Volcanic activity is the surface expression of a great deal of sub-surface processes which happen deep within the earth. When ancient volcanoes and their plumbing systems solidify they can become exposed at the surface of the earth after thousands to millions of years of erosion. The solidified magma which once forced its way into areas of the crust can be seen in many places all over the world as igneous intrusions. Recently I spent some time looking at some of these fascinating rock formations in Utah, USA. In this blog post you can find out about some of the features of dykes that can be seen in Utah and the basics of how they form.
Generally, a dyke is defined as being an igneous feature that is discordant and therefore cuts through the surrounding layers of rock.
For example, sediment is deposited in a horizontal position in defined units known as beds. A body of magma that forces its way through the beds perpendicular to the horizontal bedding planes formed by the sediments can solidify into an ignous rock that will be identified as a dyke.
A dyke can also be identified by its different characteristics compared to the rock in which it intruded, for example it may be a different colour or contain different minerals. In some cases, the harder igneous material of the dyke may protrude from the surface of the earth where the rock that once surrounded it has been removed after millions of years of weathering and erosion.
These dykes can extend for many km’s vertically and horizontally, sometimes forming large, sharp ridges across the landscape.
Diatremes – the insides of ancient volcanoes
Some dykes lead to areas where the deep-seated plumbing systems of ancient volcanoes are exposed. For example, some vertical igneous intrusions can be seen in seemingly random tall cliffs or mounds throughout Utah. These ancient plumbing systems are known as diatremes.
For example, several diatremes can be seen jutting sharply out of the landscape in Monument Valley at the Utah-Arizona boundary. In the image below, the dark band of rock running almost vertically up the height of the cliff is an example of a dyke.
A closer look
Taking a closer look at the igneous rocks that have solidified within a dyke and studying the rocks surrounding the intrusion can provide more information about how the original magma intruded and cooled.
For example, a dyke may have sharp contacts with the surrounding (host) rock with more finely crystalline edges where the magma cooled more quickly against the cooler host rock. These more rapidly cooled edges are known as chilled margins. On the other side of this contact the host rock may be more brittle with cracks or a baked appearance, for example muds may appear more like terracota/ceramics. This is where the heat from the magma within the dyke has affected the surrounding rock, heating it up and is known as a baked margin.
Often as an igneous intrusion cools it forms cooling joints, like those famously seen forming the hexagonal prisms of the Giants Causeway in Northern Ireland. In a dyke these cooling joints will form perpendicular to the angle of its edges. Therefore, a vertical dyke is most likely to form horizontal cooling joints. The formation of these cooling joints may also extend into the baked sediments surrounding the dyke.
Taking a closer look at a cross-section through a dyke it might be possible to make out bands of varying colour, crystal content or vesicle (bubble) content which are mirrored on either side of the dyke. It is possible that these represent different events of magma flow through the same dyke, with each new flow of magma chilling at its edges and the dyke itself inflating and deflating with each magma batch that flows through then solidifies.
Dykes are a common igneous intrusion that can be found all around the world, either exposed as ridges and cliffs that abruptly pierce the landscape or hidden beneath our feet. Lots of scientific research is still ongoing looking into how these intrusions form, by looking at them in the field, for example in Utah, or trying to detect their growth and development beneath currently active volcanoes!