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.