This lesson defines lava and explores the different chemical distinctions of lava. By the end of this lesson, you should be able to name the different types of lava and some of its distinctive morphologies.
Definition of Lava
You’ve likely seen the dramatic pictures of the rivers of fire flowing across the island of Hawaii. This is the image that most people think about when someone mentions lava. What many people don’t know is that this is just one type of lava.
Let’s start with the definition of lava. Lava is molten rock that erupts from volcanic vents. When lava is still within the earth, geologists call it magma.
Lava can erupt during most types of volcanic eruptions, but it’s most commonly associated with effusive eruptions. Effusive eruptions are non-explosive eruptions of lava onto the surface of the earth.Geologists classify lava based on its composition and morphology. We’ll delve into these different types of lava for the remainder of this lesson.
Composition of Lava
Chemical composition affects the behavior and temperature of the lava, in addition to affecting the manner of eruption. There are three main chemical compositions of lava: mafic, intermediate, and felsic.The lava associated with Hawaiian volcanoes are mafic lavas. Mafic lavas are lavas rich in iron and magnesium, while being low in silica.
Basalt is the solidified version of mafic lava. Mafic lavas are the most common lava type on earth. Mafic lavas are also the hottest lavas and normally erupt at over 900 degrees Celsius – that’s 1,650 degrees Fahrenheit!As it turns out, silica content is extremely important for how fluid lava is – also known as its viscosity. Viscosity is the term for lava’s resistance to flow. Silica will bond with oxygen in lava, creating long chains that prevent the lava from flowing freely.
So lavas rich in silica have a higher viscosity, or are more resistant to flowing. Since mafic lavas are low in silica, they’re the most fluid lavas with the lowest viscosity.Intermediate lavas have a medium amount of iron, magnesium, and silica. The silica content is still low enough to allow these lavas to flow, but not as fluidly as mafic lavas.
These lavas cool to form andesite. They are cooler than mafic lava at 750 – 900 degrees Celsius. What’s unique about intermediate lavas is that they only occur in volcanoes located at subduction zones, or zones above the places where oceanic tectonic plates are sinking into the mantle.Felsic lavas have the highest silica and lowest magnesium concentration. Felsic lavas are the coolest and erupt at temperatures less than 750 degrees Celsius.
The high silica concentration of felsic magmas makes them very resistant to flow. In fact, felsic lavas usually only form stiff domes or very limited flows.
Another important way we classify lavas is the morphology, or shape, of the lava. The morphology of a lava flow is heavily influenced by its composition, as we discussed earlier.Mafic lavas on dry land normally have one of two textures: ‘a’a or pahoehoe. As you may have guessed, these names are Hawaiian terms, since mafic lava is so plentiful on Hawaii.
Pahoehoe is the more fluid lava with a smooth, often ropey, texture.’A’a is coarse chunky blocks of broken lava. ‘A’a forms when lava is moving and cooling quickly, causing it to break into many pieces. Many new students remember ‘a’a because of the sound you would make if you were walking over it barefoot: ‘ah! ah!’Perhaps the most common lava type is one that we rarely see because it only erupts underwater. Pillow lavas are bulbous-shaped lava flows that form when the exterior of the lava cools immediately upon touching the cold water.
Pillow lavas are also mafic lavas.Intermediate lavas will form blocky lavas, which are higher viscosity lavas that break off into angular blocks.Felsic lavas will form lava domes.
Lava domes are mound-shaped protrusions of lava from a volcanic vent due to the extremely high viscosity of the lava. Sometimes these dome-building events can also have a limited lava flow, but usually only the eruption phase is the dome extrusion, which occurs much like you would squeeze toothpaste out of the tube.
Lava is molten rock ejected from a volcanic vent normally during effusive eruptions. Lava can take on many different compositions and morphologies. The main composition types are mafic, which are lavas rich in iron and magnesium, while being low in silica; intermediate, which are lavas with medium amounts of iron, magnesium, and silica; and felsic, which are lavas that have the highest silica and lowest magnesium concentration. The concentration of silica in these lavas directly correlates with the viscosity, or the resistance to flow, of the lava.
Mafic lavas commonly come in three morphologies: pillow lava, which is bulbous-shaped lava flows that form when the exterior of the lava cools immediately upon touching the cold water; pahoehoe; or ‘a’a. Intermediate lavas have blocky morphologies, or higher viscosity lavas that break off into angular blocks; and felsic lavas are erupted as lava domes, which are mound-shaped protrusions of lava from a volcanic vent due to the extremely high viscosity of the lava.