Answer
The model not only takes into account changes in sea level, but also changes
in land elevation. The weight of past ice sheets forced the land in many northern
areas to subside. Beneath the rigid outer 100 km of the Earth (lithosphere),
there is a layer where higher temperatures and partial melting causes the rock
to behave in a more plastic manner. This layer of the Earth, from roughly 100
to 350 km deep, is called the asthenosphere. Because of its more ductile (plastic)
nature, the asthenosphere can deform more easily than the rigid lithosphere.
As ice sheets covered parts of the lithosphere, the more mobile material in
the underlying asthenosphere shifted slightly to accommodate the ice's additional
weight. After the ice sheets melted, this material shifted back and the land
slowly rebounded. The rebound was much slower than melting of the glaciers,
so there was a time lag between the glacier's retreat and the land's rebound.
Hudson's Bay is still rebounding from the last glacial retreat, 10,000 years
ago. The bay is surrounded by a series of low ridges that formed as beaches,
during still stands in the shoreline's retreat.
Scandinavian Coastline
Knowledge of crustal rebound preceded the recognition of glacial events by
centuries. Scandinavia has slowly risen throughout its recorded history. Early
Viking law included provisions that determined who could rightfully claim the
land that had risen from the sea. The rate at which the land rose was very slow,
but even within the lifetime of an individual it was noticeable. Later in this
module there is a movie of the model centered over the North Atlantic that shows
the impact of this rebound on the Scandinavian coastline over the past few thousand
years.
Werner and Neptunism
The phenomena of the changing Scandinavian coastline was also known and documented
by 18th century naturalists - although at the time they weren't certain whether
the land was rising or sea level dropping. A renowned German geologist named
Werner, thought that the latter interpretation was correct. He combined this
apparent reduction of the world's oceans with other observations to build a
theory of Earth's past. Werner thought that the Earth had once been covered
by a vast primordial ocean, and that all rocks had been precipitated or deposited
from this slowly diminishing sea. This idea became known as Neptunism, and was
the major theory that opposed Hutton's idea of an Earth driven by internal heat.
Many of Werner's observations were valid, although a few critical interpretations
were eventually disproved. Among these was the apparent sea level fall along
the Scandinavian coast. Instead of sea level falling, the land was slowly rising,
adjusting to the reduced load of a vanished ice sheet. This rebound of Scandinavia
has created a long-term problem for the neighboring Netherlands. As part of
the asthenosphere shifted from beneath Scandinavia, it raised the Netherlands'
elevation. When the ice melted and Scandinavia began to rise, the Netherlands
began to slowly sink. A decrease in elevation over parts of the Netherlands
has been documented since at least the thirteenth century, although it had been
happening for centuries before then. Recently, this change has been accelerated
by fossil fuel production. As oil and natural gas are removed from porous rocks,
the overlying land surface can subside. The Netherlands had vast hydrocarbon
reserves, but production of this resource added to the long-term subsidence
due to glacial changes. At present a significant part of the Netherlands is
actually below sea level and this area is increasing. Considering that the Netherlands
is one of the more densely populated parts of Europe, this in not a trivial
problem.
Question:
How much does a glacier weigh? Presently there is up to 3 kilometers of ice
over parts of Antarctica. Glaciologists have estimated that 21,000 years ago,
there may have been more than 2 kilometers of ice over parts of Canada.
Density:
crustal rocks 2.7 g/cm3
glacial ice - 0.9 g/cm3
What thickness of rock would have an equivalent mass to a two-kilometer thick
ice sheet?