Reading the landscape of Etna: lava flows, caves and hornitos

At first glance, an expanse of lava can look like nothing but black rock. Then the eye starts to pick out smooth folded surfaces, sharp blocks, collapsed walls, small cones and bands of vegetation of different ages. The ground stops being a backdrop and becomes a sequence of events.

Reading the landscape of Etna means recognising the forms the volcano has built and reconstructing, at least in part, how they came about. A flow shows the direction the lava took; a cave reveals that the flow kept moving beneath an already solid crust; a hornito signals the former presence of a pressurised lava tube.

You do not need to become a volcanologist to start. A few reading keys are enough, along with the awareness that colour, shape and vegetation offer clues rather than definitive answers.

How to read a volcanic landscape: the short answer

During a hike on Etna, look at five things:

1. the shape of the lava, either smoother and folded or rough and fragmented;
2. the direction of the slope, which helps reconstruct the movement of the flow;
3. the position of cones and fractures, often connected to eruptive vents;
4. the presence of cavities, collapses and hornitos, clues to lava tubes;
5. the development of vegetation, useful for comparing surfaces of different ages.

Colour alone is not enough to date a flow. Lava can look lighter or darker depending on composition, alteration, humidity, lichens and light. To assign an age you need geological maps, historical records and scientific observation.

Lava flows: the roads the magma travelled

A flow is born when magma reaches the surface and runs downhill under gravity. Speed, temperature, viscosity, gradient and the amount of lava emitted all influence its path and final appearance.

On Etna, flows can start from the summit craters or from fractures that open on the flanks. Those emitted at high altitude often stay in uninhabited areas; flank eruptions at lower elevations can instead approach woods, roads and villages.

Looking at a solidified flow, you can pick out several parts:

• the main channel, along which most of the lava ran;
• the levées, built by accumulation and cooling along the sides of the flow;
• the lobes, the different tongues into which the flow divides;
• the front, the most advanced part, often tall and made of blocks;
• any ephemeral vents, points where the lava returns to the surface after travelling a stretch inside a tube.

The geological map of Etna is a mosaic of overlapping flows. A recent surface can interrupt a wood, skirt a rise or partly cover an older flow. The landscape therefore preserves a relative chronology: what lies on top arrived later.

Block lava and ropy lava

Not all flows have the same surface. Some are made of irregular, unstable, abrasive blocks. Others show folds, ropes and more continuous lobes, shaped while the crust was still plastic.

Volcanology often uses the Hawaiian terms ʻaʻā and pāhoehoe. The first describes scoriaceous, fragmented surfaces; the second smoother, undulating or "ropy" lava. They are two behaviours of the same general type of basaltic magma, not two completely different rocks.

A flow can also change appearance along its course. As it cools, loses gas or meets a different gradient, a surface that started out more continuous can become fragmented. This is why the form tells you above all about local flow conditions.

Why boots make the difference

Young surfaces are abrasive and often unstable. Scoria can shift under your weight and glassy edges quickly wear down light soles. Trekking shoes with good grip protect the foot and make each step steadier.

Walking off the path is not always a good idea: besides the risk of falling, you can damage lichens and pioneer plants that take years to colonise a few centimetres of lava.

Flank cones, vent alignments and craters

A flow shows where the lava passed; a cone often helps you understand where it came out.

When an eruptive fracture opens, gas fragments the magma and throws scoria, lapilli and bombs around the vent. The build-up constructs a flank pyroclastic cone, also called an adventive cone. There are hundreds of them on the slopes of Etna.

If several vents open along the same fracture, the cones can line up and form what Italian calls a bottoniera, a "buttonhole" alignment. Their direction makes visible the line of weakness the magma used to reach the surface.

These cones should not be confused with the four summit craters — North-East, South-East, Voragine and Bocca Nuova — which occupy the highest part of the volcano and are connected to the central system. The Silvestri Craters, much visited on the southern side, are also flank cones, born during the 1892 eruption.

The caves of Etna: when lava runs underground

The volcanic caves of Etna are largely lava tubes or flow galleries. They form during an effusive eruption when the surface and margins of a flow cool, creating an insulating shell, while inside the lava stays fluid and keeps moving.

When the supply drops, the level of lava in the conduit falls. If the tube empties at least partly, a cavity remains. On the walls you may find shelves marking different flow levels, solidified drips, striations and melted surfaces.

