Lava rocks! (republication)

While we are waiting for more information (and daylight) on the large Pacific eruption, here is a repost. It complements the previous post on igneousity (for which surely the ig-nobel prize was invented?). Enjoy.

Built on lava rock

What’s in a name. Would lava by any other name smell as sweet? Perhaps that is not the right question: lava is many things, but sweet-smelling it is not. It smells like a rose bush that was doused in some evil sulphurous pesticide and then put on fire. This rose also constantly explodes with a roar well above the legal maximum. No, Shakespeare’s rose is definitely the wrong analogy. The course of true lava never did run smooth.

But in geology, names are everything. The number of different minerals is staggering, and each has a different name which normally reflects its point of discovery rather than implying anything about the composition. Olivine has nothing to do with olives and perovskite is not a rock band. Do you know your cryolite from your kryptonite? How is your appetite for apatite? Studying geology must be like learning Inuit.

Volcanic rocks share this nomenclaturic nightmare. What on earth is rhyolite? What happened to komatiite? And what stuff is tuff? Even google can get confused: ask for amygdale, and it gives the right definition but images of something completely different. At least it knows not to show images of amygdala (which might require that you confirm your age). But for all of you who were confused but afraid to ask, here is help. This post gives all you need to know to understand the variety of rocks from lava, or at least help you speak like you do. Welcome to the club. We all bluffed our way into geological volcanology.

Lava is molten rock. At the end, it solidifies back into rock. So you should get back what you started with, right? Wrong. Just imagine a block of ice. Melt it, vaporize it, and let it condense and freeze, you may get snow. Both are solid water, but that is all they have in common. So too with rock, although with much more variety.

So let’s have a look at why not all lava is the same. For most of the images, if you click on them you will get full resolution. All the poor ones are from my own collection.


Basalt. Photo by the author

The type specimen of volcanic rock is basalt. It is effectively mantle material, which was melted at the bottom of the crust, ejected from a convenient hole (also called a ‘volcano’), flowed downhill for a bit and solidified there. You are looking at displaced mantle material. Most of the ocean floor consists of this stuff. It is heavy because the mantle is made of denser material than the continental crust (if it weren’t, the crust would sink down like an overweight iceberg and continental existence would be short-lived). In practice, the density of basalt is close to 3000 kg per cubic meter. The continental crust is typically around 2700 kg per cubic meter.

Basalt is the ultimate volcanic rock. Why doesn’t every old lava flow look like this? There are a number of reasons why not all lavas are the same.

1. Temperature and viscosity

The first effect is that of temperature and viscosity. I know, that is two, but they are closely related. Lava can be compared to honey. Warm honey flows easily, but put it in the fridge for a few minutes and it becomes much less keen to spread out, even though it is still a liquid. Honey is actually quite a complex substance, even though it is mainly just sugar dissolved in water. The melting temperature is around 45 C (depending on the sugar concentration): above that temperature, it does indeed behave like sugar water. Below that temperature, it should freeze but it doesn’t, at least not until the temperature goes down to about 5 C. In between, it is a super-saturated liquid, where there is more sugar dissolved in the water than fits. The excess sugar forms crystals and this turns the honey into a mush. As it cools, more and more crystals form and the honey becomes stickier and stickier. Finally, it freezes. (Strictly speaking it doesn’t fully freeze until it reaches -50 C, but below 5 C it is so sticky you hardly notice the difference.) Now turn the temperature back up – and nothing happens until you reach the melting temperature. Funny honey. (When we call a loved one ‘honey’ one does wonder which characteristic of honey is intended. This expression is definitely open to interpretation.)

The stickiness is also called ‘viscosity’. Higher viscosity means stickier, i.e. more reluctant to move.

So in what way is lava like honey? Should I call my loved one ‘lava’ or would that get my fingers burned? Well, lava is also a mix of substances, and these have different melting temperatures. As lava cools, some substance may come out of the liquid and crystallize. As more and more crystals form, the lava becomes less runny and more sticky. And this gives rise to the two Hawaiian words used to describe lava flows: Pahoehoe and A’a.



Pahoehoe is the thinner, runnier lava that happily covers large distances and creates smooth lava flow which afterwards you can walk on. It is the hot honey.

