Despite Thursday's unexplained surge in selling that drove the Dow down 900 points, the stock markets are being driven lower by fears over the global economy and the debt crisis spreading, economist Nouriel Roubini, of RGE Monitor, told CNBC Friday.
Greece is just the "tip of the iceberg" for the problem of accumulating debt, and it must be solved by raising taxes and cutting spending, if not default will follow, Roubini said.
"My concern is eventually, not this year, it may spread to Japan and in the US," he said.
