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by singwell-is off researching a lot on May 10th, 2007
Basically, a dike or dyke in geology refers to an igneous intrusion in between other rocks. The thickness of the dyke is usually much smaller than the rocks on either side. Thickness can vary from sub-centimeter scale to many meters in thickness and the lateral dimensions can extend over many kilometers.
A dike cuts across other pre-existing layers or bodies of rock, this means that a dike is always younger than the rocks that contain it. Dikes are usually high angle to near vertical in orientation ie mostly upright or near so.
A sill is a dike on its side ie it is an igneous intrusion horizontally between strata.
the first pic below is a dyke/dike and the second is a sill.
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Comments
Very good answer. However I do have two criticisms of it. First of all, you mention that dikes are always younger than the rocks containing them. This is also true of sills. If the sill is not younger than the rock layer above, then it is a buried lava flow, not a sill.
Second, sills are not necessarily horizontal. The sedimentary rocks that contain the sill could have been tilted into a vertical orientation. In such a situation, the sill would also be vertical. A sill is a fairly flat intrusion of magma that is injected between layers of sedimentary rock. the orientation of the intrusion doesn't matter as long as it is not cutting across layers.
by Glenn Blaylock on May 11th, 2007
I bow to your expertise, Glenn. Thanks for the corrections.
by singwell-is off researching a lot on May 11th, 2007