A’a is the sticky lava that refuses to go anywhere fast. It forms very uneven surfaces and is a nightmare to walk on (even after it has cooled enough). Its surface will shred your shoes – this is the one place on earth where high heels may give an advantage. A’a is the honey that has been kept too cool and really would have needed a few seconds in the microwave.

In Puna, the early eruption brought up lava that had been in storage for decades – perhaps centuries. Even though it had stayed warm (a kilometre of rock insulates pretty well), it did not stay hot. This became the sticky stuff, with high viscosity; this explains why the initial eruptions did not produce much in terms of lava flows. It built walls – not roads. Later, the new lava arrived and this was much hotter. The hot, new, all-running and dancing low-viscosity lava ran like runny honey and in no time covered huge swathes of country side.

Even though both are basalt, the two behaved very differently. Poor Puna.

2. Composition

Let me divert for a minute. Lava contains a mix of elements. The main ones are iron, magnesium, silicon, and some other things such as aluminium (called aluminum in Trump land, a spelling that was in use in the UK very briefly but was introduced to the US through the Webster dictionary of 1812.) Each of these forms minerals (mainly oxides), and each mineral has a different melting point. Hot magma contains all these elements and their minerals, although not always in the same ratios. But keep magma for a while in a storage facility (also called a magma chamber) and it begins to cool – very slowly. The first minerals to hit their melting points are iron and magnesium oxides: they form crystals and drop out of the solution. Beforehand, the magma was called mafic (for magnesium (never to be called magnesum) and iron). Now that iron and magnesium are becoming depleted, it is called andesite – you may remember this word from Puna’s infamous fissure number 17.

Store the magma for even longer, and you are left with mainly silicates, mixed with a few other elements (aluminium, calcium, sodium). This is called felsic lava (the word ‘silicic’ is also used). Rhyolite and dacite are of this form. Dacite has made an appearance in the recent Puna stories, albeit only found in deep (geothermal) drilling and not on the surface.

You can expect that this will form a sequence in temperature: mafic lavas are hotter and thus less viscous, andesite is cooler and stickier, and felsic lavas are positively cold (as lavas go) and nearly immovable. And for the most part, you would be right. Mafic flows – felsic stalls.



There are a few exceptions: sometimes magmas form by melting rocks that themselves already lack certain elements. For instance, imagine a rhyolitic magma chamber solidifying into rock. Long after, heat finds its way to the rock and melts it: the new magma will have the composition of rhyolite, but it could well be much hotter than usual, and therefore far less viscous. You can now get a rhyolitic pahoehoe, and this is for instance found along the Snake River in Trump land.

(Hint: if you want to impress your friends, using the term rhyolitic pahoehoe in Puna will do wonders. But avoid saying the andesitic a’a of Haleakala which could leave the impression that you had a drink too many.)

Iron makes the world look black. That is true in lavas as well: mafic basalt is dark to black, while the andesite is greyer and the rhyolite is a bright lava. Of course, add oxygen and over time iron turns red, like the soils of Oklahoma.

I should point out here that the world of lava is simpler than it used to be. A few billion years ago, the mantle was hotter than it is now and therefore lava was considerably hotter as well. This gave a type of lava that is ultra-mafic, an extremely magnesium-rich, which is called komatiite. They don’t make it anymore.

3. Rate of cooling

So temperature is important. But how quickly the temperature goes down is also relevant. Lava rocks that solidify fast look very different from ones that cool only slowly. The rate of cooling has three different effects.

(i) Shape

The first of these makes sense: if the cooling is fast, the flow patterns become fixed in the material. The ultimate example is pillow lava: these are erupted under water, and in consequence cool very rapidly (in the battle between the mid-oceanic ridges and the ocean, the lava has yet to win. Luckily loosing builds character and pillow lavas do have that. Although one could argue that winning builds more character.)

Pillow lavas

Petrified lava ripples

On land, cooling is slower. Thin flows on the surface cool faster than thick ones. In the picture, the difference between the thin flow in the foreground and the thicker ones in the background is notable! The former solidified fast enough that the flow patterns froze in.

