Alcoa, which produces both aluminum and parts that go into Ford pickup trucks, said on Monday that it planned to divide those two businesses.

The 127-year-old company’s commodity side will inherit Alcoa’s name. That business helped bankroll the growth of the bigger-ticket metal products business that serves markets like aerospace and automotive.

But a sharp decline in the price of aluminum, driven by weaker demand from China, has altered this relationship, yielding what many investors saw as the best possible outcome: a breakup.

Shares of Alcoa, which had fallen 40 percent this year before Monday, surged even as the broader stock market slumped. Alcoa stock ended the day up 5.9 percent.