As stocks tumbled, the market's volatility index jumped higher and the tone in options turned defensive.

The Chicago Board Options Exchange's volatility index, or Vix, tracks prices that investors are willing to pay for options on the S&P 500 and gives a read on the state of investor anxiety. It rose as much as 17% to 26.10 in the first half of Wednesday's session. The index last closed in that range in mid-July.