Page 90 - (DK) Ocean - The Definitive Visual Guide

88 COASTS AND THE SEASHORE

Coasts and Sea-level Change

A COAST IS A ZONE WHERE THE LAND MEETS THE SEA—it extends from the shoreline
inland to the first significant terrain change. Coastlines constantly alter in response to UPLIFTED TERRACE
This coastal region of New
sea-level change, land-based processes, wave action, and tides. Coasts can be classified Zealand has experienced a
localized sea-level fall in the
into many types, some of which are contrasting—for example, two opposite forms recent geological past, as the
linked to sea-level change are drowned and emergent coasts. The sea-level change itself land was significantly raised
by an earthquake. What was
may either have been global in nature (caused by a change in the volume of ocean beach is now flat clifftop.
water, for example) or only local (stemming from regional uplift or sinking of land).

Global Sea-level Change old, dense upper mantle continental
crust slow-spreading crust
The most important cause of a global change in sea level is an increase or decrease ridge
in the extent of the world’s ice sheets and glaciers. This is related to Earth’s climate.
If it cools, more water becomes locked up as ice, so there is less in the oceans. If it
heats up (global warming), the ice melts and increases the volume of ocean water.
Another cause of global sea-level change, which is also affected by climate, is a rise
ice or fall in ocean temperature. Warming lowers
reduced ocean sheet
water the density of water, so if the upper layers of the
continental crust
oceanic depressed by oceans heat up, they expand and increase the
crust ice total volume of the oceans. Any changes in the
rises
size of the ocean basins, the ocean’s containers,
also impact globally on sea levels. For example, fast-spreading younger, less dense
a change in activity at mid-ocean ridges can raised sea level ridge crust has greater
volume
have such an effect and may be important in
driving long-term sea-level change.
continental crust rises
increased due to unloading
ocean water of ice GLACIAL CYCLES OCEAN-BASIN CHANGE
During an ice age (top) A slow, global rise in sea
the volume of ocean water level can occur when new
is low as water is locked crust is produced at a fast-
up in ice sheets. When spreading mid-ocean ridge.
the ice melts (bottom), The relatively hot, buoyant
the oceans expand, raising new crust swells, pushing
sea levels globally. the ocean water upward.
oceanic crust depressed
Local Sea-level Change Drowned Coasts
Local sea-level change occurs A drowned (or submergent) coast can be the result of either global or
when a particular area of land rises local sea-level rise. There are two types—rias and fjords. In a ria coast,
or falls relative to the general sea the sea-level rise has drowned a region of coastal river valleys, forming
level. One of the main causes is a series of wide estuaries, often separated by long peninsulas.
tectonic uplifting of land, which In a fjord coast, the sea-level rise has drowned one
occurs in regions where oceanic or more deep, glacier-carved valleys. Both types are
RIA COAST
crust is being forced beneath characteristically irregular and indented. Due to The coastline around
continental crust (a process often a significant global rise in sea level over the past Hobart, in Tasmania,
associated with earthquakes). 18,000 years, drowned coasts are common Australia, was formed
Another cause is glacial rebound, worldwide. Ria coasts are particularly prevalent by a rise in sea-level
flooding a series of river
which is a gradual rise of a specific in northwestern Europe, the eastern US, and valleys. Here, the Hobart
area of land after an ice sheet that Australasia. Large numbers of fjords are present Bridge spans one such
once weighed it down has melted. in coastal Norway, Chile, Canada, and New Zealand. drowned valley.
During the last ice age, heavy
OCEAN ENVIRONMENTS SINKING ISLANDS this occurs where a heavy load of coastal sediments
ice sheets covered much of North
America and Scandinavia. Since
the ice melted, these regions have
risen, and they continue to do so
today at rates of up to a few inches
a year. In contrast, other coastal
areas are slowly sinking. Often,
These two volcanic
is pushing the underlying bedrock down. A slow
Pacific islands,
subsidence is occurring, for example, on the eastern
Rai’atea (top) and
Bora-Bora, are
coast of the US. Many volcanic islands also start to
subsiding. Locally,
subside soon after they form. This is due to the fact
the current global
that the material from which they are created cools,
rise in sea level is
compacts, and then contracts, while the sea floor
therefore slightly
exacerbated.
under them warps downward.