The debates about the adequacy of future mineral supplies that took place in the 1970s illustrate this lack of understanding. The twentieth century was an era of unprecedented metal availability due to geologic and economic circumstances. The settlement of Australia, Canada, and the western United States, and colonialera mineral exploration in Africa in the second half of the nineteenth century, led to the discovery of an unprecedented number of mineral deposits that provided abundant, high-quality minerals throughout the twentieth century. In addition, innovations in mining, such as Daniel Jacklmg's recognition that low-grade porphyry copper ores could be profitably mined by using economies of scale, as well as innovations in mineral processing and metallurgy, have reduced the cost of producing metals and added to the inventory of very large deposits that can be profitably processed. Further, advances in information processing and search technologies used in mineral exploration, including the widespread application of geophysical tools, geochemical methods, satellite imaging, and mineral deposit models, increased the efficiency of mineral exploration.