Geoscience Reference
In-Depth Information
VISUAL CONCEPT CHECK
17.1
Once you understand the glacial mass budget and the appear-
ance of a glacier, it is possible to look at a glacier and interpret
where features such as the equilibrium line and zones of ablation
and accumulation are located. Look at this image and see if you
can identify the zones of accumulation and ablation. Where is the
equilibrium line? How can you tell?
Types of Glaciers
KEY CONCEPTS TO REMEMBER ABOUT
GLACIAL FORMATION, MASS BUDGET,
AND FLOW
Now that the processes associated with glacier movement have
been discussed, let's investigate the different kinds of glaciers
by examining the environments in which they form.
1.
Glaciers form when unmelted annual snows reach a
sufficient thickness to promote the metamorphosis of
snow to firn, then to glacial ice.
Glaciers in Mountainous Regions
Perhaps the most obvious place that glaciers form is in high moun-
tainous environments that have long winters and short, cool sum-
mers. Such places are excellent locations for glacier development
because heavy orographic snows often accumulate there. Due to
the short summers, much of this snow is able to remain into the
next winter, forming the foundation of an evolving glacier.
Three fundamental kinds of ice masses occur in such
mountainous areas (Figure 17.8). An
ice cap
is a dome-shaped
sheet of ice that entirely covers less than 50,000 km² (
∙
19,000 mi
2
)
of land area. In contrast, an
ice field
is more constrained by to-
pography and forms in large basins or on top of plateaus. Many
ice fields are located in places where tall mountains, or
nunataks
(“lonely stones”), occasionally rise above the ice. Ice fields are
similar in size to ice caps and can also have many glaciers that
stream down valleys radiating away from the source area.
Ice caps and ice fields are frequently the source area for the
third kind of ice mass found in mountains,
alpine glaciers
, which
are glaciers that flow down valleys away from the high country.
2.
Glaciers originate from source areas where snow ac-
cumulation is greatest. These source areas are typically
located at high altitudes or latitudes where snow does
not melt completely in summer because of cool tem-
peratures that season.
3.
The glacial mass budget refers to the balance between
snow accumulation in the source area and the melting
of ice within the zone of ablation.
4.
Glaciers behave as a plastic that flows most quickly in
the core of the system.
5.
The interior of a glacier is always moving forward, even
though the actual front of the glacier may be retreating
because ablation exceeds accumulation in the source
area.
Ice cap
A dome-shaped sheet of ice that covers an area less
than 50,000 km
2
(
∙
19,000 mi
2
) in size.
Alpine glacier
A glacier in mountainous regions that flows
down preexisting valleys.
Ice field
A topographically constrained sheet of ice in
mountainous areas that frequently has glaciers streaming away
from it.
