A lava flow is the movement of molten rock, known as lava, that has erupted from a volcano and is spreading across the Earth’s surface. This material originates as magma, the term used for molten rock trapped beneath the planet’s crust. The transition from magma to lava is defined by this change in location, marking the moment the material is exposed to the atmosphere. At temperatures typically ranging from 700 to 1,200 degrees Celsius, the characteristics of a lava flow are governed by its composition. Understanding this substance is key to predicting how it will behave, how fast it will move, and what kind of rock it will ultimately form.
The Primary Chemical Building Blocks
The foundation of any lava flow is its chemical composition, which determines the physical properties of the solidified rock matrix. The most important component is silica, a compound of silicon and oxygen (SiO2). Lava is classified based on its silica content, which acts as the dominant control on the molten rock’s behavior.
Lavas with low silica content, typically between 45% and 55%, are known as mafic lavas, which commonly erupt as basalt. These lavas are also rich in iron and magnesium, leading to hotter, more fluid flows. Conversely, lavas with high silica content, such as those found in rhyolite, are called felsic lavas.
Beyond silica, lava contains other elements that form mineral oxides, including aluminum, calcium, sodium, and potassium. Iron and magnesium are generally found in higher concentrations in the more fluid, low-silica lavas. These elements bind together to form the silicate structure that will eventually cool and crystallize into solid rock.
The Role of Volatile Gases
A significant component of lava is the volatile gases dissolved within the molten rock. These gases are kept in solution under tremendous pressure deep beneath the surface. The most abundant volatile is water vapor (H2O), often comprising over 70% of the total gas composition.
Carbon dioxide (CO2) is typically the second most prevalent gas, making up between 10% and 40% of the gas content. Sulfur compounds, primarily sulfur dioxide (SO2) and hydrogen sulfide (H2S), are also present. As the magma rises toward the surface, the lithostatic pressure decreases significantly, causing these gases to “exsolve,” or bubble out of the liquid.
This process of gas release is similar to opening a carbonated drink, where a drop in pressure allows dissolved gas to form bubbles. The rapid expansion of these bubbles as they escape the melt drives volcanic eruptions. The resulting solidified rock often contains small holes, known as vesicles, which are the preserved remnants of these gas bubbles.
How Composition Dictates Flow Characteristics
The proportion of silica in the lava is the primary factor determining its viscosity, which is its resistance to flow. High-silica lavas are highly viscous because the silicon and oxygen atoms link together to form complex, tangled chains, a process called polymerization. This structure makes the molten rock thick and sticky, causing it to flow slowly or pile up into domes.
Low-silica, mafic lavas, which are rich in iron and magnesium, have a lower degree of polymerization. These lavas have a low viscosity, allowing them to flow more freely, sometimes traveling great distances. This difference in viscosity results in two common types of basaltic flows, both named after Hawaiian terms.
The very fluid, low-viscosity flows often form Pahoehoe (pronounced “pah-hoy-hoy”), which has a smooth, ropy, or billowy surface texture. Cooler, more viscous flows of the same composition can transition into Aa (pronounced “ah-ah”), which has a rough, jagged, and clinkery surface. This transition occurs as the lava cools and crystallizes, increasing its viscosity and causing the surface crust to break up under movement.