In nature, virtually all thorium is found as thorium-232, and it decays by emitting an alpha particle, and has a half-life of about 14.05 billion years (other, trace-level isotopes of thorium are short-lived intermediates of decay chains). It is estimated to be about four times more abundant than uranium in the Earth's crust and is a by-product of the extraction of rare earths from monazite sands. Thorium was formerly used commonly as (for example) the light source in gas mantles and as an alloying material, but these applications have declined due to concerns about its radioactivity.
Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand. There are two sets of estimates that define world thorium reserves, one set by the US Geological Survey (USGS) and the other supported by reports from the OECD and the International Atomic Energy Agency (the IAEA). Under the USGS estimate, USA, Australia and India have particularly large reserves of thorium.
Both the IAEA and OECD appear to conclude that India may actually possess the lion's share of world's thorium deposits. The Government of India's latest estimate, shared in the country's Parliament in August 2011, puts the recoverable reserve at 846,477 tonnes.
India and Australia are believed to possess about 300,000 tonnes each; i.e. each country possessing 25% of the world's thorium reserves. However, in the OECD reports, estimates of Australia's Reasonably Assured Reserves (RAR) of thorium indicate only 19,000 tonnes and not 300,000 tonnes as indicated by USGS. The two sources vary wildly for countries such as Brazil, Turkey, and Australia. However, both reports appear to show some consistency with respect to India's thorium reserve figures, with 290,000 tonnes (USGS) and 319,000 tonnes (OECD/IAEA).
The IAEA's 2005 report estimates India's reasonably assured reserves of thorium at 319,000 tonnes, but mentions recent reports of India's reserves at 650,000 tonnes.
The prevailing estimate of the economically available thorium reserves comes from the US Geological Survey, Mineral Commodity Summaries (1996–2010).
American estimates in tonnes (2010)
United States: 440,000
India: 290,000 - 650,000
South Africa: 35,000
Other Countries: 90,000
World Total: 1,300,000 - 1,660,000
Note: The OECD/NEA report notes that the estimates (that the Australian figures are based on) are subjective, due to the variability in the quality of the data, a lot of which is old and incomplete. Adding to the confusion are subjective claims made by the Australian government (in 2009, through its "Geoscience" department) that combine the reasonably assured reserves (RAR) estimates with "inferred" data (i.e. subjective guesses). This strange combined figure of RAR and "guessed" reserves yields a figure, published by the Australian government, of 489,000 tonnes. However, using the same criteria for Brazil or India would yield reserve figures of between 600,000 to 1,300,000 tonnes for Brazil and between 300,000 to 600,000 tonnes for India. Irrespective of isolated claims by the Australian government, the most credible third-party and multi-lateral reports, those of the OECD/IAEA and the USGS, consistently report high thorium reserves for India while not doing the same for Australia.
Another estimate of reasonably assured reserves (RAR) and estimated additional reserves (EAR) of thorium comes from OECD/NEA, Nuclear Energy, "Trends in Nuclear Fuel Cycle", Paris, France (2001):
IAEA Estimates in tonnes (2005)
Australia: 489,000 - 19%
USA: 400,000 - 15%
Turkey: 344,000 - 13%
India: 319,000 - 12%
Venezuela: 300,000 - 12%
Brazil: 302,000 - 12%
Norway: 132,000 - 5%
Egypt: 100,000 - 4%
Russia: 75,000 - 3%
Greenland: 54,000 - 2%
Canada: 44,000 - 2%
South Africa: 18,000 - 1%
Other countries: 33,000 - 1%
World total: 2,610,000