The place where the rock 'laterite' was named, now a National Geological Monument.
Angadipuram, in the Malappuram district of Kerala, is the birthplace of a word. It is the type locality of laterite, the place a whole family of rock is named after, because this is where a visitor first described the rock and gave it a name that stuck. That name is now used for the same material the world over.
Laterite is a soft, iron-rich, brick-red rock full of holes, the leftover crust that stays behind after a rock has been weathered for a very long time in a hot, wet climate. At Angadipuram it caps the older, harder rock underneath, a dark crystalline stone called charnockite.
To see how it forms, picture rain soaking into rock year after year in the tropics. Warm temperatures and heavy monsoon rain speed up the slow chemical attack that water makes on minerals. Two things happen together. The silica, the same stuff that makes up quartz and sand, and the soluble bases like calcium, sodium and potassium dissolve and drain away in the water. What barely dissolves, the iron and aluminium, is left in place. Over time the rock is stripped down to a residue of iron and aluminium oxides, and it is the iron oxide, the same chemical as rust, that gives laterite its red colour.
This is why laterite is a leftover rather than a new rock laid down in layers. It is not made by adding material, it is made by taking material away. The parent rock at Angadipuram is charnockite, and as the silica and bases leach out of it, the porous red cap builds up on top, sometimes several metres thick. The alternating wet and dry seasons of the Kerala coast help the process along, wetting the ground to dissolve and carry off the solubles, then drying it so the iron and aluminium set in place.
The naming happened in 1807. Francis Buchanan-Hamilton, a Scottish physician travelling through southern India for the East India Company, described the rock in his journey report and coined a name for it from the Latin later, meaning brick. He had good reason to reach for that word. Local people were cutting the rock straight out of the ground in blocks and using it to build, exactly as you would use bricks.
That building trick is the property Buchanan singled out, and it loops back to how laterite forms. Down in the ground the rock is soft enough to cut with a spade, because it is a weathered residue riddled with pore spaces rather than a hard, fused stone. Buchanan noticed that once a block is dug out and left in the air it soon hardens, in his words becoming as hard as brick and resisting air and water. The likely reason is that exposure lets the iron and aluminium oxides dry out and harden into a firm crust, cementing the block. Soft to quarry, hard once set: this is why laterite has been a favourite building stone across Kerala and the Western Ghats for centuries, and you can still see it cut in neat blocks straight from the earth today.
The Geological Survey of India marked the site as a National Geological Monument in 1979, during an international conference on how laterite forms. It is a quiet quarry, but it is where a rock that covers large parts of the tropics first got its name.
Sources
- Geological Survey of India — National Geological Monuments
- Buchanan, F. (1807), A Journey from Madras through the Countries of Mysore, Canara and Malabar — the first description of laterite
- Angadipuram Laterite, National Geo-heritage Monument (GSI, declared 1979)
- Laterite, Encyclopaedia Britannica (formation by tropical weathering and leaching)
- Lateritic Deposits: Formation, Mineralogy, Ores — Geology Science