Lava
Fresh lava from the Fagradalsfjall volcano eruption in Iceland, 2023 glows against a dark sky. This molten rock has just been expelled from the interior of Earth onto its surface. It emerges at temperatures ranging from about 700 to 1,250 degrees Celsius depending on its chemical makeup. The word lava describes this material whether it erupts directly onto land or falls back down after being ejected into the atmosphere. Solid volcanic rock resulting from subsequent cooling is often also called lava by geologists. A lava flow represents an outpouring during an effusive eruption rather than an explosive one that produces tephra.
Silicate lavas dominate the Earth's crust with oxygen and silicon as their most abundant elements. Petrologists divide these mixtures into four groups based on silica content: felsic, intermediate, mafic, and ultramafic. Felsic lavas contain more than 63% silica and include rhyolite and dacite types. These high-silica rocks are extremely viscous and usually erupt explosively to produce pyroclastic deposits. Intermediate lavas hold between 52% and 63% silica and form andesite domes on steep composite volcanoes like those in the Andes. Mafic lavas range from 45% to 52% silica and typify basaltic flows that create shield volcanoes. Ultramafic lavas drop below 45% silica and represent extreme compositions like komatiite which erupted at temperatures near 1,600 degrees Celsius. No modern komatiite lavas exist today because Earth's mantle has cooled too much to produce them.
The viscosity of most molten lava is about that of ketchup roughly 10,000 to 100,000 times thicker than water. Lava exposed to air quickly develops a solid crust that insulates the remaining liquid helping it stay hot enough to flow great distances. When highly viscous lavas erupt effusively they almost always form blocky flows or domes rather than smooth streams. A typical lava behaves as a Bingham fluid showing considerable resistance until a stress threshold called yield stress is crossed. This results in plug flow similar to toothpaste squeezed out of a tube where shear concentrates only next to the container walls. Once crystal content reaches about 60% the mixture ceases behaving like a fluid and begins acting like a solid known as crystal mush. Lava flow speeds vary based primarily on viscosity and slope with Hawaiian basaltic flows moving slowly but reaching maximum speeds up to 9 kilometers per hour on steep slopes.
Aā forms one of three basic types of flow lava characterized by a rough rubbly surface composed of broken blocks called clinker. The loose sharp spiny surface makes hiking difficult while covering a massive dense core which remains the active part of the flow. Pāhoehoe presents a smooth billowy undulating or ropy surface due to movement of very fluid lava under a congealing crust. These features arise from small lobes and toes breaking out continuously from cooled crusts forming lava tubes that maintain low viscosity over long distances. Block lava flows behave similarly to aā but their more viscous nature causes surfaces covered in smooth-sided angular fragments instead of clinkers. Pillow lava emerges when lava enters water gaining a solid crust that cracks and oozes additional large blobs as more lava advances. This structure is common underwater since most volcanoes sit near bodies of water creating distinctive pillow formations on the ocean floor.
Lava domes form prominent rounded protuberances like those found at Valles Caldera in New Mexico where extrusion of viscous felsic magma builds large pillow-like structures. Lava tubes develop when fluid lava cools on top to form an insulating crust allowing liquid to continue flowing beneath for many kilometers. Permanent lakes of lava exist only at specific sites including Mount Erebus in Antarctica Erta Ale in Ethiopia Nyiragongo in the Democratic Republic of Congo and Ambrym in Vanuau. A lava delta forms wherever sub-aerial flows enter standing bodies of water filling seabed topography such that the flow can move further offshore. Kīpukas denote elevated areas inside active volcanism appearing as forested islands surrounded by barren lava flows. Volcanoes themselves range from broad shallow shield types built by effusive basaltic eruptions to steep stratovolcanoes made of alternating ash and viscous lava layers.
On the night of the 10th of January 1977 a crater wall was breached at Mount Nyiragongo causing a fluid lava lake to drain out in under an hour. The resulting flow sped down steep slopes at up to 60 miles per hour overwhelming several villages while residents were asleep. This disaster led to the mountain being designated a Decade Volcano in 1991. In 1990 Kalapana Hawaii was destroyed by the eruption of the Kīlauea volcano leaving over 100 houses buried under fresh rock. The Nisga'a villages of Lax Ksiluux and Wii Lax K'abit in northwestern British Columbia were obliterated during the Tseax Cone eruption in the 1700s. Plymouth Montserrat became the capital and only port entry for the island before it had to be completely abandoned along with over half its population following the 1995 eruption. Goma Democratic Republic of Congo suffered destruction during the 2002 Nyiragongo eruption while Kapoho Hawaii was largely inundated in June 2018.
Common questions
What is lava and how does it form from the Earth's interior?
Lava describes molten rock expelled by a volcano during an eruption that emerges at temperatures ranging from about 700 to 1,250 degrees Celsius depending on its chemical makeup. Solid volcanic rock resulting from subsequent cooling is often also called lava by geologists.
How many groups of silicate lavas exist based on silica content?
Petrologists divide these mixtures into four groups based on silica content: felsic, intermediate, mafic, and ultramafic. Felsic lavas contain more than 63% silica while mafic lavas range from 45% to 52% silica and typify basaltic flows that create shield volcanoes.
Why do some lava flows move faster than others across steep slopes?
Lava flow speeds vary based primarily on viscosity and slope with Hawaiian basaltic flows moving slowly but reaching maximum speeds up to 9 kilometers per hour on steep slopes. A typical lava behaves as a Bingham fluid showing considerable resistance until a stress threshold called yield stress is crossed.
Which specific volcanoes have permanent lakes of lava today?
Permanent lakes of lava exist only at specific sites including Mount Erebus in Antarctica Erta Ale in Ethiopia Nyiragongo in the Democratic Republic of Congo and Ambrym in Vanuau. These locations maintain active molten reservoirs unlike other volcanic features.
What happened during the eruption of Mount Nyiragongo on the night of the 10th of January 1977?
On the night of the 10th of January 1977 a crater wall was breached at Mount Nyiragongo causing a fluid lava lake to drain out in under an hour. The resulting flow sped down steep slopes at up to 60 miles per hour overwhelming several villages while residents were asleep.