Sills and Dikes


So do sills and dikes really have a place in volcanology? The very definition of a dike establishes it as feature that forms below the Earth’s surface, yet volcanoes are defined as igneous features that do reach the surface. The fact of the matter, however, is that these features can strongly influence an eruption and begin new ones. After all, magma chambers never reach the surface, but there is no doubt as to their importance. At the surface the subject may seem entirely academic. After all, it would seem that sills and dikes have little effect on humans unless there are as-of-yet undiscovered subterranean communities existing somewhere. This is not entirely true, however, because these features may intersect the surface in places, resulting in small-scale to occasionally impressive volcanic activity (such as the Inyo Domes). In addition to this, the subject gives us valuable insight to the behavior of magma on its way to the surface (and yes, this is a very different article from the “ascent of magma” piece).

I suppose we ought to begin with the definition of a sill and dike. Both are intrusive igneous bodies; that is, they are composed of magma and do not reach the surface. Moreover, they are linear features, which distinguishes them from most magma chambers. Dikes and sills propagate through cracks in the earth; they may be hundreds of kilometers long and hundreds of meters high, but their thickness rarely exceeds a couple meters (for sills, trade height for width). A dike cuts vertically through various layers of overlying rock. A sill, on the other hand, moves horizontally between rock layers.

Sills and dikes form in extremely similar manners, but the controlling difference between the two is the type of rock through which they are traveling. Dikes may travel through pre-existing cracks and faults, where empty space facilitates the movement of magma (even more frequently, they generate their own cracks). Sills, on the other hand, travel through weak layers of rock within the earth. This may be through a low-density layer capped by a high-density layer that resists the magma’s efforts to punch through. Due to mineral composition, it may also be influenced by a lower melting temperature making it a natural path for the magma to move along.

Dikes can be centimeters to over a hundred meters thick. Moreover, they can range for distances of several hundred kilometers (though usually they are much smaller). The composition of the dike depends on the type of magma that was passing through. This can range from diabase (similar to basalt, but physically different) to rhyolite. Sills are similar in this respect and from here on, unless otherwise noted, sills will be included in the general category of dikes.

How and why they form

To begin, you have to have a subsurface magma body. This magma body, at nearly any depth, is growing in some way- probably through the addition of new magma from beneath. As the chamber becomes over-pressurized, magma needs a place to go. The wall rock of the chamber, feeling the tremendous stress of it’s expanding contents, begins to crack and fracture. This starts a process that can continue for hundreds of kilometers.

The moment that wall rock fractures, the open space between the rock (even if it’s only a millimeter or two) is filled with the pressurized gas of the chamber. This gas acts as a wedge to push the crack farther back and widen it slightly. As this gas wedge moves along, forcing the crack to grow, magma begins entering the newly widened edifice. This magma, though it is liquid, is still very dense and acts as a second wedge moving through. Moreover, this hot magma can also melt that wall rock, widening the new dike.

This rock-working duo, the gas and magma, push their way through the surrounding country-rock. Fueled by the addition of new magma behind it, the process continues with the gas wedge breaking open a path for the magma to pass through. The size of this gas wedge is theoretical, probably variable, but also probably very small. This gas is the stuff that was exsolved from the magma wedge, and so we’re not talking about enormous volumes of gas. Still, it’s presence is extremely important because if the front of the advancing magma wedge actually had to push against rock, the face of the advancing magma would harden and make the process many times more difficult.

But this is also an important concept to remember: as the magma advances further from the magma chamber, the surrounding rock gets colder. It no longer becomes an issue of melting the surrounding rock, but the surrounding rock freezing the magma. This creates an interesting situation, however: if the interior of the magma flow continued moving in a straight-line direction, the outside edge would freeze inward…and continue to freeze until the dike is literally squeezed shut. Something else must happen.

What do you know, something else does happen! Thermally driven convection. As the material along the wall cools, it sinks. The brief vacuum left by the sinking cold magma is replaced by hot magma from the interior of the dike. The brief vacuum left by this magma leaving is filled by that cold magma that has just sunk out. That magma is reheated to some degree by the surrounding interior while the hot stuff that just moved to the outside is being cooled. The process is ongoing and, while it keeps the overall flow from pinching shut, it does mean the overall magma flow is cooling. The idea is, though, that the magma cools much more uniformly. That is not to say that cooler magma isn’t still sticking to the dike walls, but that it is just happening in much smaller quantities. Also, with the exterior moving more slowly due to cooling and friction, the flow experiences shear. This shear is basically friction for fluids, and converts kinetic energy (movement) to heat. This means that, though the flow slows, it can continue moving even longer; to some extent, it is self-heating.

