Lava

Understanding Lava: The Molten Rock That Shapes Our Planet

Lava is molten rock expelled from a terrestrial planet’s interior onto its surface. It may be erupted at a volcano or through a fracture in the crust, usually at temperatures from 800 to 1,200 °C. Imagine if the Earth’s core was a giant lava lamp, with the mantle as the liquid and the crust as the glass container – that’s how lava flows out into our world. But what exactly is this molten rock, and why does it matter?

The Composition of Lava

When we talk about lava, we’re talking about a complex mixture of elements. Solidified lava on Earth’s crust is predominantly silicate minerals such as feldspars and quartz. Silicate lavas are molten mixtures dominated by oxygen and silicon, with smaller quantities of other elements. Their composition can be expressed in terms of the weight or molar mass fraction of oxides present.

Think of lava like a recipe – you have your base ingredients (silicon and oxygen), but then there’s a whole array of spices that give it its unique flavor. These include aluminum, magnesium, iron, sodium, potassium, calcium, and more. The proportions of these elements determine the type of lava we see.

Lava Types: Felsic to Ultramafic

Based on silica content, lavas are divided into four chemical types: felsic (silica > 63%), intermediate, mafic, and ultramafic. Felsic lavas have extremely high viscosity due to their high silica content, ranging from 105 Pa⋅s for hot rhyolite lava at 1,200 °C to 108 Pa⋅s for cool rhyolite lava at 800 °C. Unusually hot rhyolite lavas can flow tens of kilometers.

Imagine felsic lava as a thick, sticky honey that barely moves – it’s slow and hard to push around. On the other hand, ultramafic lava is like water – it flows freely and quickly. The difference in viscosity between these two types of lava is what makes them behave so differently.

The Behavior of Lava

Viscosity affects how lava behaves. Higher viscosity leads to more explosive eruptions, while lower viscosity allows for smoother, more fluid flows. Most lavas on Earth are mafic or ultramafic, with intermediate and felsic lavas making up smaller percentages.

Think of it like toothpaste – when you squeeze the tube, a small amount comes out as a semisolid plug because shear is concentrated in a thin layer next to the tube. This is similar to how lava behaves under pressure. The viscosity also determines the aspect, speed, and surface character of flows, creating different shapes such as high-aspect flows, domes, shield volcanoes, and stratovolcanoes.

Flow Speeds and Temperatures

The temperature of most molten lava ranges from 800°C to 1,200°C (1,470°F to 2,190°F), depending on chemical composition. Lava flows quickly develop an insulating crust and cool by slow conduction through the rocky crust.

Imagine a hot summer day where you step out of your air-conditioned home onto the pavement – it feels like stepping into a sauna. That’s how lava behaves when it comes into contact with cooler surroundings. It forms a crust that protects the inside from cooling too quickly, but eventually, it will cool down and harden.

Lava Morphology: Surface Forms

The surface form or texture of lava is crucial in understanding its behavior. More fluid basaltic lava forms flat sheet-like bodies, while viscous rhyolite lava forms knobbly, blocky masses of rock. Two main types of flow lava are ʻaʻā and pāhoehoe.

ʻAʻā is characterized by a rough or rubbly surface composed of broken lava blocks called clinker, which covers a dense core that travels downslope with cooled fragments on its surface. This produces a layer of lava fragments at the bottom and top of an ʻaʻā flow.

Pāhoehoe is basaltic lava with a smooth, billowy, undulating, or ropy surface, formed by the movement of very fluid lava under a congealing surface crust. It forms lava tubes where heat loss maintains a low viscosity and advances as a series of small lobes and toes that continually break out from a cooled crust.

These different textures are like the difference between walking on sand versus concrete – one is smooth, while the other is rough and uneven. The ʻaʻā lava is like walking through deep snow, while pāhoehoe is more like walking on a frozen lake.

Lava Flows: Destruction and Beauty

Lava flows are enormously destructive to property in their path. However, casualties are rare since flows are usually slow enough for people and animals to escape. Nevertheless, injuries and deaths have occurred due to the cutting off of escape routes, getting too close to the flow, or rapid movement of lava flows.

Imagine a river of molten rock flowing through your town – it’s not just scary but also beautiful in its own way. The destruction it causes is like a natural disaster, but the beauty lies in how it reshapes the landscape and creates new landforms.

Lava Deltas: Where Lava Meets Water

A lava delta forms wherever sub-aerial flows of lava enter standing bodies of water. A lava fountain is a volcanic phenomenon in which lava is forcefully but non-explosively ejected from a crater, vent, or fissure.

Think of a lava delta as the meeting point between land and sea – it’s like a bridge that forms when lava meets water. Lava fountains are like fireworks, shooting up into the air with force but without the explosive violence of an eruption.

The Hazards of Recent Lava Flows

Areas of recent lava flows continue to represent a hazard long after the lava has cooled. Where young flows have created new lands, land is more unstable and can break off into the sea. Flows often crack deeply, forming dangerous chasms, and a fall against ʻaʻā lava is similar to falling against broken glass.

Imagine walking on a newly formed beach – it’s like stepping on a thin layer of ice that could crack at any moment. The danger lies in the instability of the new land created by lava flows.

The Future of Lava Research

Lava tubes are known from the modern day eruptions of Kīlauea, and significant, extensive and open lava tubes of Tertiary age are known from North Queensland, Australia, some extending for 15 kilometres. Diverting a lava flow is extremely difficult, but it can be accomplished in some circumstances. The optimal design of simple, low-cost barriers that divert lava flows is an area of ongoing research.

Imagine trying to redirect a river – it’s not easy, and the same goes for lava. Scientists are working on ways to protect communities from these natural disasters by finding better methods to divert or contain lava flows.

Conclusion

Lava is more than just molten rock; it’s a force of nature that shapes our planet in both destructive and beautiful ways. From the fiery heart of the Earth, it emerges as a testament to the dynamic processes that keep our world alive. As we continue to study and understand lava, we gain insights into the very essence of our planet.