of the Phanerozoic eon.
#More to explore
Atmosphere • Continent • Earthquake
• Erosion • Geology • Gravity
• Hemisphere • Moon • Planets • Season
• Solar System • Volcano
Earth Day
Every April 22, people celebrate Earth
Day to show how much they care about
the planet. Many people use the day to
do projects that will help Earth. Some
meet to discuss how to limit pollution
and how to save endangered animals and
plants. Others plant trees, clean up parks
and beaches, or set up recycling stations.
In some places concerts and organized
walks raise money for environmental
groups.
Gaylord Nelson, a U.S. senator, founded
Earth Day. He asked Denis Hayes, a
college student, to organize the first
Earth Day events. About 20 million
Americans celebrated the first Earth Day
in 1970. The day made more Americans
aware of the damage that people had
done to the environment. In 1990
Hayes organized the first International
Earth Day. About 200 million people in
141 countries took part.
#More to explore
Pollution • Recycling
Earthquake
In an earthquake, huge masses of rock
move beneath Earth’s surface and cause
the ground to shake. Earthquakes occur
constantly around the world. Often they
are too small for people to feel at all.
Sometimes, however, earthquakes cause
great losses of life and property.
Causes
Most earthquakes are caused by changes
in Earth’s outermost shell, or crust. The
crust is not a solid mass. Instead it is
made up of about a dozen rock masses
called plates that are constantly moving.
In different places they move apart, collide,
or slide past each other. Over time
this movement causes great pressure to
build up. When the pressure becomes
too great, the rock masses suddenly shift
along a crack in the crust, called a fault.
The shifting rocks release energy in the
form of shock waves. The waves spread
through the rock in all directions, causing
an earthquake. In the most powerful
quakes, people thousands of miles away
Volunteers clear overgrowth from a hill in
an Earth Day clean-up effort.
The San
Andreas Fault
in southern
California is
about 650
miles (1,050
kilometers)
long. It can
easily be seen
from above
the ground.
BRITANNICA STUDENT ENCYCLOPEDIA Earthquake 91
from the center of the quake can feel the
ground shake.
A small number of earthquakes have
other natural or human causes. The
movement of molten rock beneath a
volcano can trigger an earthquake. The
pressure resulting from holding vast
amounts of water behind large dams can
cause minor quakes. So can underground
nuclear explosions and the digging
of mines.
Effects
An earthquake may be powerful enough
to change the surface of Earth, thrusting
up cliffs and opening huge cracks in the
ground. Earthquakes can do great damage
to buildings, bridges, railroads, and
other structures. The violent shaking
during earthquakes often causes other
Earth movements, such as avalanches.
Some quakes happen in or near oceans.
These quakes can cause huge, destructive
waves called tsunamis to sweep
ashore.
Where Earthquakes Occur
Most earthquakes take place along the
boundaries of Earth’s crustal plates. One
great earthquake belt circles the Pacific
Ocean along the mountainous west
coasts of North and South America and
runs through the island areas of Asia. It
is known as the ring of fire. The famous
San Andreas Fault of California is part
of this belt.
A second, less-active belt is between
Europe and North Africa in the Mediterranean
region and includes portions
of Asia. A small number of quakes occur
in the interior of plates rather than along
the edges.
In an earthquake, the ground can move up
and down. It can also move sideways.
An earthquake in Japan was
powerful enough to destroy an
elevated expressway.
92 Earthquake BRITANNICA STUDENT ENCYCLOPEDIA
Measuring Earthquakes
Scientists record the shock waves produced
in an earthquake with an instrument
called a seismograph. They
measure the strength of a quake in different
ways. The Richter scale measures
earthquakes based on the amount of
energy they release. The weakest earthquakes
are close to zero on the scale; the
strongest measure about 9. The Mercalli
scale measures the amount of destruction
caused by an earthquake on a scale
of 1 to 12.
#More to explore
Avalanche • Earth • Energy • Plate
Tectonics • Ring of Fire • Tsunami
• Volcano
Easter
Easter is the holiest day of the year for
Christians. It celebrates their belief in
the resurrection, or the rising from the
dead, of Jesus Christ. Jesus was the
founder of the religion of Christianity.
Easter is always observed on a Sunday in
the Spring, but its date varies.
The week before Easter Sunday is
known as HolyWeek. During this week,
Christians remember the events they
believe took place at the time of Jesus’
death. Palm Sunday recalls the entrance
of Jesus into Jerusalem shortly before his
death. Holy Thursday marks the Last
Supper of Jesus with his disciples, or
followers. Good Friday remembers the
crucifixion, when Jesus was killed by
being nailed to a cross. Christians
believe that Jesus was resurrected on the
third day after his death. Easter is therefore
a celebration of new life.