The ceiling can collapse in places, creating natural openings. These collapses help trace the route of the gallery but make the ground delicate: near the edges the rock can be unstable and the depth is not always obvious.

Caves, snow houses and human history

The caves do not only tell of eruptions. In the past some Etnean cavities were used as shelters, places of worship, stores and neviere — snow houses, where snow was packed and kept to be carried down to the valley in the warm months.

Grotta del Gelo, Grotta dei Lamponi and Grotta di Serracozzo are among the best-known names, but accessibility and difficulty vary widely. A cavity appearing on a map does not mean it is equipped or suitable to explore.

To go in you need at least a helmet, reliable lighting and proper footwear. In many cases you need to be accompanied by someone who knows the layout, the entrances and the conditions of the cave. Do not enter alone, and do not rely on your phone torch.

Hornitos: small cones without deep roots

The Spanish word hornito, "little oven", refers to a structure built on top of a flow or a lava tube. It is not an ordinary eruptive cone fed by a conduit rising from depth.

When a tube is full of gas-rich lava, incandescent material can be pushed through an opening in the roof. Still-fluid shreds pile up around the hole and, welding together, build a turret or a small hollow cone. This is why hornitos are described as rootless structures.

Their shape can be irregular: domes, pinnacles or small chimneys with welded walls. They indicate that a lava transport system once existed beneath that surface. Not every isolated rise on a flow is a hornito, though; position, internal structure and relationship with the tube all need checking.

The U.S. Geological Survey description of hornitos explains the link between these structures and full, pressurised lava tubes.

Fractures and dykes: the lines the magma followed

In the landscape you also come across fractures with no obvious cone. Some opened during intrusions or eruptions; others are linked to movements of the volcano's flanks.

When magma solidifies inside a fracture it forms a dyke, a wall of rock that can become visible once the surrounding material has been eroded. In the walls of the Valle del Bove, dykes appear as blades cutting through older sequences, showing the paths the magma followed inside earlier volcanic edifices.

A fracture should not be explored as if it were a simple crack in the ground. Depth, stability and the possible presence of gas cannot be judged by eye.

Vegetation as a clock, but not as a calendar

Life gradually returns to the lava. First come microorganisms and lichens; then, where dust and organic matter accumulate, herbaceous plants and shrubs take hold. Over time, woodland can develop.

This process lets you compare two neighbouring surfaces: as a rule, a bare flow is younger than one covered in trees. It does not, however, let you automatically fix a date. Altitude, aspect, gradient, humidity and human disturbance all speed up or slow down colonisation.

The contrast is particularly clear on the northern side, where the 2002 lavas cut across wooded areas. The edge of the flow shows, within a few metres, the difference between a mature ecosystem and a volcanic soil right at the start of its evolution.

People are part of the Etnean landscape too

Dry-stone walls, terraces, old wine presses, roads cut into the lava and rebuilt houses all tell of the relationship between communities and the volcano. At lower elevations, lava stone became a building material; volcanic soils supported vineyards, orchards and crops.

Elsewhere the landscape preserves traces of destruction: walls surrounded by flows, abandoned infrastructure, routes interrupted and then moved. Reading Etna therefore means observing geology and human adaptation together.

A checklist for your next hike

When you stop in front of a volcanic form, try asking yourself these questions:

• is it a flow, a scoria deposit or a surface covered in ash?
• which direction does the slope indicate?
• is the rise connected to a fracture, or does it sit on top of the flow?
• are there levées, channels or overlapping lobes?
• does the vegetation change abruptly along a boundary?
• can you see collapses that might indicate a lava tube?
• which elements are natural and which come from human activity?

Do not collect samples and do not approach unstable edges to test a hypothesis. A good reading of the landscape starts with respecting distance and the path.

From rock to the story of the volcano

Being able to recognise a flow, a lava tube or a hornito changes the experience of Etna. The forms are no longer isolated objects: they become parts of the same process, from the emergence of the magma to its transport, from cooling to the slow colonisation of life.

The trails of the Etna Park cross recent and historic flows, woodland and environments with no vegetation at all. To understand what you are looking at and choose a suitable route, compare the guided hikes from Guide Etna: a volcanological guide turns the clues in the ground into a story you can follow, without ever losing sight of conditions on the volcano.