Stablecoins derive their value from those reserves; if one is worth more than $43 billion – as USDC was earlier on Friday – there should be roughly that much cash or cash-like fixed-income instruments stashed somewhere backing that up. USDC's market capitalization has now slumped below $40 billion.

USDT, meanwhile, at one point spiked to $1.06 on Kraken versus the U.S. dollar, a level it almost never hits, as investors appeared to shift money away from USDC. Bitcoin rose.