This astonishing GIF comes from Nanex, and shows the amount of high-frequency trading in the stock market from January 2007 to January 2012. (Which means that the Knightmare craziness of last week is not included.)
The various colors, as identified in the legend on the right, are all the different US stock exchanges. You might think there are only two stock exchanges in the US, but you’d be wrong: there are only two exchanges where stocks are listed. There are many, many more exchanges where stocks are traded.
What we see here is relatively low levels of high-frequency trading through all of 2007. Then, in 2008, a pattern starts to emerge: a big spike right at the close, at 4pm, which is soon mirrored by another spike at the open. This is the era of traders going off to play golf in the middle of the day, because nothing interesting happens except at the beginning and the end of the trading day. But it doesn’t last long.