Graciosa's geosites: a self-guided geology loop from Porto Afonso and Ponta da Barca to the Caldeira

Graciosa is the flattest of the nine islands, which makes it an easy place to read the rocks. On a steep island, forest and cliffs hide how the ground was built. On Graciosa you can drive from one end of the story to the other in a day: an old trachyte core in the west, a field of small basalt cones in the northwest, a young lava flow in the east and a collapsed volcano with a cave under it in the southeast.

The Azores Geopark lists nine geosites on the island. This guide puts them in an order you can drive in one loop from Santa Cruz, says what to look at in each, and points out the two you should not try to enter on your own.

Graciosa is made of three pieces.

The oldest part is the trachyte of Serra Branca and Serra das Fontes in the west. Trachyte weathers to a pale, whitish rock, which is where the names Serra Branca (white hills), Pedras Brancas and Barro Branco come from, and why Graciosa is called the White Island.

The northwest is the Northwest Platform: low ground, about 50 metres above the sea on average, covering roughly the western third of the island and carrying 32 scoria cones. Most of them line up northwest to southeast, and each is the product of a single eruption.

The southeast half is the Caldeira volcano, which the Geopark calls the smallest polygenetic volcano in the Azores, meaning one that erupted many times from the same centre. Its top collapsed into the Caldeira, a depression about 1.6 by 0.8 kilometres and up to 270 metres deep.

The island is 61 square kilometres and its highest point is 405 metres. No eruption has been recorded since settlement. The last one, from Pico Timão, happened less than 2,000 years ago.

Start in town. The hill above Santa Cruz is Pico da Ajuda, a scoria cone 540 metres across at the base and 100 metres high. Its lava ran north, and part of the town is built on it. Where that flow met the sea it made the low point of Ponta da Pesqueira, a lava delta.

The two ponds in the main square, the Pauis da Vila, are brackish and rise and fall with the tide. They once watered animals and farms. On an island with almost no surface water they were worth having in the middle of town; now they are ornamental lakes. The Santa Cruz guide covers the walk up Monte da Ajuda.

Drive northwest to the lighthouse. The tower at Ponta da Barca is 23 metres tall, the highest of any lighthouse in the Azores, and has been lit since 1 February 1930.

The geology is below it. The sea has cut deep into a group of small cones here and exposed their plumbing: dykes, the sheets of rock that fed magma upwards, and necks, the solidified pipes in the middle. The Ilhéu da Baleia offshore is one of those necks. It is shaped like a whale, hence the name: baleia means whale. The Geopark also notes a field of fumaroles on the seabed off the lighthouse, which you will not see from land.

The PR04GRA trail, Das Vinhas ao Mar, passes this way between the vineyards of Bom Jesus and the sea at Barro Vermelho; the walking guide has the details.

A little further south along the northwest coast, the bay at Vitória has a long pebble beach in its northern half and a rocky shore cut in basalt flows in the southern half. A small pond, the Charco do Rossio, sits behind the north end, fed by two streams from Pico das Bichas, the largest scoria cone on the island at about 600 metres across the base.

Look for the lava near the Ermida da Senhora da Vitória. It came from Pico Barcelos, about 600 metres inland, and is probably less than 10,000 years old, one of the youngest eruptions on the whole Northwest Platform. A thermal spring is recorded about halfway along this stretch of coast.

Porto Afonso, on the west coast, is the geosite with the most colour. Between Redondo and Porto Afonso the shore changes from cliffs of lava flows to cliffs of tephra: layer on layer of ash, lapilli and volcanic bombs thrown out by small explosive eruptions. The layers vary in colour, thickness and grain size from one band to the next. Here too the sea has eaten into the cones far enough to show their feeding system. South of the bay the coast goes back to dark basalt flows.

Tephra cliffs are soft and shed stones. Look from the top or from a safe distance on the shore, and do not sit at the foot of them.

Further south, the coast rises into the Serra Branca cliffs, at times around 340 metres high, northwest of Ponta Branca. This is the oldest rock on the island: trachyte domes and thick, slow flows, pumice and dykes, with younger basalt from the Serra Dormida cones laid over the top.

