How fast does aa lava flow

The word pyroclastic comes from a Greek word that means "Rock broken by fire". When volcanoes do produce lava flows they are classified as either Pahoehoe or Aa. The lava is identical in both pahoehoe and aa lava flows, the difference comes from the amount of lava erupted and the speed of cooling.

Pahoehoe lava flows are produced from a small amount of lava that moves slowly, while aa flows usually are associated with a large volume of lava that moves swiftly. Aa flows are generally feet thick and pahoehoe flows are usually feet thick.

If the lava is very hot and has a low viscosity runny with a low gas and silica content the lava flow is called Pahoehoe. If, on the other hand, the lava has a high viscosity thick and pasty with a high gas and silica content it is called Aa. Silica is a white or colorless crystal that is present in sand and quartz. It is one of the most abundant compounds in the Earth's crust. The photograph shows a pahoehoe flow on the left and an aa flow on the right. Pahoehoe Pa-Hoy-Hoy lava flows are very hot, thin and runny.

When it cools is has a smooth to ropey texture because of the low silica content which makes it cool quickly. Pahoehoe flows creep along generally at less than 3 feet per minute but some flows have been measured at over 20 miles per hour. The terms pahoehoe and aa are from the native Hawaiian language and are now used by geologists the world over. This pahoehoe flow is advancing on the skeleton of a large mammal perhaps a horse or a cow.

The lava will engulf the animal and may fossilize the remains!! Notice how the flow advances in globs of lava. These globs of lava are called lobes. Aa lava flows are formed when the lava is produced in a manner that allows it to cool quickly.

When a fire fountain shoots the lava high into the air it cools somewhat before it can flow after landing on the surface. Aa lava also forms when there is a huge amount of lava produced or a steep slope moves the lava at high speeds. These high speeds put the lava in greater contact with the air, which makes it cool more quickly. Notice the rough and fragmented upper surface of the photo at the left.

Would you like to walk barefoot on this after it cools? A pahoehoe lava flow produced the lava tube in the picture above. A lava tube forms when the lava on the outer surface of the flow cools much faster than the inside of the flow. The outside becomes cooled hardened lava rock while the inside stays molten and also keeps flowing. If something happens to stop the flowing lava there will be nothing to fill the void and a tube is the result. No, this is not a North Dakota blizzard.

This photo is showing the ash fall from Mt. Pinatubo's Phillipines eruption in The lava type associated with submarine volcanism — pillow lava, is therefore underrepresented where ordinary people have a chance to see it. The most common way to divide lava flows into distinct types is following: Pahoehoe lava flow, Aa lava flow, Blocky lava flow, and also Pillow lava flow.

Sometimes Turbulent lava flow is also added, but the latter is only of theoretical interest to scientist because we will not see that type of lava flow in the nature. Turbulent lava flows may have been present billions of years ago when the interior of the Earth and consequently lava flows as well were significantly hotter and the composition of lava was less siliceous.

That enabled the lava to flow more easily and turbulently. The most common subaerial lava flows today are pahoehoe, aa, and blocky lavas. Lava surface is cooling very rapidly. Lava surface may cool from bright yellow to dull red within minutes. Pahoehoe is a smooth and continuous lava crust. Pahoehoe forms when the effusion rate is low and consequently the velocity of lava flow is slow 2.

Pahoehoe lava flow is usually at least 10 times slower than typical aa lava flow 5. Higher effusion rate results in lava flow being shattered which is how the rubbly and clinkery aa lava surface forms. Pahoehoe and aa lava are strikingly different in appearance, but their composition may be identical or very similar. Lava flow that was originally pahoehoe may transform into aa lava but the reverse is impossible — once lava crust is broken it can not return back to smooth and continuous form 3.

Only low-viscosity usually basaltic lava can form pahoehoe. Aa lava is much more common and is not as picky about the composition of lava flow. Aa lava can be basaltic, andesitic, tephritic, etc. Blocky lava is composed of larger blocks than aa lava and these blocks have much smoother surface.

