Geology Reference
In-Depth Information
Milankovi5 originally composed his hypothesis based on mathematical calculations and
astronomical understanding. Since then, scientists have gathered other evidence and
confirmed cycles of colder ocean temperatures occurring in sync with Milankovi5's pre-
dictions.
Rebounding isostatically
Each time an ice sheet advances across a continent, all that added weight of the ice
causes the continental crust to sink into the mantle below just a little bit deeper. (See
Chapter 9 for a description of how the crust and mantle interact.) The result of all that
ice pushing down on parts of the continent changes the relative sea level along the
coasts.
Scientists use the word
relative
because no unchanging point exists against
which to compare changes in sea level or continent level. Any change in sea level
has to be described relative to where the continent sits.
So much water is contained in the ice that you may expect the sea level to be lower, and
indeed there is less liquid water in the ocean when so much of it is frozen. But at the
same time, the continent is sinking downward under the weight of the ice, so the change
in sea level relative to the position of the continent along some coastlines is not as dra-
matic as you may expect.
When the ice sheet melts, the sea level rises relative to the continent — at first. In the
years following the disappearance of the ice sheet, the continent, which had been
pushed down by the weight of the ice, begins to slowly bounce back up (because the
weight is removed). This shift is happening right now in response to the end of the last
ice age 14,000 years ago. Continents in the northern hemisphere (including North Amer-
ica) are slowly adjusting, “floating” in the mantle — much like an ice cube that's
dropped in a glass of water floats up to the top. This movement in response to the re-
moval of the weight of the ice sheet is called
isostatic rebound.
When you drop an ice cube into a glass of water, it doesn't just sink and then bob up to a
floating position. It takes a few seconds of bobbing motion, and perhaps some tilting and
shifting, before it is balanced and floating just right. As the continents experience iso-
static rebound, they go through a similar period of adjustment, but over a much longer
time. For this reason, some regions may appear to be moving upward while others are
moving slightly downward or not moving at all.
