Volcanic Rock Formations

Volcanoes form extremely diverse landscapes. Much more than just towering craters, traces of volcanic activity can be found in a variety of different rocks. These pretty, soft-looking formations of stick lava (pahoehoe lava), these rock sculptures in which one searches for meaning like in clouds, these frozen rivers and the lunar landscapes – lava shapes various landscapes of the earth more than meets the eye.
From bright quartz to extremely dark basalt rocks, from sharply shaped gravel fields to softly shaped knitted lava – we constantly encounter the creations of volcanoes. Recognizing them helps us to understand the origin of our environment.
In this chapter we take a look at how different lavas can be categorized and identified. How they occur on land, and especially how to recognize them underwater, complements the scientific view of rock composition.

Types of lava

Many people have an image of lava that corresponds to only one type: a kind of glowing river flowing through the landscape. There is, it’s an extremely hot, fluid, heavily basaltic lava that forms fields of pahoehoe lava as it cools. But that’s not all, of course – volcanoes spit out their magma in a variety of forms.

What happens to lava as it cools?

The earth’s crust consists of about 90%, the earth’s mantle even almost completely from silicates. These make up a large part of the magma, which consists of 40-70% silicates, i.e. SiO2. There is also iron, magnesium and calcium, as well as sodium and potassium.
The type of lava differs depending on the mixing ratio of the individual substances. Lava with a high proportion of silicates is lighter in color, melts earlier and has a low density. The resulting rocks are referred to as felsites.
Lava that is low in silicate but high in magnesium and iron is significantly darker and heavier. The resulting rocks are called mafites, basalt, for example, is such a rock.

Lava rocks are characterized by the fact that they are extremely porous. This is due to the gases that drive the buoyancy of the magma, leaving the rock “riddled”

lava types

How viscous is the lava?

The different nature of the lava and the speed at which it solidifies result in very different rock formations of volcanic origin.
Lavas can be roughly divided into aa lava , a friable, very viscous lava that solidifies into sharp-edged clumps, and pahoehoe lava , a viscous, extremely hot lava that forms soft shapes. The names come from the language of the Hawaiian natives: Aa stands for wet, stony, hot, the term pahoehoe lava initially referred only to the solidified rock formation, but is now also used for lava that is still flowing. Both terms are easy to remember: they very vividly depict the noise you make when you walk over such lava…
Extremely thin lava can cover wide landscapes as flood basalt .

How big are the chunks?

The smallest particles up to 2mm in diameter are called ash . In contrast to the ash produced when burning wood, this volcanic ash is made of stone and glass and has correspondingly sharp edges. It occurs as fine dust or as coarse sand and is massively dangerous for engines, so that air traffic is restricted in the vicinity of spitting volcanoes. Where it falls, roads can get slippery, people like to put on masks, but nature is happy: many plants grow particularly well on layers of ash.
Slightly larger are the 2-64mm lapilli , small lava rocks that cluster closer to the eruption site. The largest ejected products, all over 64mm , are called lava bombs or lava blocks: bombs when they are round and were liquid when they ejected, blocks are called when they are already ejected solid. drift apart, the earth’s crust is in motion. Especially there, weak points can appear, and the pressurized magma pushes upwards.

lava formations on land

People only see what they see. One can walk and enjoy landscapes for days without knowing where they come from. But it can be fascinating to see around you where individual rock formations originated and how this patch of land you are walking on came into being.
This is about individual lava formations – you can find out what volcanoes look like in general here: Volcanic landscapes

Basalt columns

Columns of basalt form when basaltic lava – very hot and liquid – cools very slowly. During this slow cooling, four to octagonal columns usually form.
Such formations can be found all over the world: in the Ore Mountains, in the Eifel, in Ireland and Iceland, in Vietnam and the USA – a sign of the volcanic origin of a large number of landscapes worldwide.

Basalt columns in Hong Kong. Image: ImMrDrake

Lava tunnels

When thin, extremely hot lava flows, the outer layer sometimes freezes, while a lava flow remains active on the inside. Lava flows underground through such lava tubes, which can be up to 30m wide and up to 15m high, until the eruption subsides. When the lava has drained and the remains have cooled, a cave remains. These caves can be up to 50km long and often become tourist attractions.
All known cave systems of this type are geologically younger, i.e. they can be found in volcanically active zones. Very well known is the Kazumura Cave in Hawaii, with its 65.5 km length it is the longest on earth. The Cueva del Viento on Tenerife is significantly shorter, but with a length of 17 km it is still very impressive.

Thurston Lava Tunnels, Hawaii

Lava Tongues: Solidified lava flows

When relatively fluid lava flows downhill, it solidifies in large lava tongues that overlay the older landscape.
Such individual tongues on mountain slopes can be impressive: the old edge can still be seen, but is interrupted by tongues of lava.
They are also often found under water – a little history of the earth can then provide information about whether they cooled down in the water or whether the sea level rose after their formation.

