The tech community tonight experienced its own version of an earthquake. Steve Jobs has resigned as chief executive officer of Apple (AAPL). The company has promoted chief operating officer Tim Cook to the position of CEO and said that Cook will join the company's board of directors. Jobs will become Apple's chairman. Here is a copy of Jobs' resignation letter: There are no additional details yet on why Jobs is leaving, although the spot assumption is that it's related to the pancreatic cancer for which he received a liver transplant in 2009 (during which time Cook was in charge). The fact that Jobs is taking over as board chairman, rather than resigning that seat too, would seem to indicate that his condition isn't imminently debilitating -- but there also is a strong possibility that the chairmanship is more symbolic than operational. Apple shares today traded up $2.58 to close at $376.18 per share. As of 6:53pm, it already had lost $22.18 per share, or 5.9% of its value.