and slate are examples of metamorphic
rock.
The Rock Cycle
Rock is always being formed, worn
down into pieces, and then formed
again. This is called the rock cycle. Rock
wears down through erosion. Pieces of
rock then settle down and slowly
become sedimentary rock. If sedimentary
rock becomes deeply buried, it may
melt into magma. Then the magma may
return to the surface as igneous rock.
Deeply buried rock may also become
metamorphic rock. The rock cycle takes
many millions of years.
#More to explore
Erosion • Mineral • Sand • Soil
• Volcano
Rocket
Rockets are devices that produce the
force, or push, needed to move an object
forward. Rockets are used to launch
spacecraft. They are also used to shoot
missiles and fireworks.
Limestone, a sedimentary rock, makes good
paving blocks.
Slate is a metamorphic rock that splits off
into flat layers.
76 Rocket BRITANNICA STUDENT ENCYCLOPEDIA
How RocketsWork
Rockets carry fuel that is burned inside a
chamber. The fuel burns when it is
mixed with oxygen gas and ignited, or
set on fire. As the fuel burns, it gives off
hot gas that shoots out from an opening
at the back of the chamber. The force of
the gas moving backward pushes the
rocket forward. This action is called jet
propulsion.
The engines of a jet airplane also use jet
propulsion. But unlike a jet engine,
rockets carry their own oxygen supply.
This makes rockets valuable in outer
space, where there is no oxygen.
Rocket fuel can be liquid or solid. The
U.S. space shuttle uses both liquid and
solid-fuel rockets. Two solid-fuel booster
rockets launch the shuttle into space.
Three liquid-fuel rocket engines allow it
to move in and out of orbit.
History
The Chinese probably invented rockets
in the 1200s. They made rockets by
filling bamboo cases with gunpowder.
The Chinese used their rockets during
religious ceremonies and as weapons. In
the 1700s rockets were improved by
making them with metal. Rockets were
used as weapons in many wars of the
1800s and 1900s, especiallyWorldWar
II (1939–45).
In the late 1800s a Russian scientist
named Konstantin E. Tsiolkovsky got
the idea that rockets could be used to
travel into space. He did not build any
rockets, but his ideas about them were
useful. Robert H. Goddard, a U.S. engineer,
built the first liquid-fuel rocket in
1925. In October 1957 the Soviet
Union used a rocket to launch the first
spacecraft, Sputnik 1, into orbit around
Earth.
#More to explore
Space Exploration •Weapon
Rock Music
Rock is a style of popular music. Since
the 1950s, rock has probably had more
fans than any other form of music.
Rock music has had many styles and
forms over the years. One thing that
most rock music has is a strong beat.
Rock almost always focuses on the
singer or singers. It often features electric
guitars along with drums, electric bass,
and sometimes piano or organ. Some
rock musicians also use saxophones,
trumpets, and other horns in their
music. Rock music styles include country
rock, heavy metal, alternative rock,
punk rock, and many others.
All speed and
distance
records—on
land, in air,
and in
space—have
been set by
rocket systems.
Rocket fuel can be liquid or solid. When the
fuel burns, it gives off gas that pushes the
rocket forward.
BRITANNICA STUDENT ENCYCLOPEDIA Rock Music 77
The Early Days of Rock
Rock first appeared in the United States
in the 1950s. In the early days it was
called rock and roll. It developed from
other music styles, especially African
American popular music (called rhythm
and blues), gospel music, and country
music.
Chuck Berry was one of the first rockand-
roll singers and songwriters. Elvis
Presley was the first rock-and-roll superstar.
Other influential artists of the early
period included Little Richard, Bill
Haley, Bo Diddley, Fats Domino, Carl
Perkins, Jerry Lee Lewis, and Buddy
Holly. Teenagers were the biggest fans of
rock and roll. Rock-and-roll songs
expressed feelings in a way that young
people thought was powerful and truthful.
But many adults thought the music
was too wild.
Later Performers
After the late 1950s rock and roll
changed into many forms. People began
to call it simply rock music. It also
spread to other parts of the world. In the
early 1960s many British groups became
popular. These included the Beatles, the
Rolling Stones, and the Who. U.S. rock
stars of the 1960s included the Beach
Boys, Bob Dylan, Janis Joplin, the
Grateful Dead, Jimi Hendrix, and Led
Zeppelin.
Bruce Springsteen, Aerosmith, Kiss, the
Police, the Talking Heads, and the Clash
were just a few of the rock artists that
appeared in the 1970s. In the 1980s
such rock artists as Madonna, Prince,
R.E.M., Guns N’ Roses, U2, and the
Red Hot Chili Peppers became famous.
In the 1990s and early 21st century Nirvana,
Pearl Jam, Radiohead, No Doubt,
Coldplay, andWilco were some of the
top rock artists.
#More to explore
Beatles, The • Blues • Country Music
• Gospel Music • Popular Music
• Presley, Elvis
Rocky
Mountains
The Rocky Mountains are a massive
mountain range of western North
America. They are called the Rockies for
short. The Rockies include some of
North America’s highest peaks. In the
past they formed a great barrier to
