Earthscience Geoscience Lesson
A sheet intrusion is where a mass of molten magma, takes advantage of a pre-existing linear feature in a host rock, such as a long rupture or fault, and forces its way into these spaces. Thus the magma, intruded between existing rocks, solidifies into large thin sheets of igneous rock referred to as a sill. The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across pre-existing rocks, in contrast to dikes which do cut across older rocks. Sills are always parallel to beds (layers) of the surrounding country rock. Usually they are in a horizontal orientation, although tectonic processes can cause rotation of sills into near vertical orientations. They can be confused with solidified lava flows; however, there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock. On both the "upper" and "lower" contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere. Because sills generally form at depth, the pressure of overlying rock prevents this from happening much, if at all. Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.
At the posted coordinates you can observe the cross section of a sill that has been exposed in a rock cut created to make way for the new Route 1 Gateway Highway.
To log this earthcache
You will need a measuring stick.
Please email via my profile email the answers to the following questions
1.What is the average thickness of the sill?
2.What is the total length of the sill? ( you may pace the distance, measure your pace, then do the math or use your GPS)
3.In your own words please describe the difference between the sill rock and the rock in which the sill has intruded? ( i.e. color, grain size, strength)