Pele’s hair

An extreme case is that of liquid droplets flying through the air. As they cool they solidify, and form long streamers. This creates the strangest rock of all, with the evocative name of Pele’s hair. You have to feel sorry for her hair dresser! What kind of comb would be needed? This lady is not for brushing! Pele’s hair can be found especially around lava fountains. But beware: the hair strands can be needle-sharp and should be handled only with thick gloves.

The strings come from wind acting on the flying droplets. If there is too little wind, you don’t get strings but tear-shaped droplets (very much like the shape of a drop of water falling from a leaky tap), about a centimetre across. These are called, not entirely surprisingly, Pele’s tears (although with the lady’s reputation, the need for tears seems minimal.)

(ii) Glass

So much for the first of the three effects. The second one is very different: this is when lava is cooled so quickly that it briefly forms a supercooled liquid, i.e. a liquid below its melting temperature. You can create this yourself by putting distilled water in the freezer. It will remain a liquid even though its temperature drops far below freezing. But disturb it ever so slightly, and it freezes over instantly. If you do this with molten rock, and let it suddenly solidify well below its melting temperature, it can create a glass. (The temperature below which this can happen is called the glass transition temperature, which is different for each material.) The trick is to make it solidify all at once, with as few separate crystals as possible.

An easier way to do this is by starting out with a lava which contains a bit of water. Water lowers the melting temperature, and so the lava can be cooler whilst still a liquid. Now evaporate the water (as can happen as the lava becomes exposed to air). Suddenly the melting temperature goes up, and the lava finds itself caught out, being a liquid well below its new melting temperature.


A well-known example of a rock that formed in this way is obsidian, a black rock of volcanic glass. It forms from silicate-rich melts, i.e. from rhyolites. Obsidian was sought-after in the stone age as it can be used for cutting (including the careless owner).

It should be harder to make a glass out of basalt, because it is hotter to begin with and readily forms crystals while cooling. But Hawaiian volcanoes manage it quite easily. Basaltic glass is called tachylite. It can be a thin edge on a crystallized lava flow, but on Hawaii it can form thick layers. And now you will not be surprised to know that Pele’s hair also consists of strands of this glass. Ouch again.

Perhaps the most dangerous of all is when lava meets the ocean. The instant cooling forms small particles of glass, and the rising steam carries them away. The white plumes of Puna, where the lava comes over the sea cliffs, are pretty only from a safe distance. There are several reasons why you shouldn’t breath in the stuff – the tiny glass particles among them.

(iii) Crystals

The third effect is that of crystallization. As lava cools, crystals begin to form. The slower the cooling, the larger the crystals can become. These crystals (or their absence) are easily recognized by eye.

Compare the following wo rocks. Both are obsidian, and thus formed through rapid (instantaneous) cooling. The left one looks ‘normal’: a hard glass, albeit with a greenish tint. The one on the right contains a host of crystals, a feldspar to be precise. What happened? It spend some time cooling slowly, allowing the crystals to form, before it suddenly cooled very fast and let the remainder turn to glass. The texture shows that it cooled in two distinct phases, one slow, one fast.

Two types of obsidian, with and without crystals


The next one is pitchstone: also a rhyolitic glass, like obsidian, but containing a larger fraction of minute crystals. It gives the rock a dull appearance. The crystals are very small, and this shows that the initial cooling was fairly fast. Pitchstone contains a bit more water than normal obsidian, and so the rock has a lower melting temperature. The various minerals with the highest melting temperature had time to form small crystals before the remainder turned to glass.


And finally the other extreme: this is a rock with enormous crystals which must have cooled very slowly. In fact this particular rock, a pegmatite, would have formed in the deep crust, where the cooling was so slow that the single crystal could take centuries to form, before the surrounding magma finally turned to stone.


Now we know what volcanic rocks look like. But often, volcanic rocks look very different, striated or welded. They also come in a range of sizes, from the island-sized flows of Mauna Loa to the ash of Mount St Helens. What causes that difference?

Fragmentation of the lava comes mainly from explosions. Rock does not easily explode, of course: it lacks suitable chemistry, and lava is nowhere near hot enough to vaporize rock. The explosions come from trapped volatiles. Big explosions come from volatiles in magma which suddenly decide to become a gas, need a thousand time more volume for this, and end up blowing apart complete mountains. But it can also come from trapped vegetation underneath a lava flow, or even in one infamous incident, trapped snow. These kind of explosions produce flying lava bombs – the name is not entirely accurate, as the bombs are ejected by the explosion – they do not explode themselves.

