LATEST Archaean and earliest Palaeoproterozoic times (from 2.6 to 2.2 billion years ago) have generally been viewed as a largely quiescent period of Earth history; the geological record indicates the very slow deposition of pelagic and chemical sediments, and bears only a limited record of magmatic and tectonic activity. Such quiescence is consistent with the contention that the Earth's main banded iron formations (BIFs)-finely laminated chemical sedimentary rocks, rich in iron oxide-formed slowly as oxygen abundances in the oceans gradually increased, thus reducing the capacity of sea water to retain dissolved iron. Here we show that a large igneous province, comprising >30,000 km3 of dolerite, basalt and rhyolite, accompanied deposition of a Hamersley Province BIF 2,449 ± 3 million years ago. This observation indicates that Hamersley BIFs formed during a major tectono-magmatic event and were deposited very much faster than previously thought, at similar rates to (or faster than) modern pelagic sediments. Thus the largest Palaeoproterozoic BIFs, rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence, are more likely to have formed as a result of an increased supply of suboxic iron- and silica-rich sea water upwelling onto continental shelves during a pulse (or pulses) of increased submarine magmatic and hydrothermal activity.