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There has been a great deal of tectonic activity in the Mekong basin, starting tens of millions of years. Some of this activity was accompanied by extensive lava flows, particularly during the lower and middle Pleistocene, a geological era that lies back some 40 million years. The largest remnants from that time period consist of sheets of basaltic lava east of Pakse along the Mekong in southern Lao PDR, now known as the Bolovens Plateau. Today, the plateau is still a very prominent, saucer-shaped topographic feature, as the volcanic rock material resists erosion. Consequently, the plateau rises more than 3250 feet (1000 metres) above the Mekong and stretches over an area of more than 100 by 125 km. It is one of the highest regions of the Mekong Basin and receives more than 4000 mm of rainfall per year, the highest in Lao PDR. Although reputed to have rich soil, the region is sparsely populated. Similar, albeit much smaller, outcrops of Quarternary basalt flows can be found throughout the southern portion of the Mekong basin.
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