The explosions produce fragments of a variety of sizes. They are distinguished by size.

Particles smaller than 2 mm are called ash.
Up to 6.5 cm it is lapilli.
Above that size it is a lava bomb.
All together it is called tephra.

Lava bombs of two different sizes

What goes up must come down – as true in volcanics as it is in politics. It is important to know that a lava bomb is made of lava – it will happily start a fire if it lands on flammable material; the danger is not just in being hit by a projectile with the size and speed of a cannon ball (although that is not entirely without danger either). They can be as large as 5 meters (although 20 centimeter is a more typical size, luckily), be ejected at a speed of 200 meters per second, and can travel considerable distances through the air: up to 5 kilometers. Lava bombs may also arrive as tachylite. Ouch!

Lapilli tuff

Smaller fragments travel further than large ones because they benefit from the lift from rising hot air of the eruption. These fragments also are much more voluminous, and the ash can cover large areas in a blanket that is centimeters to meters thick. It welds together, either through heat or over time. The welded layer is called tuff.

The name is not fully appropriate because it is the softest rock created by volcanic eruptions. The tuff can embed lapilli fragments and even lava bombs if not too far from the eruption site. Whereas the lava bombs have the composition of the lava, the tuff will often have a composition close to that of the mountain. If lava, it is often rhyolitic (because that explodes well). Tuff is often light coloured, and can be almost white. The best place to find tuff is in local buildings: it is everyone’s favourite building material. Although, be aware, if your local buildings use it, somewhere in the area is a mountain which made it, and which may have its own building demolition program.

A tuff sandwich. Source:

The tuff in this image is layered between lapilli (which falls first) and ignimbrite, which is debris from a pyroclastic flow. The tuff can be recognized by its smoother texture.

Gas content

The final piece of the puzzle is the gas content of the erupted lava. If the gas content is high, the lava becomes frothy, and when it solidifies it has lots of holes, like a swiss cheese. There can be so many holes that the rock weighs less than water, and floats: this is called pumice, and it forms especially well after an underwater eruption where the water provides more gas than the lava can cope with. The sea can become covered by rafts of pumice kilometres wide: after Krakatoa, they made local sea travel almost impossible for months.

A floating island of pumice

Scoria. Source: wikpedia (Jonathan Zander)

If the stone has lots of holes but not enough to float it is called scoria or cinder (the two words are interchangeable; ‘cinder’ is older). This is where Cinderella got her name from. (No, I don’t know either. And nowadays she would be called Scorella.) These form especially during volcanic explosions as the gas-rich lava is shot out from the vent.


But gas bubbles can also form underground, and leave magma that solidified under ground with holes. Over time, the holes may become filled with water and the water can deposit new minerals, often a calcite. These rocks are called amygdales.

But it can get even better. The rocks can have very large holes inside, and over time, anything can happen in those holes. Open the rock and you may have the surprise of a life time, with wondrous crystals and colours. Here is an example, an oversized amethyst. What causes the different colours? That, my dear Watson, is elementary. But the science of volcanic gems is a different topic – a different post, perhaps.

An oversized amethyst

Albert, May 2018

343 thoughts on “Lava rocks! (republication)

  1. Long time lurker first time commenter here. Thank you for an excellent site! Been enjoying your articles and discussions almost daily.
    I came across this article by google, and thought it described the event pretty well for a layman. Most of its information has already been here, but not collected in one place. As nobody has yet linked it here (or at least I haven’t seen it), I thought I might as well do it. I have seen the writer’s name appear in some plots here, too.

    • Thank you. Yes, this is by a VC contributor, one of our friends. It is also a very good article.

  2. Massive thought that this has been blasted away without any explosives 😮
    Don’t mess with steam.

    So then, “who”‘s next?

  3. Albert…Where is the location of the photo which caps your excellent lava review. It suggestive of either Plymouth, Monteseratte, or Paricutingiro, Mexico, both of ehich were subsumed.

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