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EducationHow did this happen?

13th Feb 2020 16:43 UTCLarry Maltby Expert

I have been wondering how this horizontal Diorite Sill formed. It is about 100 feet wide and very long. What are the mechanics that caused this? The only thing that I can think of is that it may have happened when the strata was in a vertical position. A massive fault occurred and opened similar to a small riff during the Lewis Overthrust. How did the gap stay open while the intrusion took place? Any ideas?

Here is a quote from The Geologic Story of Glacier National Park Special Bulletin No. 3:

“This imposing layer of rock, unlike the lava, never reached the surface in a molten state, but was intruded between beds of sedimentary rook and thus became a sill instead of a flow. We need only a glance to determine its intrusive nature. Wherever it occurs it is bordered at top and bottom by thinner gray layers. These are Siyeh limestone which was changed to marble by the tremendous heat of the diorite during its intrusion. This effect is termed contact metamorphism by geologists. Because this contact-metamorphosed zone is at both top and bottom of the sill we know the latter was intruded into the adjacent rocks. Lava flows, even though covered later by sediments, of course alter only the underlying rocks.”

13th Feb 2020 17:20 UTCNathalie Brandes Expert

A number of things influence the emplacement of sills, including deformation of the host rock and how easily fractures propagate in it. The magma does not enter an open space, but rather forces its way into the rock. This is why things like the strength and rheology of the host rock are important. Other important factors researchers have studied include interface strength between rock layers, tectonic stresses, and the physical properties of the magma including magma buoyancy.
Here are a few links to papers discussing sill emplacement:


I hope this helps!

14th Feb 2020 13:34 UTCLarry Maltby Expert

Thanks for the comments and references. I now have some direction and “key words” for research. It is cold up here in Michigan, below zero last night, a good time to stay in at the computer.

14th Feb 2020 03:02 UTCDoug Daniels

I think Nathalie has pretty well given an explanation of sill emplacement (including the links, which I'm too lazy to look at right now).  As far as the Lewis Overthrust contributing...maybe.  Any stresses passed through the area may have weakened a certain bedding plane, allowing the magma to force its way between them (note that we rarely, if ever, find the conduit up to the sill).  And, if the overthrust put those beds into a vertical position, followed by sill intrusion.... how do the beds then become horizontal again?  I know that, over time, rock layers can be "flexible", but in my experience, not in that way!

14th Feb 2020 13:38 UTCLarry Maltby Expert

I took another look at the illustration shown above and it proves your point exactly. If there was folding during the overthrust it is now eroded away. The limestone that contains the sill is roughly horizontal and always has been. My curiosity is now focused on, what does a lava front look like during intrusion and what happened to 100 feet of limestone? It seems like there are two possibilities. The intruding lava followed a weakened bedding plane and as a huge wedge pried all of the limestone above the bedding plane upward 100 feet (not likely), or the lava melted 100 feet of limestone consuming it to be recrystallized in the diorite (also not likely). Anyway, I will pursue this over the winter and perhaps find an answer (still another thing not likely). Thanks for the help.

The illustration is from the National Park Service in 1920, here is the link:

14th Feb 2020 17:08 UTCJamison K. Brizendine Expert

One of the dangers of using a 1920s geologic interpretation was that the geosynclinal theory was still in vogue. It wasn't until the 1950s and 1960s until the "modern" plate tectonic theory was accepted. 

14th Feb 2020 17:43 UTCRichard Gunter Expert

Diorite sills are fairly common world-wide in the correct geological environments and they all look similar to the photograph. Very often the white "rim" on the sill has masses of contact metamorphic minerals, illustrating the intense temperature gradient during emplacement. The Scawt Hill locality in Northern Ireland is a good example.

A more unusual example of a sill is the very volatile rich silicio-carbonatite sill emplaced in the Francon Quarry, Montreal. Here the volatiles were retained in the sill and crystallized in vugs next to the top and bottom contacts of the sill. As I remember it the Francon sill did not have any reaction rim between it and the surrounding limestone.

14th Feb 2020 19:48 UTCRichard Gibson

The age of the Purcell sill is about 1,200 million years, so its intrusion had nothing to do with the Lewis overthrust, which happened about 65 million years ago. 

16th Feb 2020 12:47 UTCLarry Maltby Expert

Thanks, good comments all.

I am looking for a professional paper with a hypothetical illustration of a lava front intruding into a preexisting rock formation with a force diagram that answers the question: “Did the lava push apart the preexisting rock like a wedge or did the lava melt and consume the preexisting rock?” Or some of both?

If I eventually find this I will post it here. Thanks again for the help.

16th Feb 2020 21:25 UTCRalph Bottrill Manager

I actually went to a talk a few weeks ago by Sandy Cruden ( Natalie refers to one of his papers) on this topic, specifically the formation of dolerite sills, and he had some great images and information. He has a few publications out on the topic ( from granites to basaltic magmas) , you should be able to contact him at the Uni of Melbourne, or at least find his publications there. It does seem to be a mixture of processes, but largely a matter of the magma rising (partly by dykes, also stoping and assimilation) to a level where the overlying rock pressure balances the pressure of the rising magma. The magma then “seeks” weaknesses in bending planes and largely bulldozes its way along them, lifting to roof as it goes,  till it can find a fault or fracture along which it may rise again. But there are various geo dynamic processes happening, so best to read some of his works.

17th Feb 2020 23:25 UTCLarry Maltby Expert


I spent the morning looking for papers and books that were associated with Alexander Cruden’s name and I found a bunch of them. They provided detail for the summary that you wrote above. The breakthrough for me was the concept that the fluid pressure in the lava seeks equilibrium with the pressure caused by the weight of the overburden and that process is capable of lifting the overburden. I have learned a lot from this thread.

17th Feb 2020 16:55 UTCGregg Little

While we are at it I might as well throw in laccolith.  I think of them as probably starting as a sill with lateral spreading over a limited area along with doming of the overlying strata caused by the intrusive exceeding the confining pressure of the overlying rocks.  Just a little to the east of your photo of the sill in Glacier National Park and in northern Montana is a large laccolith centered around Kevin and Oilmont.  This doming structure formed many oil traps in the overlying sedimentary rocks leading to Montana's oil rush in the early 1920's.

Five years ago I spent two winters with a Calgary company drilling the structure for economic deposits of CO2.  Only one well penetrated the very top of the intrusive which appeared to be granodioritic in composition with a potassic alteration overprint.

17th Feb 2020 23:26 UTCLarry Maltby Expert


Most of the papers that I have been reading included information about laccoliths. I see the similarity. Sounds like you had a neat assignment there, beautiful mountains and complex geology. Thanks again to all of you for helping me do my “homework”.

18th Feb 2020 17:09 UTCLarry Maltby Expert

Here is an interesting photo that I found this morning. The label reads: “Top of the diorite sill of Blackfoot Glacier. The man is standing on the sill. Light rock overlying the sill is contact-metamorphosed Siyeh Limestone”. This is located about 15 miles south of Many Glacier where I photographed the sill. The trail to this spot requires a hike of 14 miles with ropes and experienced climbers.

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