How Does Lava Move When It Is Very Thick?

Lava is one of the most fascinating and dangerous substances on Earth. When it erupts from a volcano, its movement can vary drastically depending on its viscosity, or thickness. Thick lava, known as "viscous lava," moves in unique and often dramatic ways that captivate scientists and observers alike. To truly understand how thick lava moves, it's essential to first grasp how its properties differ from more fluid types of lava.

What is Viscous Lava?

Viscosity refers to a fluid’s resistance to flow. Thick or viscous lava has a higher silica content, which makes it denser and more resistant to movement compared to the more fluid types of lava like basaltic lava. The higher the silica content, the thicker the lava becomes, slowing down its flow. This characteristic causes viscous lava to behave in ways that can be both mesmerizing and terrifying.

Unlike the fast-moving rivers of lava often seen in movies, thick lava can barely crawl along the surface, moving at speeds that can be as slow as a few meters per day. However, its slow speed does not diminish its destructive power. In fact, it often leads to more intense pressure building beneath the surface, causing violent eruptions and massive explosions.

Blocky Flows and Spines: How Viscous Lava Moves

The movement of thick lava is characterized by blocky, fragmented surfaces known as “block lava.” Unlike the smooth flows of more fluid lava, viscous lava moves like a bulldozer, piling up rocks and boulders in its path. The lava cools quickly on the surface, forming a solid crust that breaks apart as the molten material underneath continues to push forward. This process forms what is called a "lava spine," where the lava towers upwards like a slow-growing skyscraper of molten rock.

As the lava moves, it often creates steep-sided domes, which can reach hundreds of feet in height. These domes grow slowly over time, often collapsing in on themselves when they become too large to support their own weight. The result is a cycle of growth and collapse that can continue for months or even years, as the lava oozes out and piles up in a dramatic and often unpredictable fashion.

Explosive Eruptions and Lava Domes

Because thick lava is slow to release gas, it tends to build up extreme internal pressure. The gas is trapped within the molten rock, unable to escape easily due to the high viscosity. This pressure can result in violent, explosive eruptions that send ash, rocks, and molten lava flying into the air. These eruptions are some of the most dangerous volcanic events, capable of causing widespread destruction.

One of the most famous examples of thick lava causing explosive eruptions is the eruption of Mount St. Helens in 1980. The eruption was so powerful that it blasted away the entire top of the mountain, sending an enormous column of ash and gas 80,000 feet into the sky. This eruption was caused by the slow-moving, viscous lava that had been building pressure beneath the surface for weeks.

Thick lava often creates lava domes, which are large, dome-shaped structures that form over time as the lava slowly extrudes from the volcano. These domes can collapse without warning, sending pyroclastic flows — fast-moving avalanches of hot gas and volcanic material — down the slopes of the volcano, posing significant risks to nearby communities.

How Does Temperature Affect Thick Lava Movement?

Temperature plays a crucial role in how thick lava moves. As lava cools, it becomes more viscous and resistant to flow. The outer layer of thick lava cools quickly upon exposure to the atmosphere, forming a hardened crust. Beneath this crust, the lava remains molten and continues to flow slowly. The cooler the lava gets, the slower it moves, sometimes coming to a complete stop before reaching the base of the volcano.

However, the interior of the lava can remain hot for long periods, allowing it to continue moving under the crust. This creates a strange, almost mechanical movement where the crust cracks and shifts as the molten core pushes forward. This kind of movement is often seen in what are known as "a'a" flows, where the lava moves in jagged chunks rather than smooth, continuous streams.

Viscous Lava in Different Volcanic Regions

Thick lava is most commonly associated with stratovolcanoes, which are large, steep volcanoes built from layers of lava, ash, and volcanic debris. These volcanoes tend to erupt explosively, due in part to the viscous nature of their lava. Famous stratovolcanoes with thick lava flows include Mount Fuji in Japan, Mount Vesuvius in Italy, and Mount Pinatubo in the Philippines.

In these regions, the lava's high viscosity often leads to the formation of massive volcanic domes and spines, which grow slowly over time. The lava in these areas can remain active for years, causing ongoing volcanic hazards long after the initial eruption has ended.

The Danger of Thick Lava to Human Settlements

While thick lava moves slowly, its unpredictable nature makes it particularly dangerous to nearby human settlements. It can flow through populated areas, burying homes, roads, and infrastructure under tons of rock and ash. The slow speed of the lava gives people time to evacuate, but the long-term impact on the land can be devastating.

Furthermore, the potential for explosive eruptions poses a significant threat to nearby communities. Volcanic eruptions can send ash clouds high into the atmosphere, disrupting air travel, and causing widespread damage to agriculture, water supplies, and the environment. The combination of slow-moving, thick lava and explosive eruptions creates a deadly mix that has claimed countless lives throughout history.

How Can We Predict the Movement of Thick Lava?

Predicting how thick lava will move is a challenging task for scientists. Because of its high viscosity, thick lava can change direction unexpectedly, and its flow can stop and start without warning. Modern technology, including satellite imagery and ground-based monitoring systems, allows scientists to track the movement of lava in real-time. These tools provide critical data that can help predict where the lava will go next and how fast it will move.

One of the key factors in predicting lava movement is understanding the internal dynamics of the volcano. By monitoring gas emissions, seismic activity, and temperature changes, scientists can get a better sense of how much pressure is building beneath the surface and when an eruption is likely to occur. This data is crucial for issuing timely warnings to nearby communities and minimizing the impact of volcanic activity.

Conclusion

The movement of thick lava is a slow but powerful process, capable of reshaping entire landscapes and posing serious risks to human life. Its high viscosity makes it unique, creating blocky flows, lava spines, and explosive eruptions. While its slow speed might seem less threatening, the dangers associated with thick lava, including volcanic domes and pyroclastic flows, make it one of the most hazardous types of volcanic activity. Advances in technology are helping scientists better understand and predict the movement of thick lava, but its unpredictable nature continues to be a challenge.