Hole in the Clouds

The Madagascans have a word for it

Nov 3, 2009

Tsingys. Which means: the kind of place where you don't want to walk barefoot.

We use the German word, karst, as a general term for tsingys and other less extreme landscapes carved by the chemical interaction of limestone and rainwater. Limestone is oceanic in origin, formed at the bottom of the sea from the shells of dead sea creatures. When tectonic forces thrust the seafloor up onto dry land, rainwater immediately begins chewing away at it, in a chemical reaction something like the vinegar-and-baking-soda volcano of an elementary school science project.

Monsoon rains have been attacking the Jurassic limestone bedrock of western Madagascar for millions of years, dissolving channels in the surface and opening up caverns underground. Eventually, as the caverns expand, the rock above tthem collapses, forming sinkholes. The sinkholes enlarge along fissues and underground drainage channels, eventually forming steep-sided  "solution valleys." The rains continue to eat away at the rock between the valleys, until all that is left is raggedy spikes. Tsingys.

It's so hard to get around in this landscape that the flora and fauna have yet to be catalogued. Even animals and plants have a hard time traveling here; they live in micro-ecosystems that have evolved in isolation from one another as well as from the rest of the world.

Ten percent of the earth's surface is karst, but most of it is too young or too arid to develop the extreme features of the tsingys. But all karst is evolving slowly or rapidly toward the kind of landscape seen here. It will be kind of a shame in a few million years when Florida gets to looking like this; who's going to want to visit beaches where you can no longer walk barefoot?

River delta

May 24, 2010

The Bestiboka River reaches the sea in the Mozambique Channel, along the northwestern coast of the island of Madagascar. There at the mouth of the river, ocean tides push saltwater upstream, slowing or even halting the downstream flow of the muddy river water; wherever the river pauses, sand and silt drop to the bottom of the bay, piling up into sandbars and islands.

In Madagascar's tropical climate, new sandbars quickly acquire a fringe of bright green mangrove scrub, which stabilizes the sediment and also shelters baby shrimp and other aquatic critters. Bombetoka Bay, the estuary here, is highly productive, especially for shrimp. The rectangular pens near the top of this picture are commercial shrimp farms.

The mangrove swamps along the lower reaches of the river trap vast quantities of sediment pouring down from upstream, which keeps the water clean and free of mud as it enters the bay; without this mangrove filtering, Madagascar's huge coral reefs just offshore (off the top edge of this picture) would soon die, smothered by sand.

The Baobabs of Madagascar, #1

Aug 24, 2012

The Baobabs of Madagascar, #2

Aug 25, 2012

Among the finalists for this year's National Geographic photo competition is this shot of a baobab grove near the town of Morondava in western Madagascar. 

Baobabs are unusual trees, with swollen trunks that store water, allowing them to survive long periods of drought. Some species of baobab can grow without soil, drilling their roots directly into bare limestone, and some are so tolerant of salt water they can grow within a few feet of the ocean. 

The trees in this picture are believed to be many thousands of years old, but baobab wood does not produce annual growth rings, making age calculations rather speculative. 

Baobabs produce fruit with a flavor that is described as very tart and grapefruit-like. The fruit pulp is a common ingredient in many regional dishes and is being studied by international food companies as a possible additive to Western-style foods and beverages, such as fruit smoothies. "It brings an interesting and exotic flavor," said PhytoTrade spokesperson Lucy Welford. "Now that we've had a lot of interest in Europe, I think there might be a knock-on effect in the U.S."