remains frozen from one year to the
next. Glaciers also slowly flow over the
land. Thousands of years ago, large parts
of the world were covered with glaciers.
At times glaciers covered about 30 percent
of Earth’s surface. At other times
there were fewer glaciers than there are
today.
Where Glaciers Are Found
Glaciers exist all over the world today.
Most of Greenland is covered by thick
sheets of glacier ice, as is Antarctica.
There are smaller glaciers in the Rocky
Mountains of North America, in the
European Alps, in the Andes of South
America, and even at mountainous
points along the equator.
The giraffe is built for reaching the treetops.
Glaciers cover about 10 percent of Earth’s
surface.
116 Girl Scouts and Girl Guides BRITANNICA STUDENT ENCYCLOPEDIA
How Glaciers Form
Glaciers are formed when snow is
packed down into ice. Snow crystals
accumulate year by year in areas where
the temperatures never become high
enough to completely melt the snow
and ice. When a large amount has
accumulated, the snow is under great
pressure from the weight. Eventually
the snow becomes packed so tightly
together that it turns into ice. It usually
takes several years for glacier ice to
form. In some cases, however, it may
take thousands of years.
Effect on the Environment
Glacier ice today stores about three
fourths of all the freshwater in the
world. Many people believe that if temperatures
around the world increased
enough that the glaciers melted, sea
levels would increase greatly and flood
coastal areas. The world’s sea levels are
already rising about 0.08 inch (2 millimeters)
every year. Scientists believe
that melting glaciers cause much of
this rise.
Despite this activity, it is unclear how
much sea levels might eventually rise
because of melting glaciers. Some scientists
believe that if temperatures on
Earth were to rise dramatically, the
warming of the air would cause moisture
to form in the atmosphere. This moisture
would fall as rain and snow, which
could balance out any melting that
might occur.
#More to explore
Antarctica • Greenland
Gladiator
Gladiators were professional fighters in
ancient Rome. Their fierce battles—
sometimes to the death—thrilled stadium
crowds.
Gladiators were usually slaves and criminals.
They trained at special schools.
They used many kinds of weapons.
Some fought with swords, while others
used knives or tridents (three-pointed
spears). Some had to battle wild animals.
The shows were held in huge arenas
called amphitheaters. A fight usually
went on until one gladiator was
wounded. The crowd then decided
whether the loser had fought bravely
enough to be allowed to live.
The winner received palm branches and
sometimes also money. A champion
might be allowed to retire. Sometimes
he gained his freedom.
A gladiator spears a leopard in a Roman
artwork.
BRITANNICA STUDENT ENCYCLOPEDIA Gladiator 117
The first known gladiator show in Rome
took place in 264 BC. Some later shows
went on for weeks and used thousands
of gladiators. Shows were held throughout
the Roman Empire.
Gladiator shows gradually became
unpopular. Constantine I outlawed
them in AD 325. But they may have
continued for at least 100 years after
that.
#More to explore
Rome, Ancient
Glass
Glass is a useful and unique material. It
usually lets light shine through, but it
blocks liquids and air. Glass is easily
breakable, but it can also be made very
strong. And glass can be formed into all
kinds of shapes.
Glass feels hard like a solid, but it is
built more like a liquid. The tiny particles
that make up true solids are
arranged in a specific order. The particles
in glass are arranged randomly, as
in a liquid.
How Glass Is Made
The main ingredient for glass is pure
silica, or sand. It takes very high temperatures
to make glass from sand. By
adding certain chemicals to the silica,
the process needs much less heat.
Chemicals can also make the glass stronger
or add colors to it.
The silica and chemicals are called the
batch. To begin, glassmakers add to the
batch some glass that has already been
made. This scrap glass helps the silica to
melt.
Once the batch is melted, glassmakers
remove any bubbles or streaks. Then the
melted glass can be molded into shapes
or rolled into sheets and allowed to
harden.
Modern machines can quickly and easily
create huge numbers of glass items. But
artists still create unique items through a
method called glassblowing. Glassblowers
blow air through a tube into melted
glass to create different shapes.
Uses
People use glass in countless ways. Glass
windows allow light to pass through but
keep out cold and rain. Electric lightbulbs
are made of glass. Glass containers
store all kinds of things, including food,
drinks, and chemicals. Glass mirrors
Onion-shaped pieces of blown
glass float on a pond. They are
part of an art display.
118 Glass BRITANNICA STUDENT ENCYCLOPEDIA
reflect images. Lenses made of highquality
glass are used in microscopes and
telescopes. Fiber-optic cables are bundles
of tiny glass threads that carry information
in the form of light. These cables
are used to link telephones and computers
over long distances.
History
