Xitle Eruption: The Lava That Buried Cuicuilco in Ancient Mexico

The Xitle eruption was a volcanic eruption in the southern Valley of Mexico, dated by radiocarbon to roughly AD 245–315. Lava from the small Xitle cone flowed across the valley floor and buried Cuicuilco, one of the oldest and largest early settlements in the region, along with its famous round pyramid. The rock it left behind still shapes the landscape of southern Mexico City today.

Cuicuilco before the eruption

Cuicuilco grew up near the southern shore of the lake system that once filled the Valley of Mexico. People lived there from at least around 800 BC, and between about 600 BC and the 1st century AD it became one of the two main centres of the valley, alongside the young city of Teotihuacan to the north.

Population estimates vary widely, but at its peak Cuicuilco may have had as many as 20,000 inhabitants. Its people farmed maize and other crops on the fertile land below the volcanic mountains of the Sierra Chichinautzin and built platforms, houses and ceremonial structures.

The round pyramid

Cuicuilco’s best known monument is a large circular platform built in several stages, probably beginning in the first millennium BC. It measures about 110 metres across and rises some 20 metres in a series of tiers, faced with stones and filled with earth. Ramps led to an altar or temple on the top.

Circular platforms are unusual in Mesoamerica, where later builders preferred square or rectangular pyramids. The Cuicuilco structure is one of the earliest large public monuments in central Mexico, built centuries before the Pyramid of the Sun at Teotihuacan.

The eruption

Xitle is a small cinder cone on the northern slopes of the much larger Ajusco volcano. It is a monogenetic volcano, meaning it erupted in a single episode and then fell silent. The eruption began with explosive activity that built the cone and spread ash, followed by the outpouring of fluid basalt lava.

The lava flowed north and north-east for more than 10 kilometres, filling valleys and covering an area usually estimated at about 70–80 square kilometres. Around Cuicuilco, the lava is several metres thick and in places reaches around 10 metres. It surrounded the round pyramid, covering its lower levels, and buried houses and fields across the area.

Dating and the decline of Cuicuilco

For much of the 20th century, the eruption was dated much earlier, sometimes to before the start of the AD era, and was thought to have destroyed a thriving city. This led to the popular idea that refugees from Cuicuilco helped build up Teotihuacan.

In 2000 the volcanologist Claus Siebe published new radiocarbon dates from soils and charcoal beneath the lava, placing the eruption at about AD 280, with an uncertainty of several decades. This means Teotihuacan was already a large city by the time Xitle erupted. Many researchers now think Cuicuilco had declined earlier, perhaps partly because of eruptions of Popocatépetl in the 1st century AD, which affected settlements across the region. The Xitle lava then sealed the remains of a much reduced settlement.

Rediscovery

The lava field, known as the Pedregal (“stony ground”), long hid Cuicuilco. In the early 1920s the American archaeologist Byron Cummings excavated the round pyramid, cutting through the lava to expose its sides. He recognised that the structure was older than the lava, and therefore very ancient, although his own estimates of its age were later shown to be too high.

Cuicuilco and the Pedregal today

In the 20th century Mexico City spread across the Pedregal. The main campus of the National Autonomous University of Mexico (UNAM), the Ciudad Universitaria, was built on the lava in the 1950s and was listed as a UNESCO World Heritage Site in 2007. The Olympic Village for the 1968 Games was built near Cuicuilco.

The Cuicuilco archaeological zone, with its round pyramid and a small site museum, is now surrounded by roads, housing and commercial buildings. Archaeologists believe many more remains still lie beneath the lava and the modern city.

Timeline

• c. 800 BC: Settlement at Cuicuilco is established.
• c. 600 BC – 1st century AD: Cuicuilco grows into a major centre and its round pyramid is built in stages.
• 1st century AD: Eruptions of Popocatépetl affect the region; Cuicuilco may already be declining.
• c. AD 245–315: Xitle erupts and lava buries Cuicuilco.
• 1922–1925: Byron Cummings excavates the round pyramid.
• 1950s: UNAM’s Ciudad Universitaria is built on the Xitle lava.
• 2000: Claus Siebe publishes radiocarbon dates placing the eruption around AD 280.

Why it matters

The Xitle eruption shows how natural disasters shaped early Mesoamerica, and how new scientific dating can change long-accepted stories. Cuicuilco, preserved by the very lava that ended it, remains a rare window into the Valley of Mexico before the rise of great cities such as Teotihuacan. It also connects to the earlier traditions of the Olmec civilization and other Formative peoples who first built monumental centres in Mexico.