Figure 8.19: Vesuvius is a dormant volcano near the city of Naples. Although it shows no
current signs of eruption, it could one day become active. ( 19 )
Earthquakes
As magma beneath a volcano pushes upward, it shakes the ground and causes earthquakes.
Although earthquakes probably occur every day near a volcano, the quantity and size of
the earthquakes increases before an eruption. In fact, a volcano that is about to erupt may
produce a continuous string of earthquakes, as magma moving underground creates stress
on the neighboring rocks. In order to measure these earthquakes, scientists use seismographs
that record the length and strength of each earthquake.
Slope Deformation
Allthatmagmaandgaspushingupwardscanmakethegroundorthevolcano’sslopebeginto
swell. Sometimes, ground swelling reveals huge changes in the shape of a volcano. Most cases
of ground deformation are subtle, though, and can only be detected by tiltmeters, which are
instruments that measure the angle of the slope of a volcano. Additionally, ground swelling
may cause increased rock falls and landslides.
Gas Emissions
Oftentimes, gases are able to escape a volcano before magma reaches the surface in an
eruption. So, scientists can measure gas output, or gas emissions, in vents on or around the
volcano. Gases, like sulfur dioxide (SO 2 ), carbon dioxide (CO 2 ), hydrochloric acid (HCl) and
even water vapor can be measured at the site or, in some cases, at a distance with satellites.