Lava Flow Facts For Kids

Lava is molten, or partly molten, rock that comes from deep inside a planet or moon and reaches the surface. It can pour out of a volcano or squeeze through cracks in the ground. Lava can flow on land or under the sea, and sometimes it is thrown up into the air before falling back down as glowing pieces.

When lava cools and hardens, it becomes solid rock. People often call that solid rock “lava” too. Lava can look different depending on how hot it is, what it’s made of, and how it flows.

Lava is very hot—usually between about 800 and 1,200 °C (roughly 1,470 to 2,190 °F). It reaches the surface when pressure or cracks let it escape. Sometimes lava flows out gently, and other times it is thrown up before landing again.

Lava can travel far because a thin crust forms on top as the surface cools. That crust acts like an insulating blanket, keeping the inside lava hot so it keeps moving. Lava also needs a bit of a push to start flowing; scientists call this behavior a Bingham fluid, which means it won’t move until force overcomes a tiny sticking point.

Viscosity is a word that tells how sticky or runny lava is. Most molten lava is about as thick as ketchup and is 10,000 to 100,000 times thicker than water. Hotter lava and some kinds of rock make it runnier, while cooler lava and different minerals make it thicker.

Lava doesn’t always shear smoothly. Often it flows with most of the motion in a thin layer near the surface. This is called plug flow, and it is like squeezing toothpaste from a tube—the outside moves slower while a chunk of the middle slides along more quickly.

ʻAʻā is a type of basaltic lava that makes very rough, broken surfaces. As it moves, the top breaks into jagged pieces called clinker. These sharp pieces ride along on the flow and push forward as the lava keeps moving.

Under the clinker there is a denser, still-hot interior. At the front, chunks tumble and get buried, so the flow builds up layers of broken rock on top and bottom. Sometimes bits stick together into round clumps called accretionary lava balls, which can grow as large as about 3 metres—about the height of a tall person. ʻAʻā is usually thicker and rougher than smoother pāhoehoe lava.

Pāhoehoe is a kind of lava that looks smooth, billowy, or like twisted ropes. It usually comes from runny, basalt lava that spreads out in thin sheets. As the front cools, small lobes and “toes” push forward and break out from a thin crust. This makes the surface look like big ropes or wrinkles—lava sculpture that artists might admire.

Because pāhoehoe stays hot and runny (often more than one thousand degrees Celsius), it can travel far. If it cools and becomes thicker, it can change into a rougher type of lava called ʻaʻā. Some pāhoehoe flows are short, but a few have flowed tens of kilometres.

Block lava flows come from thicker, stickier lava that makes chunky, angular blocks on the surface. These flows are common on steep, cone-shaped volcanoes. Instead of smooth ropes, the top of the flow breaks into strong, blocky pieces with sharp edges and flat faces.

Under the broken surface, hot molten rock keeps moving. The flow pushes forward slowly as the new lava slides over the pile of broken blocks at the front. Because the lava is thicker, block lava moves much more slowly than runny types like pāhoehoe.

Lava tubes form when a smooth layer of lava cools on top and makes a hard roof while liquid lava keeps flowing below. The hardened roof acts like a blanket, keeping the lava underneath hot and able to travel far from its source. Over time this can make long tunnel-like passages underground.

When the eruption ends or the lava slows, the tunnels can drain and leave open caves. People have explored many tubes safely with guides. Some modern examples are found at Kīlauea in Hawaii, and older tubes exist in places like North Queensland, Australia.

Lava domes form when very thick, sticky magma pushes out slowly and piles up near the vent. Instead of flowing far, this lava balloons into a rounded, bumpy mound that can grow taller and wider. As it inflates, the dome’s surface often cracks and becomes covered by broken rock and ash.

If a dome grows on a slope, parts can squeeze out in short, thick streams called coulées (dome flows). These flows travel only a few kilometres but are tall and chunky instead of thin and runny. Famous dome-forming events include domes that grew after eruptions at places like Mount St. Helens.

A cinder cone is a small, steep hill that forms when bits of hot rock and glassy ash fly out of a volcano and pile up around the vent. These bits are called tephra (rock and ash pieces). Cinder cones can throw out hot fragments that fall nearby and make loose, crumbly slopes that are unsafe to walk on.

A spatter cone grows when molten blobs of lava land close together and weld into a mound. Spatter cones can make fast, rough lava that flows a short distance and can burn or cover nearby land. Both cone types can block rivers or roads and bury fields, so they are dangerous to people and towns close by.

Lava can cover buildings and fields, so some towns have been badly damaged or lost. In the 1700s a thick lava flow from Tseax Cone in British Columbia buried the Nisga'a villages of Lax Ksiluux and Wii Lax K'abit.

Other places show different outcomes. Garachico in Tenerife was destroyed by lava in 1706 but later rebuilt. Keawaiki, Hawaii, was covered in 1859 and was abandoned. San Sebastiano al Vesuvio, near Mount Vesuvius, was destroyed in 1944 and later rebuilt. Koae and Kapoho in Hawaii were buried in a 1960 Kīlauea eruption and were abandoned. Sometimes people rebuild; other times the land is too changed.