A few times on Britarch the question of why archaeology gets buried has
come up. In a few cases such as wind blown sand, the reason is obvious,
but I've never really understood how a building can just appear to sink
into the ground. How now I think I've an idea why!
Last night I was thinking of three things:
* Kostas theory that the Stones of Stonehenge were moved there by ice
* The Caterpillar effect (which affects tectonic plate movement over
ice-ages - which is supported by increases in volcanic activity &
modulation of mid-oceanic ridge formation)
* A question I was asked about whether the Caterpillar effect could be
seen on yearly temperature changes.
And it clicked!!
Every day the earth heats up and cools. And similarly over a period of a
year. Each time the earth goes through this cycle, the particles of the
earth expand and contract causing a regularly movement of the earth akin
to giving it a shake. Now one "shake" each day is hardly going to make
the earth move much for us humans who don't stand in one place very long
at all. But for a rock or a building, this constant daily and yearly
"shaking" effectively turns the earth into something quicksand.
That could explain why many surface objects sink! They are literally
sinking due to regular thermal movement of the top layers!
However ... the "shaking" rapidly reduces in intensity as we go down
into the soil. Daily changes only have a significant change within the
top 20cm. Yearly within the top few meters. So, in principle a rock or
other feature, ought to fairly rapidly descend the first 20cm or so into
soil, and then only very very gradually sink further. And indeed at some
point (10,000yrs?) it will reach the bedrock
Has anyone done any studies into the average depth buildings sink (or as
it might be phrased, earth covers) a building, a standing stone or
Also, presumably, thermal expansion and contraction is an important
effect on standing structures. It would probably leading to regular
movement in walls, which over time would "grind down" presumably leading
to instability and collapse?
CATERPILLAR EFFECT (For any who have not heard about it)
Each glacial cycle the earth heats up and cools around 5-8C. Just as
the yearly cyclic changes in heating and cooling and thus expansion and
contraction reach a couple of meters into the ground, the ice-age cycle
reaches a few kilometres into the ground. This causes the whole surface
of the earth to expand and contract successively pushing against each
other and then contracting. The expansion per unit of rock is small, but
because (surface) rock is largely incompressible, the result is that the
rock is forced to expand and over 40,000km of the earth's circumference
the surface expansion is equivalent to a couple of kms of movement over
an ice-age cycle.
Similar effects are seen of lake & sea ice - where pressure ridges form
due to daily or other changes in the air temperature. However in the
crust, rather than pushing up to form a ridge, expansion (usually)
causes crust to be pushed down when it is heated. This pushes rock down
to where part is thermally decomposed giving rise to a sharp increase in
volcanic activity as we come out of an ice-age ( Lund et al (2016)
<http://science.sciencemag.org/content/351/6272/478>). However on
contraction, the location of the effect is different: the crust tends to
break apart at mid-oceanic ridges. This increase the rate of mid-oceanic
ridge formation as shown by *Maya Tolstoy (Link).
*Thus over each ice-age cycle, rocks pushes out from the mid-ocean
ridges and down into the ground and subduction zones. And then it pulls
away from the mid-oceanic ridges that then fill in. This results is a
creepy effect like the movement of some caterpillars: hence the nickname.