Parícutin

One afternoon a farmer was working in his cornfield.

The ground in front of him cracked open. Smoke came out, and it smelled like rotten eggs. By the next day, a little hill of rock had grown there. It kept on growing!

For weeks before the eruption, people nearby heard rumbling like thunder, even though the sky was clear. These were earthquakes deep underground. Then, on February 20, 1943, a farmer named Dionisio Pulido and his wife Paula were burning brush in their cornfield when the ground swelled up and split open into a crack. Gas hissed out of it, smelling of rotten eggs.

Within hours the crack became a small volcano. Hot lumps of rock flew out and piled up around the opening. After the first day the cone was already about 30 meters high, and after a month it was 148 meters high.

San Diego State University's account describes three weeks of thunder-like rumbling from earthquakes at depth; UNAM's guide says the birth was preceded by 45 days of rising regional seismic activity. On February 20, 1943, Dionisio Pulido and his wife Paula saw the ground in their cornfield swell and split into a fissure, with a hissing release of gas that smelled of rotten eggs, the smell of hydrogen sulfide (H₂S). UNAM says the vent first formed an east-west fissure that threw out ash, sulfur-rich gases and glowing bombs.

Cones grow fast in this style of eruption. UNAM's figures: about 30 m after the first day, 148 m by the end of the first month and 336 m by the end of the first year, then slow growth to 424 m in 1952. (San Diego State gives 50 m after 24 hours; the early heights were estimates.) Ash columns rose up to 8 km, and fine ash reached Mexico City, 350 km to the east, in early April 1943.

Parícutin is not alone.

There are many, many other small volcanoes around it. Deep under Mexico, a piece of the ocean floor slides down under the land. Scientists link the volcanoes here to that sliding plate.

Parícutin is the youngest volcano in the Michoacán-Guanajuato volcanic field, a wide area of central Mexico with more than a thousand volcanoes. Many of them are small cones like Parícutin. Each one had just one eruption. Volcanoes like this are called monogenetic, which means "born once".

Why are there so many? Off Mexico's Pacific coast, a plate of the ocean floor, the Cocos plate, is sliding down underneath North America. Scientists link the rock of Parícutin to this sliding plate.

The IUGS describes the Michoacán-Guanajuato volcanic field as one of the most extensive monogenetic fields in the world, with more than 1,100 Quaternary volcanic centers (Hasenaka and Carmichael, 1985); UNAM's guide counts more than 1,200 volcanic edifices, including about 300 medium-sized shields, around 25 maars and two older stratovolcanoes. Each monogenetic center erupts once.

The chemistry of Parícutin's rocks, which range from andesitic basalt to andesite, is linked to the subduction of the oceanic Cocos plate beneath North America along the Middle America Trench. UNAM notes that Parícutin and Jorullo (1759–1774) are the only monogenetic volcanoes formed in the Trans-Mexican Volcanic Belt since the Spanish conquest, that another monogenetic eruption in the field is very likely, and that a monitoring network set up after recent earthquake swarms has detected about 100,000 earthquakes.

The ash covered the fields and forests around the volcano.

Now pine and oak forests grow in the area. Purépecha people live in the villages nearby. Many of them still speak their own language.

Ash from the eruption fell over a wide area. UNAM scientists say that plants were completely destroyed across about 300 square kilometers, where the ash was more than 15 centimeters deep, and farming and nature were badly hit. Rain and wind then moved the ash around, carving gullies and even piling up dunes.

Today the land around Parícutin has forests of pine and oak, and farms grow avocado trees. The nearby villages are home to Purépecha communities, and many people there still speak the Purépecha language.

UNAM's guide reports that vegetation was completely destroyed over about 300 km², buried under more than 15 cm of ash, with a large impact on ecology and agriculture. By 1946 the new deposits were being reworked: landslides, mudflows and water erosion in the May–October rainy season, and wind ripples and dunes in the November–April dry season. More than 70% of the ash thickness around the volcano was reworked while the eruption was still going on.

The area is now characterized by pine and oak forest and avocado plantations, and the villages, including Angahuan, 23 km northwest of Uruapan, are home to Purépecha communities, many of whom still speak their native language. Heat lingers too: gases still escape from fumaroles on Sapichu and at the summit of the cone, leaving whitish coatings.

Many people visit Parícutin on horses.

The ride starts in a village called Angahuan. Then people walk up the cone. The rocks are loose, so walk slowly. It can be windy and cold at the top.

No paved road reaches Parícutin, because it is surrounded by rough lava. The easiest way, a UNAM guide says, is to ride a horse from the village of Angahuan, about three hours to the base of the cone. From there, climbing to the crater rim takes 30 to 60 minutes. The whole route is about 11 kilometers.

The slopes are made of loose scoria and ash, so the climb is hard work; take it slowly and stay on the path. The rim is about 2,800 meters above sea level, and UNAM's guide warns that the wind is often strong and the air cool up there, so bring a jacket. On the way back you can see the ruins of the old church of San Juan Parangaricutiro.

UNAM's field guide describes the standard route: Parícutin cannot be reached by paved road because it is surrounded by blocky lava, so horses are the easiest way in: from Angahuan, about 3 hours' ride to the western base of the cone, then a 30–60 minute climb to the crater rim, about 11 km and 4 hours in all. The cone stands 220 m above the surrounding lavas, because the flows buried its base, with a base width of 950 m and a crater 250 m across and about 40 m deep.

The guide calls the climb tedious on loose scoria and ash, warns of strong wind and cool temperatures at about 2,800 m, and notes that hot fumaroles remain on the cone and in the lava field. Visitors should stay on the trail, keep away from steaming ground, and carry warm clothing. The ruined church of San Juan Parangaricutiro, still standing in the lava, is a stop on the way back.