1. 22. Sills and laccoliths Dan Barker April 2009 Salisbury Crags sill and Arthur's Seat, Edinburgh

2. Sills are tabular intrusive bodies in which the top and bottom contacts are generally parallel to bedding planes in the wall rock. They cannot everywhere be parallel, if the body is intrusive, because magma had to cut through the wall rock at some point. Laccoliths are like sills, except the top contact is arched. Some laccoliths started out as sills, but as the thin outer margins cooled and crystallized, no more magma could be injected in those margins, so magma pooled up near the center and raised the roof. Other laccoliths grew as stacks of sills successively injected above, between, or below others.

3. The first example is the sill forming Salisbury Crags in the Royal Park in the eastern part of Edinburgh, Scotland. Much of the jointed western face was quarried away and carried to London to pave the streets.

4. At first glance, this view of the base of the sill does not look important, but at two places (arrowed) the sill cuts across bedding planes in underlying sedimentary rocks.

5. A closeup of one of the arrowed portions shows why this is one of the most important contacts in the history of geology.

6. It was here in the late 18th century that James Hutton recognized that the sill rock cut across the layers at its base. The dark rock could not be a sedimentary deposit, as Neptunists claimed, but represented mobile material injected from below.

7. The base of the sill (dark red) over sedimentary rocks (gray) is intermittently exposed for about one kilometer.

8. In most places the contact (arrow) is parallel to bedding planes

9. but in others it cuts across them, showing that the sill is intrusive.

10. Another well exposed sill is at Shonkin Sag, MT. At its eastern edge the sill sent "fingers" (arrows) into the wall rock. Sill Wall rock

11. In detail, the basal contact is discordant at low angles to the bedding planes,

12. and the sill has baked the underlying shale.

13. Remnants of baked shale at the top prove that this is not a thick lava flow.

14. Like many dikes, sills can also be composite. A light-colored syenite layer is near the top of the sill at Shonkin Sag.

15. The Rubh an Eirannach sill on the east coast of the Isle of Skye, Scotland, is composite with internal gradations.

16. The top of the sill is mafic

17. as is the base,

18. but the interior grades through intermediate compositions

19. to a felsic center. Apparently, after mafic magma intruded, a felsic magma invaded the still hot center of the sill and the two partially mixed to form the intermediate rock and the felsic center.

20. A sill near Lajitas, Texas, shows a different kind of zoning.

21. The sill (dark) is exposed in three dimensions along the floor and walls of a canyon and its tributaries.

22. The top contact of the sill with the overlying Cretaceous Boquillas Formation shows some thermal metamorphism, and the sill rock is fine- grained and lacks large plagioclase crystals.

23. There is a vertical step in the roof of the sill, but strata in the overlying Boquillas Formation are not displaced.

24. A Texas microfossil in a slab of Boquillas Formation detached from the top of the sill.

25. Xenoliths choke some parts of the sill. Most are quartzite and other Paleozoic rocks in the subsurface, but some are coarse- grained cumulates from the magma reservoir that fed the sill.

26. Here the fine-grained rock at the top of the sill changes abruptly into xenolith-rich igneous rock.

27. A xenolith-rich layer pinches out toward the left.

28. Generalized block diagram of the Lajitas sill. Green xenoliths are Boquillas, blue are Paleozoic quartzite,and red are igneous cumulates.

29. Quartzite xenoliths were infiltrated by magma, and tridymite blades, now inverted to quartz, grew at high temperature.

30. A dike of mafic magma cut a quartzite xenolith and was rapidly chilled. This suggests that the quartzite was the wall rock of the shallow magma chamber, and that the quartzite and igneous xenoliths came from the same depth.

33. A sill cutting hyaloclastite, Iceland

34. Baked zone below the sill.

35. Thin sill carrying mantle xenoliths, Elephant Butte Reservoir, NM. D. K. Bailey for scale.

36. Drumadoon sill, Isle of Arran, Scotland, taking a nose dive in the foreground.

37. Sierra Prieta sill, Diablo Plateau, TX, rides up and over an earlier sill (arrow).

38. Sierra Tinaja Pinta complex, Diablo Plateau, TX. Three sills (strike and dip symbols) dip away from a central intrusion (X). X

39. Ground-based view of Sierra Tinaja Pinta complex from the west. Layers are in Cretaceous limestone.

40. A laccolith, San Antonio Mtn, Diablo Plateau, TX-NM.

41. A rhyolite laccolith, Sierra Blanca, TX

42. Once considered a laccolith, the Torres del Paine in Chilean Patagonia is now known to be a stack of three sills, the first intruded at the top and the last at the bottom.

43. Another view of the Torres del Paine complex. The contacts between the three sills are invisible at this distance.