Pahoehoe is also known as ropy lava and it has several more varieties named entrail, festooned, filamented, sharkskin, shelly, etc 4. Forming basaltic pahoehoe lava flow in Hawaii.

Such lava flows move slowly and are not overly dangerous when compared with some other volcanic phenomena. However, their destructive power is practically unstoppable.

You can easily walk away from a lava flow like that in most cases, but you will not be able to save your house if it happens to be on the way of such a slow-moving disaster. Here is all that is left of a house engulfed by a pahoehoe lava flow.

Although it is possible to predict which regions are in danger of being covered by lava flows, people, if allowed, stubbornly continue to build their houses in these districts. Destruction that inevitably follows keeps the attention of public focused on these flows and provides money for scientists studying them.

Lava flows are often imagined as fiery rivers of molten rock. That is indeed often the case, but it is also very common that there is no easily defined boundary of an active lava flow. The lava often flows largely underground. When I was hiking in Hawaii with a clear target in my mind to find an active lava flow I was perplexed and amazed when I suddenly discovered that I am actually standing right in the middle of such a flow.

There were no river of lava. The red glowing material was just seeping out of the ground here and there, some in front of me and some already behind. This is potentially very dangerous situation not so much because of lava itself because it moves slowly and contrary to popular belief it is impossible to sink into it.

But because of toxic volcanic gases that are involved with all types of lava flows. I tried to be very careful, trying to observe the flows so that the wind was blowing the gas away from me. Here is a picture of volcanic gas seeping out from the ground. If you are also trying to find flowing lava, always try to keep a safe distance with such a thick cocktail of hazardous gases and pay attention to the wind direction.

Walking on a pahoehoe lava flow is relatively easy, especially when compared with walking on an aa-type lava flow which is practically impossible undertaking. However, hiking on a smooth lava surface is not without some nasty surprises. As mentioned earlier, lava crust solidifies quickly, but the interior keeps moving which often leaves empty space beneath the thin surface.

It can easily break when you step on it which may result in badly scratched legs because the edges of a broken lava flow are razor sharp. Or even worse, you might have a broken leg which understandably is a major problem if you are miles away from nearest roads. Through practice hikers will learn relatively quickly where it is safer to step and where it pays to be extra careful.

Generally speaking, smaller lava lobes are safer and larger gently sloping, but otherwise relatively smooth parts are the most dangerous ones. This image shows also that voids in lava flows are a common feature. Such a long tunnel-like caves are called lava tunnels. Large amount of lava can move through these tunnels which makes it easier for a volcano to grow itself laterally larger.

This is a Thurston lava tunnel in Hawaii. Lava tunnels are especially common within silica-poor basaltic lavas. Examples like that can be also described as ropy lava which is a subtype of pahoehoe. The wrinkled surface of ropy lava is a result of the interior of the lava flow moving more quickly than the exterior.

You can see the formation process in the video below:. The formation process of basaltic ropy pahoehoe lava. Light-colored mound in the foreground is composed of pumiceous lapilli. Picture taken in Tenerife, Canary Islands. Aa lava is a rough rubbly crust of a lava flow.

It is a major lava flow type. Technical Announcements. Employees in the News. Emergency Management. Survey Manual. Lava flows typically move slowly enough to outrun them, but they will destroy everything in their path. Lava flows are streams of molten rock that pour or ooze from an erupting vent. Lava is erupted during either nonexplosive activity or explosive lava fountains.

The speed at which lava moves across the ground depends on several factors, including 1 type of lava erupted and its viscosity; 2 steepness of the ground over which it travels; 3 whether the lava flows as a broad sheet, through a confined channel, or down a lava tube; and 4 rate of lava production at the vent.

Lava flow moving into the town of Kalapana, Hawaii Island. Flow front is center, silver-black mass with rising fume. Buildings and lagoon in this photograph were completely buried within one month.