Lava Flow Hawaii - Image: Brocken Inaglory


Every type of lava, yes, every type of rock, is subject to the eternal cycle of erosion as soon as it is formed. On land, for example, this happens through the weather, running water, seasonal temperature fluctuations, and vegetation. And under water, for example, water movement, dissolution processes, and also biological activities drive erosion.
These processes soften over time the once harsh volcanic landforms, and eventually vegetation covers even what were once mighty lava fields and scree slopes.
However, especially in very dry (arid) areas of the world, there are also volcanic landscape forms that are subject to much less erosion. In these landscapes, lava flows, cinder cones, and block heaps remain visible for much longer. Such landscapes are quite attractive, and sometimes home to otherwise rare species. But they are also very difficult to traverse, which has earned them their Spanish proper name, Malpais.

From the seabed? – Restingolitas

A strange phenomenon occurred during the 2010 eruption of the underwater volcano Tagoro on El Hierro that has geologists in turmoil. Masses of lava balls were floating on the water, which were obviously (including the gas inclusions) less dense than the water. Inside these spheres are made of very light silicates, white and very porous, the outer wall and some thin layers inside are dark lava.
It is not entirely clear how these balls are formed. It is probably a kind of “lava balloon”, i.e. gases that have remained trapped in cooled lava. Sometimes sediment is also “captured” in the process: the white silicates in the core, which can sometimes be directly assigned to the local seabed.
These little spheres were named after the place La Restinga on El Hierro. This location was partially evacuated during the eruption. They are now called “restingolitas” not only in the vernacular, but also in the specialist literature.
Also on La Palma they have occurred sporadically. The fact that they appear on another very young island supports the thesis that the bright parts are material from the sea floor.



Lava under water

Anyone who has ever dived in a volcanic area knows how interesting such regions are. Underwater, lava forms unique rock formations that are not only pretty to look at, but also provide living space for sea creatures.
Particularly impressive is of course lava, which is still quite new to the ocean. Seeing life settle there and ecosystems thrive is a very special adventure. Lava formations are often found where no volcano has been active for a long time – there they are much older and owe their current existence below sea level to the rise in sea level after the last ice age.

Old Lava: Submerged by sea level rise

Volcanoes, of course, were active at times long before mankind appeared, and their traces are found buried deep under newer layers of earth and also under water. Just very hard lava has been preserved as the water level has risen by over 120 meters since the end of the last glacial maximum some 20,000 years ago. Smaller and less resistant areas have been removed, leaving impressive formations.

Sea level rise over the last 24,000 years - Graphic: Climate Wiki

Pillow lava

The unbroken favorite lava of the rock gazers among the divers is probably the pillow lava. It is formed when semi-liquid lava is rapidly cooled – as happens in the ocean. It then forms a bubble, a hard shell inside which the lava is still liquid. If more lava follows, the bubble breaks at the thinnest point and another one forms. This creates lava formations that, from a little further away, can easily be mistaken for elephants, rhinos, mammoths and dragons….
To see this form of lava you have to dive or snorkel: it doesn’t solidify so quickly on land. They are found where lava has flowed from land into water, but also at very great depths on mid-ocean ridges. There it stands as evidence of undersea volcanism. Subglacial volcanoes, i.e. eruptions under ice, can also form this form of lava. The cinder cones that tower up and also look as beautiful as a volcano from a picture book are called cinder volcanoes. They are formed by the spat out pyroclastics, which pile up and form a steep cone of lava boulders and ash. A round crater forms at the top, which repeatedly collapses and builds up again during an eruption. They usually do not stand alone, but, for example, as flank volcanoes on the slopes of stratovolcanoes.

lava flows

Lava flows that solidify under water remain in their form as a typical “river”. Sometimes they form interesting archways and small tunnels that are characteristic of underwater volcanic landscapes.
The big question when looking at lava flows is always: how old are they? To find out, only a look at the history of the earth helps.
On a small island in the Atlantic, La Palma, we find underwater forms from various geological ages. A look at the historical volcanic eruptions at least reveals that the streams formed from pillow lava in Las Cabras and Los Molinos poured into the water during the 1971 eruption of the Teneguia. Other lava tongues, as shown here in the pictures, must have been formed long before – they are also very eroded and overgrown. However, they can be found close to the surface of the water, which indicates that they were formed well after the last ice age.

Basalt columns under water

We also encounter basalt formations under water. Most of the time we find them in the shallow water, a clear sign of how the sea level has risen in this place.
This slowly solidifying lava is often buried beneath other lava fields. As extremely hard rock, however, they remain as erosion wears away the rock around the pillars.

Underwater lava tubes

Once lava flows into the sea, it cools quickly – causing lava tubes to end in coastal areas. Directly on the coast in volcanic areas but at shallow depths there are many remains of lava tubes and a few meters of cave. These formations can often be dived without major complications, offer a little adventure and a view of sea creatures that feel more comfortable in the dark: shrimp, anmonas and lobsters cavort in the walls, and sometimes you can also find sleeping rays in such caves..

Quiz: Volcanic Rock Formations

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