A major strength of the Soviet economy was its enormous supply of oil and gas, which became much more valuable as exports after the world price of oil skyrocketed in the 1970s. As Daniel Yergin notes, the Soviet economy in its final decades was "heavily dependent on vast natural resources–oil and gas in particular". World oil prices collapsed in 1986, putting heavy pressure on the economy. After Mikhail Gorbachev came to power in 1985, he began a process of economic liberalization by dismantling the command economy and moving towards a mixed economy. At its dissolution at the end of 1991, the Soviet Union begat a Russian Federation with a growing pile of $66 billion in external debt and with barely a few billion dollars in net gold and foreign exchange reserves.