Sills in Little Black Mountain, Utah

6. Adventures in Utah – sills

Millions of years ago magma forced its way through the rocks underlying Utah. Today these intrusions and rocks are exposed at the surface of the earth and allow us to study processes that would otherwise be practically invisible. These igneous intrusions can take on many forms and understanding the ways in which they form and behave is still ongoing in scientific research. Following on from the previous post about dykes this week I will show you the amazing features presented by the sills exposed in Utah, USA.

Sills in Little Black Mountain, Utah
Black sills created the amazing topography of Little Black Mountain, Utah. Credit: S.Wanmer 2016


Sills are generally described as being concordant with the host rock that they intrude. This means that they lie parallel with the planes (bedding) of the rocks that they are found in. Often sills intrude into sedimentary rocks made up of clearly defined layers of rock and the sill between these beds looks as if it was part of the sedimentary sequence all along.

Observations of sills in the field, however, show that this is not always the case. For example, sills may jump between bed boundaries and exploit multiple surfaces within a unit of host rock.  In this case the general definition of a sill, as being a concordant igneous intrusion, is challenged. At the moment this is becoming the subject of much debate and discussion in the geoscience community, pushing us deeper into our understandings of earths processes.

Sills in sedimentary rocks in Utah
Sills (black) follow and jump between the sub-horizontal boundaries of the sedimentary rocks that they have intruded. Credit: S.Wanmer 2016

Sill or lava?

You may be wondering how you can tell the difference between a sill which intruded into a rock after the rock formed and a lava which would have originally flowed on the surface but may later have been buried by sediments being deposited on top of it. From a distance the two may look very similar but closer inspection can reveal the truth.

A sill forms when magma forces its way through a pre-existing rock at depth beneath the surface. Once it has intruded into the host rock it slowly begins to cool and solidify. A lava forms when magma is erupted onto the surface of the earth and moves along the ground. Once the lava has erupted it slowly begins to cool and solidify into solid rock.

A sill will be contact with rock on all sides, where-as a lava will only be in contact with the surface that it is travelling across and the rest of it will be surrounded by air. This means that a the temperature of a sill can affect the rock surrounding it in all orientations, where-as, a lava will only impact the rock below it.

When magma or lava come into contact with pre-existing rock it is possible to heat that rock and bake it. The cooler rocks can also chill the outer surface of the magma or lava. This forms baked and chilled margins. A sill, therefore, will be surrounded by baked and chilled margins, where-as, a lava will only have one baked and chilled margin at its base.

This is the main way of identifying whether you are looking at a sill (igneous intrusion) or a lava (igneous/volcanic extrusion).

Sill or lava? baked margins
Sill or lava? Sills might have a baked margin above and below them, but a lava will only bake the rock beneath it

Another determining feature that sets sills and lavas apart is the nature of their upper surfaces. A sill will generally have quite smooth surfaces on all sides. In contrast a lava can form a rough, jagged, surface of clinker. This clinkery surface of a lava might be removed by erosion meaning that it might not be present once the rock has been burried and then eroded and exposed on the present land surface. So this determining characteristic might not always be present!

However, sills and lavas have many similar features. They can both display columnar jointing, formed as a pattern of cooling cracks penetrate through the solidifying molten rock. Each individual sill or lava can be formed of several layers showing that the magma, or the lava, was emplaced in stages or inflated and deflated with pulses of new magma or lava.

Lavas often contain vesicles – solidified gas bubbles – but sills can also contain vesicles depending on whether the pressure allowed gases to escape (exsolve) from within the magma or depending on reactions with the rock or any fluids within the rock that it intruded. It is possible that sills will not contain any vesicles but this is also possible in lavas where all of the gases have been able to escape from the lava or the lava has been very gas-poor.


Sills are igneous intrusions that generally intrude parallel to the layering of the surrounding host rocks. They can be identified in the field where they have a chilled and baked margin at every contact with the host rock. And, the general definition of a sill is being challenged in geoscience today after observations that sills can jump between layers of host rock and do not necessarily behave in a nice concordant fashion!

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