Devils Tower (Wyoming, USA): Igneous Formations
1. Zoom from Uluru, Australia, to Devils Tower (Wyoming)
2. Let’s say you are a Hollywood movie producer, and you are making a movie
about aliens that come to Earth to communicate with humans. You want to
have the meeting place somewhere spectacular and strange, but still familiar
to moviegoers. What you choose? Devils Tower, of course (Steven Spielberg,
Close Encounters of the Third Kind).
3. This lecture is along similar lines to the previous one, but Devils Tower
doesn’t look anything like Ayers Rock. And in fact, the two have almost
nothing in common (other than they are both really big rocks sticking up
above the surrounding land).
4. Examine Devils Tower
a. (aside – was going to be “Devil’s Tower,” America’s first National
Monument, declared by Teddy Roosevelt in 1906. But the official
proclamation mistakenly left off the apostrophe, so it has been Devils
Tower ever since)
b. Rises nearly vertically 386 m (1267 ft) above the surrounding land; is
flat-topped; seems to be made of large columns that can be up to 8
feet in diameter; the columns often taper from the bottom to the top;
the columns are usually 5 or 6-sided, but can be more or less; There
are many columns that have broken off and have fallen to the base,
suggesting that the tower was once even wider at the top.
c. So, how does such a tower form, and why does it seem to be made of
separate columns?
5. As you can probably guess, this is a result of differential erosion, like with
Ayers Rock. (explain)
a. So, what is Devils Tower made of, that it is so tougher than the
surrounding rocks? It is a rock called phonolite, which is a type of
igneous rock in between a granite and a basalt in composition.
i. Igneous Rock – cooled from magma; we have dealing with a lot
sedimentary rocks in recent lectures (limestones, sandstones,
shales). Igneous rock is MUCH stronger – crystals perfectly
interlocking when it xllizes.
ii. Phonolite is rare, but commonly associated with melting in the
middle of a continent by rock underneath, which melts some of
the continental rock that combines with the rising hot rock
from below.
iii. Grey or Grey-green appearance; stands out from the
surrounding sedimentary rocks, which are alternating layers of
sandstones, shales, limestones, and gypsums.
iv. Surrounding rocks formed during about 225- 100 Ma;
sometimes in shallow seas, sometimes above sea level.
v. During the mountain-building process that began in the
western US about 70 my ago and uplifted the Rocky Mountains,
magma intruded into and through the crust in many places,
and there were many episodes of volcanism.
vi. Devils Tower formed from magma that pushed its way up
through the surrounding sed rx about 40 Ma.
b. But here we have to pause in our story, because what happened next
is not known. There are two competing theories:
i. Devils Tower is the conduit of an ancient volcano. The top of
the volcano was once much higher, but the top of it has since
eroded away. However, the igneous rock of the volcanic pipe
was harder than the surrounding sediments, so after 10s of
millions of years, it stands up above the surrounding rx.
1. Uncommon, but not unheard of. For example,
Edinburgh Castle (Scotland) is built on top of an ancient
volcanic plug.
ii. Or, Devils Tower is the remains of a laccolith: a plug of magma
that pushed its way in between sedimentary layers of rocks but
never made it to the surface, and entirely cooled underground.
Then erosion left it exposed (just as with the volcanic conduit).
iii. If it was a volcano, then there should be ash and lava that
erupted at the surface, but this rock isn’t found. However,
because all of the overlying layers of rock have been eroded
and washed away, its not clear that any of this lava an ash
would still be around even if it did occur!
iv. This is an important point. Sometimes the evidence left over
after so many millions of years is not sufficient to determine
something’s geologic history with total certainty. In this case,
geologists all agree that Devils Tower cooled underground
from liquid magma, but whether it ever formed a volcano or
not is still debated.
v. This is a good thing.
1. Shows that science is vibrant, changing, improving our
understanding over time.
2. If it weren’t, and we had it all figured out  I’d be out of
a job.
6. However, one thing that isn’t debated, as strange as it seems, is how the
columns formed.
a. Explain cooling, shrinking, constrained in volume, so cracking occurs
throughout the volume.
b. If cooling occurs very slowly, then you can get nearly perfect
hexagons.
c. Show this for Devils postpile, Korean shore, Hughes Mtn, Tenerife,
Columbia River: happens all the time!
d. Why hexagons? Explain how it maximizes shrinkage with the fewest
cracks (minimum energy: it is hard to break rock!)
e. Also see hexagons in tiling, honeycombs, salt flats, mud cracks, desret
pavement, etc.!
f. Frequent example of the fundamental nature of mathematics. But this
isn’t really correct, as nature doesn’t know about math. It is really a
perfect example of thermodynamics – things happen in the easiest
way possible. Nature is lazy!!!
7. As with Ayers Rock, the Tower is sacred to several tribes, including the
Lakota Sioux, Cheyenne and Kiowa.
a. Creation myths are many and varied, but often involve people being
chased by bears or a giant bear, calling to gods for help, and being
lifted up high in the air for protection. The columns are the scratches
and claw marks of the bears.
b. This creates a conflict with tourism, especially with climbing. Tower is
a favorite technical rock climbing destination in the country, even the
world. Thousands climb it each year.
c. Local native American tribes have asked that people not climb in June,
when there are native ceremonies around the tower. Most people
oblige, though some still climb then.
8. Top 5:
a. Shiprock (new Mexico) – ancient volcanic conduit, with radiating
dikes
b. Giant’s Causeway (Northern Ireland)
c. Fingal’s Cave (Staffa, Scotland)
d. Los Organos (Canary Islands)
Questions:
1) Though perfect hexagonal basaltic columns would be the easiest way for the
cooling rock to contract, this rarely happens, and the columns are usually a
combination of columns ranging from 4-sided to 7-sided. Why?
2) Why do you think the columns only form vertically? Why aren’t there
horizontal cracks?