1.4-million-year-old lava flows whose chemistry reveals they bubbled up from a mantle that barely melted. A geological oddity under stable continental crust.
A geologically quiet eruption
The Bolaven Plateau in southern Laos is a vast highland of rolling hills and coffee plantations covering 6,000 square kilometers. This entire landscape is built from ancient basaltic lava flows. The volcanic field has a long history, with eruptions occurring episodically for the last 16 million years. Some of the prominent flows are dated to around 1.2 to 1.4 million years ago.
This volcanic activity is a geological puzzle. The Bolaven Plateau is an intra-plate volcanic field, meaning it is located far from the edges of tectonic plates where most volcanoes form. The region shows no obvious signs of the continental stretching that typically allows magma to rise in such settings. The lava itself provides the clues, pointing to an origin deep within the Earth's mantle under conditions that should not produce magma easily.
A tale of two melts
Geochemical analysis of the lava rocks reveals two distinct types of magma. The most unusual are alkaline lavas, which are the product of a very low degree of partial melting. Their chemistry indicates they formed deep within the Earth's upper mantle, at depths of 85 kilometers or more, inside the garnet stability field. At these depths, only a very small fraction of the mantle rock melted to create this magma.
This contrasts with the more common tholeiitic basalt flows found across the plateau. These lavas formed from a much higher degree of melting—greater than 10 percent—at shallower depths of less than 70 kilometers. The presence of the deep, low-degree melt is unexpected for a stable continental interior. It suggests a complex thermal or chemical anomaly in the mantle that could generate magma without a major heat source like a mantle plume or significant tectonic activity. The stacked layers of both lava types can reach a combined thickness of up to 250 meters in certain areas.
💡Fun Facts
The alkaline magma originated more than 85 kilometers beneath the surface, a depth where intense pressure usually keeps the mantle solid.
The entire Bolaven volcanic field covers an area of approximately 6,000 square kilometers.
A separate scientific theory proposes that the Bolaven Plateau completely buries a massive, 790,000-year-old meteorite impact crater.
In some parts of the plateau, the accumulated lava flows from different eruptions have a combined thickness of 250 meters.
Accessible 24/7; the plateau is a large geographical region.
Admission
Free to visit the region.
Accessibility
The Bolaven Plateau is an elevated region with paved roads connecting major towns like Pakse and Paksong. The terrain consists of rolling hills, agricultural land, and some forest. Off-road areas can be rugged.