Earthquakes
An earthquake is the shaking and trembling that result from the sudden
movement of part of the Earth’s crust.
Scientists estimate that more than a million earthquakes occur each year, but
only about 20 of them cause significant damage.
What causes earthquakes? Most earthquakes happen at faults. Faults are
breaks in the Earth’s crust where the surrounding rock has moved or shifted.
Earthquakes usually occur at faults that are along plate boundaries. The plates of
the Earth push together, pull apart, or slide past one another. As these plates move,
they put pressure on themselves and each other. When the forces get great enough,
the crust is forced to break. The stress is released as energy moves through the
Earth in the form of waves, called seismic waves.
Almost 80% of the Earth’s earthquakes occur along the Ring of Fire, which
circles the Pacific Ocean.
Plate movements usually happen far below ground. The point beneath the
Earth’s surface where the rocks break is called the focus – the origin of the
earthquake. Directly above this, on the Earth’s surface, is the epicenter. During an
earthquake, the most violent shaking is found at the epicenter.
Earthquakes also occur on the floor of the ocean. These earthquakes often
produce giant waves called tsunamis.
A seismograph measures the strength and duration of an earthquake. The
size of the earthquake is called its magnitude. The strength of the earthquake is
rated using a Richter Scale.
Volcanoes
A volcano is an opening in the Earth’s crust through which steam, lava, and
ashes erupt.
What causes volcanoes? When magma (molten rock) gets close to the
surface, it collects in magma chambers. Magma and hot gasses continue to collect in
these magma chambers until the pressure gets too great for the chamber to hold any
more. Then it erupts. When magma reaches the Earth’s surface, it is called lava.
Where do volcanoes occur?
 Divergent plate boundaries: the plates of the Earth pull apart. Magma seeps
up and fills the gap. Most volcanoes occur at divergent plate boundaries.
Most of them are located under the ocean
 Convergent plate boundaries: the plates of the Earth come together. Once
plate slides under the other (subduction) leaving a gap for magma to leak out

Hot Spots: localized areas of hot, rising molten magma material. The magma
melts away the oceanic crust and erupts to build an active volcanic island.
 The Ring of Fire surrounds the Pacific Ocean, formed at convergent
boundaries. The Ring of Fire has more earthquakes and volcanoes than any
other place on Earth.
How are volcanoes classified?
 Active volcanoes: erupt continually or periodically (every once in a while)
 Dormant volcanoes: have erupted during modern times, but are now
inactive
 Extinct volcanoes: have not erupted within modern history and are not
expected to ever erupt again.