Well that solves that…except…that large rhyolite dikes, which are much cooler and more viscous, have been found to extend dozens of kilometers. Now I don’t care how you look at it, that just doesn’t seem right. Even from the vent, rhyolite is usually extruded like toothpaste from a tube. Now we find rhyolite squeezing through dikes for dozens of kilometers without freezing, being clogged, or stopped? That just doesn’t make any sense. Unless…

Unless we have bimodal flow. We know about it now, and engineers have tested it. Bi means two and modal refers to “states” or “types.” That is, through the dike we have two types of magma flowing. One is high viscosity, the other low. You might think that the high and low viscosity magmas would mix, but remember that the high viscosity one is not very fluid at all compared to the low. That means it behaves more like modeling clay in a river of mud (what would you do without these off-the-wall analogies?). Through this same dike we have the two magmas; for our example, basalt and rhyolite. Why do we have the two? Perhaps the crack in the magma chamber is pulling rhyolitic mush-zone material out and also some basaltic chamber interior. As the material enters the dike, it probably flows rather chaotically. Soon, however, the low viscosity (highly fluid) magma surrounds the rhyolite. This means only fluid magma is contacting the walls, which means the high viscosity magma remains insulated and is moving with almost no friction to slow it down; the basalt acts like a lubricant along the walls.

As bizarre or unusual as that may sound, it’s really not at all. The petroleum industry has utilized this phenomenon for years by injecting water into oil pipelines, allowing the oil to move much more quickly from point A to point B. If any two magmas are injected, at the same time, into a dike, this effect will always occur. This event may also be present during sizeable eruptions, with a lower viscosity magma coating the walls of the conduit. There is some good evidence for this occurring, although it is not widely acknowledged at the moment.

Practical Applications: The Real World

So we know how the processes work, but where do we find these? The following are a couple of examples that illustrate the points made earlier:

The Basement Sill, of the Dry Valleys Region, Antarctica, is an enormous structure. Sill thickness (remember, sills form as sheets rather than vertical cracks) ranges from one centimeter at the edges to 700m around the center. Individual arms of the sill extend over 100 km. While sills and dikes differ as to their functions (sills tend to store magma while dikes transport it elsewhere), their mode of propagation is probably very similar. Many people believe sills are more closely analogous to magma chambers.

The Inyo Domes rhyolitic dike extends at least 7 km, probably more (not including vertical distance from the magma chamber). Inyo Domes has provided a wealth of information on dike formation since that dike punctured the surface in four different locations (not including blast craters). The resulting rhyolite/obsidian domes formed in chronological order along the dike; that is, though this occurred long before volcanologists were around, we know that the domes formed as the dike moved outward. Looking at the lava composition of the domes, we can see a transition from crystal-rich lava to crystal-poor at the end. The first dome is Deadman Dome, which displays both crystal-rich and crystal-poor lavas. What does this mean? Well, the crystal-rich magma was extruded from the edges of the magma chamber, into the dike. After a time, however, the crystal poor chamber interior began to be drawn into it. The crystal-poor magma, being of lower viscosity, lubricated the rhyolitic flow. A similar situation is seen at the next vent, Glass Creek Dome. At the final vent, Obsidian Dome, however, we almost strictly see crystal-poor rhyolite from the chamber interior being extruded. Regardless, this rhyolite is still lubricated by lower viscosity (and silica) rhyodacite (a transitional lava between rhyolite and dacite).

Dikes occur on Hawaiian basaltic shields as well. An intrusion near Puhimau crater has resulted in a massive tree-kill zone due to escaping magmatic gasses. In reality, however, the large rift-zones on Hawaiian volcanoes are really nothing more than enormous dikes. These dikes travel for kilometers; where the dike punctures the surface, there is an erupting vent. The most active dike on Kilauea currently is the East Rift Zone. Along this rift, the dike currently reaches the surface at Pu’u O’o vent where the eruption has occurred off-and-on for nearly 20 years (as of this writing). Over the last 50 years, the dike has also punctured through the surface at Kupaianaha, Mauna Ulu, and at Napau Crater (for a single day in 1997). Because of the magma’s high fluidity, it is doubtful that bimodal flow occurs here.

When looking at solidified dikes and sills, there may be initial difficulty in telling the two apart in places that have experienced structural deformation. Take, for example, a region that has been heavily folded. In some places the dike, which originally cut upward, may be tilted to a horizontal angle, making it appear to be a sill. Likewise, a sill may be tilted vertically to look like a dike. In these situations, careful analysis of the surrounding wall rock is vital to understanding what happened.


Well, this concludes our article on sills and dikes. Hopefully you have a much better understanding of what dikes are and how they’re important. You should also have a much better understanding of how they form and just how magma flows before it reaches the surface. Toward the end, we discussed several brief examples of dikes which should give you some context. If you’d like to learn more on the subject, I would suggest researching textbooks or scientific journals.