Basaltic lava flows are one of the most common types of lava flows. Basalt is a type of volcanic rock that is composed mainly of the elements silicon and magnesium, and iron and other metals.
When basaltic lava cools and hardens, it forms an extensive layer of material known as sediment. Lava can flow in many ways, which can be divided into two main categories: effusive and explosive.
Effusive lava flows occur when the lava flow is low in gas content and thus low in explosion risk. This allows the lava to flow a greater distance before cooling and solidifying. Explosive lava flows occur when the flowing lava is high in gas content, making it volatile and explosive.
Basaltic effusive lava flows typically have a fairly smooth, fragmented surface due to how easily the surface material breaks down under pressure from advancing lava.
Lower flow temperatures
Basaltic lava typically has a temperature range of 700 to 1100°F (371 to 693°C). This is much lower than the temperatures needed to melt most rock types.
However, as lava flows down the volcano, it comes into contact with cooler air and water, which cause it to cool and harden. If the air and water are warm enough, the lava can cool and solidify while still flowing.
When this happens, you get a smooth, unfragmented surface on the lava flow. The higher the temperature of the flowing lava, the less chance of this happening. Lower temperature flows have a higher chance of having a smooth surface due to more time for air and water to cool it down.
There are ways to increase your chances of getting a smooth surface on your flow! Try working on lowering your flow temperature.
Higher flow temperatures
When lava cools and hardens, it does so in a specific formation depending on the temperature of the lava and its flow rate. When lava has a slower flow rate and is hotter, it produces more continuous, dense rock.
Lower flow temperatures result in less-dense, less-continuous rocks due to the liquid nature of the lava. These are classified as basaltic lava flows. Basalt is a common mineral found in the earth’s crust, which explains why this type of lava flow is so common.
As mentioned before, these flows can have ropy surfaces or relatively smooth surfaces depending on the cooling process. If the surface of the flow cools faster than the inside does, you will get a ropy surface. If internal parts cool faster than the surface does, you will get a smoother surface.
Coarse-grained basaltic lava
Volcanologists can tell a lot about a lava flow based on how it looks. If it has a rough, fragmented surface, then it is likely very hot lava that rapidly cooled as it flowed away from the source vent.
If the surface is smooth and ropy, then it is likely cooler lava that slowly cooled as it flowed. These are both very important distinctions, as they tell researchers more about how volcanoes function.
How fast or slow something moves or cools can influence what other elements are in the lava and how much rock is left in the material. These details are important for mining and mining research.
There is one more type of coarse-grained basaltic lava flow that has a fairly smooth, unfragmented ropey surface: ultra-mafic flows! These have very high concentrations of magnesium and very low concentrations of silicon and calcium.
Fine-grained basaltic lava
Another type of lava flow is fine-grained basaltic lava. This type of lava is known for its very smooth surfaces. It also has a tendency to form very thin, frag free surfaces as well.
Because it is fine-grained, it flows faster and thinner than non-fine grained lava. This can be due to different temperatures or volume amounts of lava.
Fine-grained basaltic lava flows can have very smooth, uninterrupted surfaces due to the speed at which it cools. As the surface cools, it hardens and prevents any cracks or disruptions in the surface.
Like wax, as the fine-grained basaltic lava cools slowly, it maintains its shape and does not crack or break down. This gives the surface a very smooth texture.
Ropy surface from fragmented crevices
A ropy surface can be produced by lava that has a fairly smooth, unfragmented, ropey surface. Lava of this texture flows down the mountain in long, flowing streams.
As it flows, it leaves behind a thin layer of solidified lava. This layer is very thick and can be anywhere from a few inches to hundreds of feet thick.
Below this layer is what is called the bottomed lava flow. This is when the liquid lava has cooled and hardened into a solid rock base. The bottomed lava flow can be very smooth or very rough, depending on how much movement the liquid lava had as it cooled.
These two textures are both signs of basaltic lavas and can be seen at many volcanoes around the world. Some examples are Mount Etna in Italy, Mt. Yasur in New Caledonia, and Mount Meager in Canada.
Lava with lower silica content will have a rougher surface texture
As mentioned before, the lava flow texture depends on the ratio of lava flow components: olivine, clinopyroxene, Fe-Ti oxides, and gabbroic material.
The olivine content in the lava directly affects the smoothness of the surface. Higher olivine content lava flows will have a rougher surface due to more fragmented lava.
Clinopyroxene content in the lava also influences surface texture. Lava with higher clinopyroxene content will have a smoother surface since it is denser.
The remaining components influence surface texture as well: if there are more Fe-Ti oxides or gabbroic material, the surface will be more fragmented.
Surface texture can also be affected by how long it takes for the flow to cool down and solidify. Slower cooling may cause more rough surfaces due to incomplete crystalization.
Lava with higher silica content will have a smoother surface texture
A lava flow’s surface texture is dependent on the viscosity of the lava and the ratio of silicon to aluminum in the lava. Higher viscosity lava, lave with higher silicon content, will have a smoother surface due to its thicker nature.
Viscosity is a measure of fluid resistance to flow and changes in direction. Higher viscosity fluids require more force to change their rate of flow and direction.
As seen in orange plateau lavas, lower aluminum content lowers the cohesion between crystals, thus breaking down the texture of the lava surface. A more fractured surface appears more smooth than a fragmented one!
These two components work against each other, making it hard to determine whether a lava flow has high or low silicon and aluminum content based solely on its texture.
Volcanoes that produce more viscous lava also tend to produce smoother flows
As mentioned before, viscosity is a measurement of how resistant liquid (or in this case, molten rock) is to flow.
Thicker lava flows take longer to cool and harden, which explains why some lava flows are fractured and have jagged, sharp edges.
Lava with a low viscosity flows very quickly, which is why these types of lava flows are known as “lait” or “laitue” (French for lettuce) due to their shape.
The average thickness of a lava flow cannot be determined unless you have access to the original source volcano crater. However, if you see a very thick lava flow that has smooth edges and a rounded surface, it is likely that it took longer to cool and solidify.