At Baía do Filipe you can see where the old trachyte ends and the Caldeira volcano begins. The rocks here include tuffs formed underwater, from when the Caldeira volcano was still erupting under the sea. The second stage of the Grande Rota round the island crosses Serra Branca, if you would rather see it on foot.

Inland in the west of the island is the Caldeirinha de Pêro Botelho: a pit inside the crater of a young spatter cone. The Geopark gives it as 25 metres deep with a chamber about 25 by 7 metres at the bottom. It was first explored in 1964 by the caving club Os Montanheiros. It is for equipped cavers only. Look into it from the rim and stay back from the edge.

The PR01GRA trail starts here. It is 11.3 kilometres, about three hours, graded medium, and crosses Pico Timão on the way to Praia. Pico Timão is about 550 metres across and 120 metres high, and it is the vent of the island's last eruption. Its thick, rough aa lava ran about 4 kilometres east and covered 3.1 square kilometres. Where it reached the sea it pushed out a point about 1,600 metres long between Lagoa-Arrochela and the sand beach at Praia. One older cone, Pico dos Alhos at Boavista, was left as an island inside the flow; geologists call that a kipuka.

Drop to the south coast at Folga. The bay sits in a low strip of land running northeast to southwest that was once a sea channel between the old western island and the young Caldeira volcano. The channel began to close about 31,000 years ago. Near the Folga lighthouse you can see deposits from the Caldeira volcano's early underwater eruptions.

PR03GRA, the island's only trail graded easy, runs 2.1 kilometres from Luz down to the Portinho da Folga in about an hour.

About 12,000 years ago a lava lake filled the Caldeira and spilled over the lower northwest rim. The smooth, ropy pahoehoe lava ran south to Folga and Luz and north towards Lagoa in Praia parish, and still covers about 4.2 square kilometres of the island.

Inside the crater is the Furna do Enxofre, the cave with a lake and a mud fumarole that you reach down a tower with a spiral staircase of 183 steps. It has its own visitor centre with set opening hours, and in winter it closes on some days; see the Furna do Enxofre guide for times and tickets.

The Furna do Abel, also called Furna do Lavar, is a lava tube formed by that overflow. It is 96.3 metres long, up to 6.2 metres high and 8 metres wide, and three holes in the roof let in daylight along most of its length. A stone altar and cross inside are left from a mass and a baptism held there in the 1970s. It is not a show cave. Ask at the Furna do Enxofre visitor centre before you go looking for it.

Finish in the southeast corner. The cliffs at Carapacho and Ponta da Restinga show the earliest, underwater stages of the Caldeira volcano. The Ilhéu de Baixo offshore is what remains of a submarine volcano. The Geopark gives the thermal water used at the Carapacho spa as 40 °C, and the Carapacho guide covers the baths and sea pools.

From Carapacho the road runs back north through Praia, across the Pico Timão flow, to Santa Cruz.

The whole loop fits in a day by car, with a stop at each site. Graciosa's buses do not reach several of these points, so you will need a hire car or a taxi; the getting around guide has the options.

• Morning: Santa Cruz, Ponta da Barca, Vitória, Porto Afonso.
• Midday: Serra Branca, the Caldeirinha and Folga.
• Afternoon: the Caldeira and the Furna do Enxofre, then Carapacho for a bath at the end.

Check the Furna do Enxofre's closing time that day and leave it enough of the afternoon, especially from November to April, when it has closing days. If you have two days, walk PR01GRA one morning and drive the rest the next.

How many geosites does Graciosa have?

Nine, in the Azores Geopark list: the Caldeira and Furna do Enxofre, the Caldeirinha de Pêro Botelho, Ponta da Barca and Ilhéu da Baleia, Porto Afonso and Redondo, Carapacho with Ponta da Restinga and the Ilhéu de Baixo, the Serra Branca cliffs and Baía do Filipe, Baía da Vitória, the Pico Timão eruption and Santa Cruz da Graciosa.

When did Graciosa last erupt?

Less than 2,000 years ago, from Pico Timão. Nothing has erupted since the island was settled.

Can I go down into the Caldeirinha de Pêro Botelho?

No, not without caving gear and experience. It is a vertical pit about 25 metres deep. Look from the rim.

Do I need a car for the geosites?

In practice, yes. The sites are spread round the coast and the buses miss several of them. A taxi for the day is the alternative.