Lava: The Science, Composition, and Dynamics of Molten Rock
When molten material is expelled from the interior of a terrestrial planet or moon onto its surface, it is known as lava. Whether it erupts from a volcano or through a fracture in the Earth's crust, this liquid rock can emerge on land, underwater, or even be ejected into the atmosphere. Once this material cools and solidifies, it becomes volcanic rock.
Lava typically erupts at temperatures ranging from 800 to 1,200 °C (1,470 to 2,190 °F). While an explosive eruption produces tephra (a mixture of volcanic ash and fragments), an effusive eruption results in a lava flow—a steady outpouring of molten rock.
Fresh lava from Fagradalsfjall volcano eruption in Iceland, 2023
Key Facts
- Temperature: Most lava ranges between 800 °C and 1,200 °C.
- Viscosity: Molten lava is roughly 10,000 to 100,000 times more viscous than water, similar to the consistency of ketchup.
- Composition: Lava types are primarily classified by their silica (SiO2) content.
- Flow Dynamics: A solid crust often forms on the surface of a flow, acting as insulation that allows the interior to remain liquid and continue moving.
Chemical Composition and Lava Types
The behavior of lava is largely dictated by its chemical makeup, specifically its silica content. Silica levels determine how thick (viscous) the lava is and whether an eruption will be slow and flowing or violent and explosive.
Felsic Lavas
Felsic (or silicic) lavas have a silica content greater than 63%. This includes rhyolite and dacite. Because of the high silica content, these lavas are extremely viscous. While they usually erupt explosively, they can occasionally form lava domes, spines, or thick, short flows known as coulees. These flows often fragment into block lava and may contain obsidian.
A forested lava dome in the midst of the Valle Grande, the largest meadow in the Valles Caldera National Preserve, New Mexico, United States
Intermediate Lavas
Intermediate or andesitic lavas contain between 52% and 63% silica. They are generally richer in magnesium and iron than felsic lavas and are often hotter, ranging from 850 to 1,100 °C. With a viscosity somewhat greater than smooth peanut butter, they tend to form andesite domes and block lavas, often found on steep composite volcanoes like those in the Andes.
Arenal Volcano, Costa Rica, is a stratovolcano.
Mafic Lavas
Mafic (or basaltic) lavas are characterized by high magnesium and iron oxide content, with silica levels between 45% and 52%. These are the least viscous of the common silicate lavas, with a consistency similar to ketchup. Because they flow easily, they often create low-profile shield volcanoes or vast flood basalts. Common forms include ʻaʻā and pāhoehoe, or pillow lavas when erupted underwater.
Pillow lava on the ocean floor near Hawaii
Pāhoehoe lava from Kīlauea volcano, Hawaii, United States
Toes of a pāhoehoe advance across a road in Kalapana on the east rift zone of Kīlauea Volcano in Hawaii, United States
Glowing ʻaʻā flow front advancing over pāhoehoe on the coastal plain of Kīlauea in Hawaii, United States
Comparison of Lava Compositions
| Component | Nephelinite | Tholeiitic Basalt | Andesite | Rhyolite |
|---|---|---|---|---|
| SiO2 | 39.7 | 53.8 | 60.0 | 73.2 |
| Al2O3 | 11.4 | 13.9 | 16.0 | 14.0 |
| FeO | 5.3 | 2.6 | 1.9 | 0.6 |
| MgO | 12.1 | 4.1 | 3.9 | 0.4 |
| CaO | 12.8 | 7.9 | 5.9 | 1.3 |
Rheology and Flow Speed
Rheology refers to the study of how matter flows. For lava, viscosity is the primary driver of behavior. While composition is the main factor, temperature and shear rate also play roles. Generally, lava flows slowly; for example, Hawaiian basaltic flows typically move at 0.40 km/h (0.25 mph).
However, speed can increase significantly on steep slopes, reaching up to 48 km/h (30 mph). In extreme cases, such as the collapse of a lava lake at Mount Nyiragongo, speeds have been recorded as high as 97 km/h (60 mph).
Lava fountain at Kīlauea
Morphology: Pāhoehoe vs. ʻAʻā
Basaltic lavas often manifest in two distinct textures:
- Pāhoehoe: This lava advances in small lobes or "toes" and often forms lava tubes. It has a smooth or billowy surface.
- ʻAʻā: This lava is more viscous and has a rough, jagged, or clinkery surface.
As pāhoehoe flows lose heat and increase in viscosity, they can transition into ʻaʻā. This transition typically occurs at temperatures between 1,170 and 1,200 °C.
Block lava at Fantastic Lava Beds near Cinder Cone in Lassen Volcanic National Park
Shiprock, New Mexico, United States: a volcanic neck in the distance, with a radiating dike on its south side
Lava Hazards and Impact
Lava flows are highly destructive to property and infrastructure. While casualties are relatively rare because flows are often slow enough to allow for evacuation, they can be deadly if escape routes are cut off or if the flow is exceptionally fast, as seen in the 1977 Nyiragongo eruption.
Lava can easily destroy entire towns. This picture shows one of over 100 houses destroyed by the lava flow in Kalapana, Hawaii, United States, in 1990.
When lava enters the ocean, it can create new land, but it also produces laze—an acid haze formed by the interaction of molten rock and cold seawater.
Lava entering the sea to expand the big island of Hawaii, Hawaiʻi Volcanoes National Park
Lava enters the Pacific at the Big Island of Hawaii.
Frequently Asked Questions
What is the difference between magma and lava?
Magma is molten rock located beneath the surface of a planet, while lava is the term used once that molten material is expelled onto the surface.
Why does lava flow for long distances?
As lava is exposed to air, it quickly develops a solid outer crust. This crust acts as an insulator, keeping the interior liquid and maintaining a low enough viscosity to allow the flow to continue moving.
Can lava exist in a liquid state for a long time?
Yes. Because of the insulating properties of the surface crust, residual liquid can remain at great depths for many years. For example, liquid lava was still present 80 meters deep in the Kilauea Iki lava lake nineteen years after its 1959 eruption.
What are lava fountains?
Lava fountains are volcanic phenomena where lava is forcefully but non-explosively ejected from a vent or fissure. They can appear as continuous jets or short pulses.
Are all lava flows slow?
Most are slow enough for people to escape, but they can move very quickly on steep slopes or during sudden collapses, sometimes reaching speeds up to 100 km/h.