Explain why do some volcanoes erupt pyroclastic flows while others produce lava flows?

In short (click here for detailed version)

Some volcanoes produce pyroclastic flows, also known as pyroclasts, when an intense eruption violently ejects volcanic materials, such as ash and rocks, rapidly interacting with the air to form a hot and destructive plume. Other volcanoes emit lava flows when fluid and viscous lava slowly flows down the sides of the volcano, typically during less explosive eruptions.

Explain why do some volcanoes erupt pyroclastic flows while others produce lava flows?
In detail, for those interested!

Chemical composition of magma

The major difference mainly comes from the silicon present in the magma. Simply put: the more silica (silicon dioxide) the magma contains, the thicker and stickier it becomes. This "sticky" magma easily traps volcanic gases, leading to a build-up of pressure. When this pressure becomes too great, it violently explodes, resulting in destructive pyroclastic flows. In contrast, when the magma has low silica content, it is fluid, and the gases can escape gently, producing rather beautiful and tranquil lava flows, like those seen in Hawaii. These fluid flows are often rich in iron and magnesium, unlike highly siliceous explosive magmas.

Influence of magma viscosity

The viscosity of magma, in other words its ability to flow easily or very difficultly, greatly affects the type of eruption a volcano will produce. If the magma is very viscous, imagine thick caramel that sticks to your fingers; the trapped gases will struggle to escape: as a result, the pressure builds up until everything explodes violently, generating pyroclastic flows, those powerful burning clouds that rush down the slopes of the volcano at high speed. Conversely, a well-fluid magma, like hot honey, gently releases the gases that escape without causing strong explosions; the result is rather beautiful, tranquil flows of glowing lava that slowly slide down the slopes without rushing. Everything thus depends on the chemical composition of the magma, which directly influences this famous viscosity.

Role of volcanic gases and internal pressure

In the magma, there are dissolved gases that behave somewhat like the bubbles in a pressurized soda bottle. As long as they remain trapped underground, everything is fine, but when the pressure suddenly drops, these gases can escape, causing a large explosion. If the magma is very viscous, like thick paste, the gases have difficulty escaping and end up causing a violent rupture: this results in the dreaded pyroclastic flows. On the other hand, if the magma is fluid, the gases are released more easily, and you just get calm lava flows. It all comes down to how much gas is trapped and how easily it can escape to the surface.

Impact of the geological structure of the volcano

The shape and internal structure of a volcano greatly influence the type of eruption. Volcanoes with a narrow or blocked conduit build up a lot of pressure: when it cracks, it often results in an explosive pyroclastic flow. Those with a wide and clear conduit allow the magma to rise more gently to the surface, producing fluid lava flows that calmly descend the slopes of the volcano. Steep-sloped volcanoes facilitate rapid avalanches of hot materials, easily forming pyroclastic flows. In contrast, flat and wide volcanoes, known as shield volcanoes, tend to favor slow and long lava flows that are less spectacular and travel far without exploding.

Effects of magma-water interactions

When water comes into contact with molten magma, it suddenly changes from a liquid state to a gaseous state. This very rapid transformation causes a violent expansion: this is a phreatomagmatic explosion. That’s why some volcanoes explode violently when their magma encounters groundwater, glaciers, or even the sea. The result: the formation of large quantities of ash, pulverized debris, and formidable pyroclastic flows. Conversely, in the absence of water, magma flows gently in the form of peaceful lava flows. It is this sudden contact between hot magma and cold water that enhances the explosive nature of eruptions and their unpredictable character.

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Frequently Asked Questions (FAQ)

1

Do magma-water interactions increase the explosiveness of eruptions?

Yes, when hot magma encounters groundwater or surface water, it causes instant vaporization, creating a violent expansion of gases that triggers particularly intense and explosive eruptions.

2

What are the specific dangers associated with lava flows?

Despite their relative slowness, lava flows can destroy infrastructure, cause fires, and permanently alter landscapes. However, their slow advance usually allows time for people to evacuate.

3

Can we predict if a volcano will produce pyroclastic flows during an eruption?

Volcanologists can identify indicators such as the composition of the magma, the geological structure of the volcano, and precursor signs (earthquakes, increased gas emissions) to assess the risk of pyroclastic flow formation. However, accurate forecasting remains challenging.

4

Why do some volcanoes have fluid lava while others do not?

The fluidity or viscosity of lava mainly depends on its chemical composition, particularly its silica (silicon dioxide) content. Low silica content produces fluid lavas and gentle flows, while high silica content results in thick lavas that lead to more explosive eruptions.

5

What exactly is a pyroclastic flow?

A pyroclastic flow is an extremely hot and fast-moving mixture of volcanic gases, ash, and rock fragments that rushes down the slope of a volcano at high speed, reaching temperatures that can exceed 500°C and speeds greater than 100 km/h.

Natural Sciences : Geology

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