Magma is a molten material that is present beneath the surface of the Earth. As a result of volcanic activity, magma is extruded to the surface and once this material reaches the surface, it is known as lava. The lava will eventually cool on the surface and form igneous rock. The type of lava that will come to surface is dependent on the composition of the magma that it came from. There are three main types of lava and these include mafic, intermediate, and felsic.
Mafic lava will come from mafic magma and is more commonly known as basaltic lava. This type of lava is the most common by volume. Basaltic lava has lower dissolved gas content and also lower amounts of silica than the other lava types. The silica content in basaltic lava is around 50% and it has large amounts of iron and magnesium. The lower silica content makes basaltic lava less viscous, meaning that it will flow much easier and is reminiscent of a heavy cream consistency. The lava flows will therefore be thin and widespread. The temperature of this lava will exceed 950 degrees Celsius. Due to its lower viscosity, basaltic lava can flow for more than 10 miles across the surface.
Basaltic lava is also has three different types of lava flows. These flow lavas are A’a, Pahoehoe, and pillow lava. They are basically just a description of what the lava looks like as it flows over the surface. A’a lava is very rough and its surface is comprised of broken pieces of cooled lava blocks called clinker. Pahoehoe forms a very smooth surface as it moves across the land and advances in a serious of continuously forming loves. Pillow lava is basalitic lava that emerges underneath the surface of water. Because of contact with the water, it cools rapidly and gains a solid crust but this crust cracks and forms series of large ‘pillow’ or blobs of cooled igneous rock.
Intermediate lava is also commonly called andesic lava. Andesic lava commonly emerges to the surface at plate boundaries in the Earth’s crust. This kind of lava has high amounts of iron and magnesium but also larger amounts of dissolved gases and silicates than the basaltic lava. The composition of silicates in andesic lava is about 60% and the additional 10% from the basaltic level of silicates make this type of lava more viscous than the basaltic lava and therefore it is thicker and doesn’t flow as well. The temperature range of this lava is between 750-950 degrees Celsius so it is cooler than the basaltic lava.
Andesitic lava has two common lava flows associated with it and they are block lava flow andesitic lava domes. As a result of the slower moving lava and extra gas content, the lava will be much rougher than the A’a lava and have many large rocks in it. The slower moving lava will have more time to cool but will still move and so it will have much more cooled lava traveling with it and these pieces will get larger. Also, because of the increased dissolved gas in the lava, it will likely have a more powerful and explosive eruption. Before eruption, some of the dissolved gas will be given off while it is still magma and increase pressure which can result in an explosive eruption that can throw material a good distance away from the site as well as cause material to break apart and get swept away by the lava flow. The dome feature is created due to extrusion of viscous lava that will quickly harden and begin the formation of a dome.
Felsic lava comprises two lava types known as rhyolite and dacite. Sometimes it is just referred to as rhyolite lava. Feslic lava has more silica than any other type of lava. Rhyolite has around 70% silica while dacite has around 65%. This makes felsic lava the most viscous and slowest type of lava. It also has high amounts of aluminum, sodium, potassium, calcium, and the lowest amounts of iron and magnesium. The temperature range for felsic lava is typically 650-750 degrees Celsius however some felsic lava can reach temperatures of 950 degrees Celsius in which case the lava would be less viscous because of the temperature and could travel distances over 5 miles.
Felsic lava is associated with lava domes and lava spines. These two features are due to the extrusion of viscous lava through an opening and it steadily builds up to either a spine or a dome. The spine will form due to a volcanic vent whereas the dome will be due to a volcano. These features are typically associated with pyroclastic deposits that could easily cause pyroclastic flows during an eruption. Also given the increased amount of dissolved gas in the lava and therefore the magma, the forming of a dome could result in a much larger and explosive eruption because of building pressure.