1. 20. Joints in igneous rocks
Dan Barker
March, 2009

2. Joints are systematically oriented fractures along
which no detectable slip has taken place.
Joints are responses to stress caused by
1) Flexure
2) Unloading
3) Volume decrease (shrinking during cooling)
Flexure results in orthogonal joints, with three
principal sets of planes perpendicular to each other.
Unloading by erosion causes subhorizontal or
gently curving joints, called "sheet structure", roughly
parallel to the overlying topographic surface.
Shrinking during cooling causes joints that are
prismatic, elongated blocks with 3 to 8 sides
(usually 6). Many geologists call these "columnar
joints", although most architectural columns are circular
in cross section.

3. Orthogonal joints, Danbury granite, GA

4. More complex jointing in granite, Joshua Tree National
Monument, CA

5. Orthogonal joints, Aeolian Buttes, CA. Prospector with
rifle (arrow) for scale.

6. Sheet structure (also called exfoliation), Enchanted RockTX

7. Another view of Enchanted Rock and its sheet structure.

8. As granite is unloaded, it expands, resulting in "pop up"
or tent structures. Enchanted Rock.

9. Sheet structures combined with orthogonal joints make
quarrying granite much easier. Marble Falls, TX

10. Sheet structure,
Carmolli Brothers
Quarry, Elberton, GA

11. Sheet structure, Granite Mountain, GA

12. Orthogonal joints are most interesting to
structural geologists, and sheet structure
to geomorphologists, hydrogeologists, and
quarry engineers. Both kinds of joints form
long after the igneous rock solidified.
Prismatic joints, on the other hand, form
while the rock is cooling, and from their
orientations the original forms of partially
eroded intrusive and extrusive bodies can
be inferred. A good working assumption is
that the long axes of prismatic joints are
perpendicular to cooling isotherms, so joints
point in the direction of maximum heat loss.

13. Prismatic joint blocks
in a parking area, le
Pas de Cere, Cantal
volcano, France

14. Prismatic joint blocks used as road posts, S of Axum,
Ethiopia

15. The most famous example of prismatic joints is the Giant's
Causeway, Northern Ireland. A basalt lava flow filled a river
valley, and most of the cooling occurred from the top
surface, resulting in vertical prisms.

16. Another view of the Giant's Causeway.

17. Curved ball-and-socket joints cross-cut the prismatic joints
Curved ball-and-socket joints cross-cut the prismatic joints. Giant's Causeway.

18. Both the prismatic and ball-and-socket joints are shown here.

19. Rainy days make for treacherous footing.

20. Another example, made famous by
Stephen Spielberg, is Devil's Tower, WY.

21. Prismatic joints are roughly
vertical in the top two thirds
of the Tower, but diverge and
become nearly horizontal at
the base. The most plausible
Interpretation is that cooling
Isotherms were horizontal
near the top, where heat was
lost in a vertical direction, but
were vertical lower down,
where most heat was lost
through the vertical sides of
the feeder.

22. Another view of
diverging joints
near the base of
Devil's Tower.

23. Prismatic joints in a 120 m thick rhyolite flow, Hell's Gate, Kenya

24. Fischer's Tower is
another landmark in
Hell's Gate, Kenya.
Note the diverging
joints near the base.

25. Kilt Rock, on the east coast of the Isle of Skye, Scotland,
is an example of a sill with prismatic joints vertical from
top to bottom, because all cooling took place at the top
and bottom contacts.

26. Prismatic joints also aid in resource extraction. For at least
2500 years, lava has been dug from underground workings
and made into millstones for grinding grain. Mendig, Germany.

27. Prismatic joints in the Salisbury Crags sill, Edinburgh, helped
In 18th century extraction of paving stones for the streets of
London.

28. Workers in the Weibern quarry, Germany, used prismatic
joints from a welded pyroclastic flow for artistic purposes.

29. Another pyroclastic flow deposit with prismatic joints is
the Green Tuff, Pantelleria, here overlain by aa.

30. Prismatic joints are channels for alteration; eroded top of
the Green Tuff, Pantelleria,

31. Prismatic joints, Howenegg, Germany

32. Prismatic joints in a thick
lava flow, Mont Rodeix,
Auvergne, France

33. Prismatic joints in lava flow, Mont Rodeix, Auvergne

34. Prismatic joints
perpendicular to
cooling surfaces,
andesite dikes at
Moiwa, Hokkaido

35. Successive fillings of a paleovalley by lava flows, Hochsimmer
NW of Mayen, Germany

36. Basalt lava filled
a steep-walled
valley, Gembudo,
Honshu

37. Prismatic joints in this lava flow pass downward into pillows.
Vancouver Island, BC

38. Prismatic joints in a breadcrust bomb, Fossa di Vulcano. The
ultimate in joints perpendicular to cooling surfaces.

39. Kaldaksvisi Canyon, Iceland, entrenched in jointed basalt.

40. Growth of prismatic joints occurs in short spurts, as
indicated by these "chisel marks", Takurayama, Honshu

41. In some examples, growth spurts show as pinch-and-swell
structures, rather than sharp "chisel marks". Hochtsberg
quarry, Germany.

42. Not all prismatic joint patterns are easily interpreted. Note
the "chisel marks". Gembudo, Honshu.

43. entablature
colonnade
entablature
colonnade
One popular generalization is that, in many lava flows,
prismatic joints (the "colonnade") are overlain by platy
or unoriented joints (the "entablature"). Columbia River
Basalts, Clearwater River, ID.

44. Entablature Colonnade The terms come from architecture; neoclassical
building, Bristol, England. In nature, the entablature forms
when water gets into unoriented fractures in the
top part of a cooling flow.

45. entablature colonnade
A typical sequence in Columbia River basalt flows.

46. When lava flows are stacked and lack entablatures, does
this mean that the top of each flow was eroded before
the next one erupted? Dettifoss, Iceland.

47. At Hjalparfoss, Iceland, there is an entablature in the
top left, but joints of the colonnade diverge downward, and those in the upper right terminate against a vanished vertical surface (ice?).

48. Joints in a dike, Hljodarkletter, Iceland.