Saprolite - Definition, Description and Locations

Definition, Description and Locations

Saprolite (from Greek σαπρος = putrid + λιθος = rock) is a chemically weathered rock (literally, it means "rotten rock"). More intense weathering results in a continuous transition from saprolite to laterite.

Saprolites form in the lower zones of soil horizons and represent deep weathering of the bedrock surface. In lateritic regoliths – regoliths are the loose layer of rocks that rest on the bedrock – saprolite may be overlain by upper horizons of residual laterite; most of the original profile is preserved by residual soils or transported overburden. Weathering formed thin kaolinitic saprolites 1,000 to 500 million years ago; thick kaolinitic saprolites 200 to 65 million years ago; and medium-thick immature saprolites 5 million years ago. The general structure of kaolinite has silicate sheets bonded to aluminium hydroxide layers.

Iron compounds are the primary coloring agents in saprolites. At most outcrops the color comes from ferric compounds; the color relates to the mineralogy and particle size. Submicron-sized goethite is yellow; coarse goethite is brown. Submicron-sized hematite is red; coarse hematite is gray to black.

Regoliths vary from a few meters to over 150 m (490 ft) thick, depending on the age of the land surface, tectonic activity, climate, climate history and the composition of the bedrock. Although these deeply weathered terrains now occur in a wide variety of climates ranging from warm humid to arid, tropical to temperate, they were formed under similar conditions in the past. In parts of Africa, India, South America, Australia and southeast Asia, regolith has been forming continuously for over 100 million years. Deeply weathered regoliths are widespread in the inter-tropical belt, particularly on the continental landmasses between latitudes 35°N and 35°S. Similar weathered regoliths exist at much higher latitudes – 35–42°S in southeast Australia (Victoria and Tasmania), 40–45°N in the United States (Oregon and Wisconsin) and 55°N in Europe (Northern Ireland, Germany) – although these are not regionally extensive.