A lake in Ghana formed by an asteroid impact about a million years ago that has been recording African climate in its sediments ever since. Each layer of mud preserves evidence of past monsoons and droughts, creating a climate record that stretches back a million years.
NASA Earth Observatory images by Michala Garrison, using Landsat data from the U.S. Geological Survey. Story by Adam Voiland., Public domain, via Wikimedia Commons
A climate archive from an asteroid
About 1.07 million years ago, an asteroid struck what is now Ghana, hitting 2-billion-year-old crystalline rocks. The impact formed the Bosumtwi crater, a well-preserved structure with a diameter of about 10.5 kilometers. This event was so powerful that it created tektites—natural glass formed from ejected earth—that are found 350 kilometers away in Côte d'Ivoire. Over time, rainfall filled the crater, creating Lake Bosumtwi, Ghana’s only natural lake.
The lake has no outlet, meaning water and sediment that flow in become trapped. This unique feature makes it a perfect natural archive. Year after year, layers of sediment settle at the bottom, preserving a continuous record of West Africa's environmental history. The total depth of the crater is around 750 meters, including a thick accumulation of these post-impact sediments. These layers of mud act as a record of past climate, documenting shifts in rainfall, vegetation, and atmospheric conditions for over a million years.
Drilling deep into Africa's past
In 2004, the International Continental Scientific Drilling Program (ICDP) undertook a major project at Lake Bosumtwi. Using a floating platform called the GLAD800, scientists drilled sixteen holes at six different locations across the lake. They extracted sediment cores and also drilled into the impact-brecciated rock beneath, reaching a maximum depth of 540 meters. In total, the project recovered about 2.2 kilometers of core material.
These sediment cores contain paleoclimatic information. Scientists analyze pollen trapped in the layers to reconstruct past vegetation, showing shifts between rainforest and grassland. Chemical proxies, such as the ratio of certain elements, indicate the intensity of past monsoons and periods of drought. For example, analysis of nitrogen isotopes helps show changes in lake level and moisture availability. The cores provide one of the longest and highest-resolution terrestrial records of climate change from sub-Saharan Africa, showing how the region responded to global climate cycles like ice ages and warmer interglacial periods. Data from the cores have helped correlate West African climate shifts with events recorded in Antarctic ice cores, showing connections between high-latitude and tropical climate systems.
💡Fun Facts
The Ashanti people consider Lake Bosumtwi sacred, a place where the souls of the dead bid farewell to the goddess Asase Ya.
It is traditionally forbidden to let iron touch the lake's water, so local fishermen use large wooden planks called "padua" instead of conventional boats.
The impact that formed the crater was so energetic it created a strewn field of tektites (impact glass) in neighboring Côte d'Ivoire.
The crater is one of only about 190 confirmed impact structures on Earth.
The lake is a natural area and generally accessible at all times, though visitor services and activities have specific hours, typically during the day.
Admission
A small entrance fee is charged at a checkpoint on the road to the lake, which helps support the local communities.
Accessibility
The lake is in a crater surrounded by steep hills. A paved road leads down to the main village of Abono. The terrain around the lake can be uneven, but resorts and villages are accessible.
