Volcano Science Made Easy
This volcano glossary includes photos and videos to help you understand the volcanology terms I'm using in my Volcanoes Are Hot Stuff! series on volcano news, history and amazing videos.
Types of Volcanoes - Shield Volcanoes, Stratovolcanoes, Cinder Cones and Calderas
The stickiness of lava -- its viscosity -- affects the shape of volcanoes and the way they erupt. Sticky, cookie-dough-like lava tends to move slowly, pile up higher, plug up volcanic plumbing, and let go in violent explosions. Runny, low-viscosity lava flows out like wax from a soft candle, spreading out in a puddle and venting off gas so it doesn't explode.
Stratovolcanoes, also called composite volcanoes, form rugged, steep mountains. They consist of layers of rocky debris, ash, pyroclastic flows, and pipe-like "sills" and "dikes" of solidified magma that provide a skeleton. The lava from these volcanoes is mostly andesite, which is cooler and more brittle than basalt (see below). The plumbing of stratovolcanoes may be blocked by "lava plugs" under which the pressure builds, then explodes!
St. Helens, Apr 1980, CVO/USGS
Cinder cones sometimes grow on side vents of other volcanoes; other times they sprout on their own. They are steep hill-sized cones around a single vent. They toss up congealed lava in ribbons and cinders (gravel-sized chunks). If the gas pressure subsides, lava flows may ooze out through the base. Cinder cones sometimes change to shield volcanoes.
Kapoho, Hawai'i, HVO/USGS
Lava domes are piles of highly-viscous lava that slowly squeeze out like dried-up toothpaste. Inside, they may be molten. They are brittle and unstable, and often explode or collapse in pyroclastic flows. Since Mount St. Helens blew its top in 1980, many lava domes have formed and collapsed in the large crater left by that eruption.
Lava dome in Mt. St. Helens CVO/USGS
Calderas are collapsed areas over drained magma chambers, often circular. Shield volcanoes' shallow calderas sometimes refill with a lava lake when the magma returns. Stratovolcano calderas may develop lava domes, stiff rocky plugs that slowly swell as new lava pushes up from below. Old calderas may collect water, forming lakes that may be heated, highly acidic or full of sulfur from rising gases.
Crater Lake, Oregon, CVO/USGS
Rhyolite caldera complexes are the scars left by huge, ancient eruptions. They build up massive amounts of pressure and explode violently, leaving behind a vast collapsed area over gigantic magma chambers. There are often multiple caldera scars, ordinary-sized stratovolcanoes, and lava domes in these complexes. Geologists have discovered deep layers of ash and flows spreading out from them for thousands of miles. Luckily, the last such eruption was in 83 AD. Unluckily, Yellowstone is one of these monsters.
100km long, 30km wide, Lake Toba is the ancient rhyolite caldera left over from a 73,000 year old cataclysmic eruption. (Read more about it). The island in the middle of Lake Toba is a lava dome that has uplifted over a remnant of magma in the ancient magma chamber.
Types of Volcanic Debris
Ash, Pyroclastic Flows, Lahars
While shield volcanoes can sometimes get blocked up and explode, these three kinds of volcanic debris are more typical of stratovolcanoes. See the videos following this section for examples.
Volcanic Ash is created when old, solidified lava and rocks are pulverized to dust by a violent eruption and hurled up into the sky in huge columns resembling smoke. While ordinary ash is made of burned-up wood and other combustibles, volcanic ash is made of ground-up rock, and its grains are jagged and glassy. It can travel for thousands of miles or fall like snow. Not only does volcanic ash gum up machinery and air filters, but it turns into concrete-like muck if it gets wet. (In fact, Romans used it for concrete.)
Highly-magnified volcanic ash
Credit: Mark Wilson
Videos of Pyroclastic Flows and Lahars - Deadly Volcanic Hazards
The first two videos show pyroclastic flows; the second two are lahars; the last is ash.
Types of Volcanic Eruptions
Strombolian, Vulcanian, Plinian Eruptions
Here's a few more terms for different types of eruptions that you'll often see in the news or on volcano websites.
