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In February of 1942, 
Mexican farmer Dionisio Pulido

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thought he heard thunder 
coming from his cornfield.

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However, the sound wasn’t coming 
from the sky.

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The source was a large, smoking crack 
emitting gas and ejecting rocks.

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This fissure would come to be known as 
the volcano Paricutin,

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and over the next 9 years, its lava 
and ash would cover over 200 square km.

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But where did this new volcano come from,

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and what triggered 
its unpredictable eruption?

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The story of any volcano 
begins with magma.

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Often, this molten rock forms 
in areas where ocean water

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is able to slip into the Earth’s mantle 
and lower the layer’s melting point.

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The resulting magma typically remains 
under the Earth’s surface

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thanks to the delicate balance 
of three geological factors.

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The first is lithostatic pressure.

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This is the weight of the Earth’s crust 
pushing down on the magma below.

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Magma pushes back with the second factor, 
magmastatic pressure.

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The battle between these forces 
strains the third factor:

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the rock strength of the Earth’s crust.

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Usually, the rock is strong enough 
and heavy enough

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to keep the magma in place.

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But when this equilibrium is thrown off, 
the consequences can be explosive.

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One of the most common causes 
of an eruption

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is an increase 
in magmastatic pressure.

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Magma contains various elements 
and compounds,

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many of which are dissolved 
in the molten rock.

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At high enough concentrations, compounds 
like water or sulfur no longer dissolve,

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and instead form 
high-pressure gas bubbles.

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When these bubbles reach the surface,

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they can burst with the force 
of a gunshot.

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And when millions of bubbles 
explode simultaneously,

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the energy can send plumes of ash 
into the stratosphere.

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But before they pop, they act 
like bubbles of C02 in a shaken soda.

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Their presence lowers 
the magma’s density,

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and increases the buoyant force 
pushing upward through the crust.

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Many geologists believe this process 
was behind the Paricutin eruption

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in Mexico.

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There are two known natural causes 
for these buoyant bubbles.

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Sometimes, new magma 
from deeper underground

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brings additional gassy compounds 
into the mix.

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But bubbles can also form 
when magma begins to cool.

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In its molten state, magma is a mixture
of dissolved gases and melted minerals.

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As the molten rock hardens, some of those 
minerals solidify into crystals.

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This process doesn’t incorporate 
many of the dissolved gasses,

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resulting in a higher concentration 
of the compounds

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that form explosive bubbles.

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Not all eruptions are due 
to rising magmastatic pressure—

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sometimes the weight of the rock 
above can become dangerously low.

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Landslides can remove massive quantities 
of rock from atop a magma chamber,

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dropping the lithostatic pressure 
and instantly triggering an eruption.

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This process is known as “unloading”

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and it’s been responsible 
for numerous eruptions,

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including the sudden explosion 
of Mount St. Helens in 1980.

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But unloading can also happen 
over longer periods of time

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due to erosion or melting glaciers.

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In fact, many geologists 
are worried that glacial melt

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caused by climate change 
could increase volcanic activity.

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Finally, eruptions can occur when 
the rock layer is no longer strong enough

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to hold back the magma below.

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Acidic gases and heat escaping from magma

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can corrode rock through a process 
called hydrothermal alteration,

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gradually turning hard stone 
into soft clay.

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The rock layer could also be weakened 
by tectonic activity.

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Earthquakes can create fissures 
allowing magma to escape to the surface,

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and the Earth’s crust 
can be stretched thin

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as continental plates 
shift away from each other.

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Unfortunately, knowing 
what causes eruptions

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doesn’t make them easy to predict.

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While scientists can roughly determine 
the strength and weight

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of the Earth’s crust,

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the depth and heat of magma chambers 
makes measuring changes

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in magmastatic pressure very difficult.

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But volcanologists are constantly 
exploring new technology

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to conquer this rocky terrain.

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Advances in thermal imaging 
have allowed scientists

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to detect subterranean hotspots.

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Spectrometers can analyze 
gases escaping magma.

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And lasers can precisely track the impact 
of rising magma on a volcano’s shape.

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Hopefully, these tools will help us better
understand these volatile vents

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and their explosive eruptions.