentioned above.
Yep – I love dikes.