Volcanic eruption explained Steven Anderson
In February of 1942,
Mexican farmer Dionisio Pulido
thought he heard thunder
coming from his cornfield.
However, the sound wasn’t coming
from the sky.
The source was a large, smoking crack
emitting gas and ejecting rocks.
This fissure would come to be known as
the volcano Paricutin,
and over the next 9 years, its lava
and ash would cover over 200 square km.
But where did this new volcano come from,
and what triggered
its unpredictable eruption?
The story of any volcano
begins with magma.
Often, this molten rock forms
in areas where ocean water
is able to slip into the Earth’s mantle
and lower the layer’s melting point.
The resulting magma typically remains
under the Earth’s surface
thanks to the delicate balance
of three geological factors.
The first is lithostatic pressure.
This is the weight of the Earth’s crust
pushing down on the magma below.
Magma pushes back with the second factor,
magmastatic pressure.
The battle between these forces
strains the third factor:
the rock strength of the Earth’s crust.
Usually, the rock is strong enough
and heavy enough
to keep the magma in place.
But when this equilibrium is thrown off,
the consequences can be explosive.
One of the most common causes
of an eruption
is an increase
in magmastatic pressure.
Magma contains various elements
and compounds,
many of which are dissolved
in the molten rock.
At high enough concentrations, compounds
like water or sulfur no longer dissolve,
and instead form
high-pressure gas bubbles.
When these bubbles reach the surface,
they can burst with the force
of a gunshot.
And when millions of bubbles
explode simultaneously,
the energy can send plumes of ash
into the stratosphere.
But before they pop, they act
like bubbles of C02 in a shaken soda.
Their presence lowers
the magma’s density,
and increases the buoyant force
pushing upward through the crust.
Many geologists believe this process
was behind the Paricutin eruption
in Mexico.
There are two known natural causes
for these buoyant bubbles.
Sometimes, new magma
from deeper underground
brings additional gassy compounds
into the mix.
But bubbles can also form
when magma begins to cool.
In its molten state, magma is a mixture
of dissolved gases and melted minerals.
As the molten rock hardens, some of those
minerals solidify into crystals.
This process doesn’t incorporate
many of the dissolved gasses,
resulting in a higher concentration
of the compounds
that form explosive bubbles.
Not all eruptions are due
to rising magmastatic pressure—
sometimes the weight of the rock
above can become dangerously low.
Landslides can remove massive quantities
of rock from atop a magma chamber,
dropping the lithostatic pressure
and instantly triggering an eruption.
This process is known as “unloading”
and it’s been responsible
for numerous eruptions,
including the sudden explosion
of Mount St. Helens in 1980.
But unloading can also happen
over longer periods of time
due to erosion or melting glaciers.
In fact, many geologists
are worried that glacial melt
caused by climate change
could increase volcanic activity.
Finally, eruptions can occur when
the rock layer is no longer strong enough
to hold back the magma below.
Acidic gases and heat escaping from magma
can corrode rock through a process
called hydrothermal alteration,
gradually turning hard stone
into soft clay.
The rock layer could also be weakened
by tectonic activity.
Earthquakes can create fissures
allowing magma to escape to the surface,
and the Earth’s crust
can be stretched thin
as continental plates
shift away from each other.
Unfortunately, knowing
what causes eruptions
doesn’t make them easy to predict.
While scientists can roughly determine
the strength and weight
of the Earth’s crust,
the depth and heat of magma chambers
makes measuring changes
in magmastatic pressure very difficult.
But volcanologists are constantly
exploring new technology
to conquer this rocky terrain.
Advances in thermal imaging
have allowed scientists
to detect subterranean hotspots.
Spectrometers can analyze
gases escaping magma.
And lasers can precisely track the impact
of rising magma on a volcano’s shape.
Hopefully, these tools will help us better
understand these volatile vents
and their explosive eruptions.