Strombolian eruptions, named after a volcano in Sicily, look like a giant sputtering blowtorch. They shoot out a burst of lava blobs with a loud noise, die down, and then go "foosh" again after a few seconds or minutes. Large gas bubbles in the volcano's plumbing cause the characteristic pistoning action. These aren't very dangerous, since the lava lands around the cone, not miles away. Many Mt. Etna videos show Strombolian eruptions.
Stromboli Volcano by m. aquila
Vulcanian Eruptions are more violent: they happen when a volcano wakes up and and blasts through a lava plug or dome, sending ash and chunks of magma and rock into the air. They may have small pyroclastic flows (avalanches of hot gases, ash, and rocks) down the sides of the volcano. A vulcanian eruption may continue for some time, spreading ash over a broad area. Its eruptions can reach 5-10 km high, as opposed to to...
Sakurajima Volcano by Kimon Berlin
Videos: Types of Volcanic Eruptions - Strombolian (1 & 2), Volcanian (3 & 4), Plinian
Types of Lava Flows
The Difference Between A'A and Pahoehoe
Stratovolcanoes produce lava flows as well, but these are what builds shield volcanoes:
Pahoehoe (Pah-HO-ee-HO-ee) is fast-moving, runny lava. Its leading edge flows forward in lobes or "toes" like puddling wax. Often, the surface of a pahoehoe flow will solidify, creating a naturally insulated pipe that can carry lava for miles. If all the lava flows out of a tube, it may leave behind a large round tunnel!
Ropy Pahoehoe, HVO/USGS
A'a (below) is slightly cooler, clumpy lava that piles up in large, slow-moving walls. Chunks of lava and cinders tumble down the front face and get swept under like the tread of a tank. Tour guides often tell visitors that "a'a" was named for the sound you make when walking over it barefoot.*
Photo by J.D. Griggs, Hawai'i Volcano Observatory/USGS
*Squidoo member June, who lives on the Big Island of Hawai'i, explains the real meaning of these Hawai'ian words:
"Pahoehoe means smooth, unbroken or satin like. The "Aa" lava is the stony, rough lava. The joke about the sound you make when walking on it barefoot was created for the tourist. The spelling "a'a" has too many meanings to be listed here, but all of the various definitions are dependent upon the context in which the word is used, but none of them being lava. The closest would be to burn; glowing; fire; staring, as eyes (a'a maka) or the throat is on fire with thirst (Ua `a'a ka pu'u )."
Video: A'a and Pahoehoe - Can You Tell Which Is Which?
Highlight for answer: (The lava flow eating the car is a'a. The brief clips of runny stuff at the beginning and the timelapse sequence at the end are pahoehoe.)
Ellen BrundigeScoria, or volcanic cinders, are what most people think of when they say "lava rock." It's basaltic, meaning it was formed by fairly liquid lava, and contains lots of bubbles. Ranging from black to red to white, it's commonly used in garden landscaping. Here's a beach pebble of scoria I found on the volcanic island of Thera. Fresh scoria is craggy and irregularly shaped; this piece has been rounded by the ocean.
Ellen BrundigePumice is essentially solidified lava foam. Very frothy lava, full of gas bubbles, turns into a soft rock riddled with holes like a sponge. Like the hard foam used to make Boogie Boards, there's so many air (or gas) pockets in pumice that it can sometimes float. In 1883, sailors reported seeing huge rafts of pumice floating in the Indian Ocean. At right, I've floated a piece of pumice from Thera in a flower vase. See my Youtube video, Pumice: The Rock That Floats or my Why Do Things Float? page for more info.
NOTE: Technically, "Lava" only refers to volcanic rock coming out of a volcano while it's still molten. After it cools, it is called "igneous" rock from IGNIS, the Latin word for fire.
Self-Test: Can You Recognize Any of These? - National Geographic Photo Gallery
Among other things, this photo gallery shows: several stratovolcanoes, a Vulcanian eruption, a few Plinian eruptions, two stills of Strombolian eruptions, at least one cinder cone, a lava tube with a "skylight" (a hole in the top), several calderas, and solidified pahoehoe.
More Volcano Terms
Did I miss a volcano term you're curious about? See the USGS Volcanoes Photo Glossary for answers to all your burning questions.