In a move that would permanently ban oil drilling along more than 50 miles of Northern California coast, the Obama administration announced plans Thursday to expand two Northern California marine sanctuaries, extending them up the rugged Sonoma and Mendocino coast.

The announcement, at a news conference in Washington, D.C., with members of the Bay Area congressional delegation and officials from NOAA, the National Oceanic and Atmospheric Administration, marks the largest expansion of national marine sanctuaries in California in 20 years — since President George. H.W. Bush established the Monterey Bay National Marine Sanctuary in 1992.

“This area is a national treasure. It needs and it deserves permanent protection from oil and gas exploration,” said Rep. Lynn Woolsey, D-San Rafael, who led efforts to expand the sanctuaries.

“This is a matter of economic common sense. Jobs and livelihoods hang in the balance,” she said. “No one is going to vacation on the Sonoma coast if they are looking at oil derricks.”

While national marine sanctuaries ban oil drilling and other extractive activities, and set rules on such practices as sewage dumping by cruise ships, they do not ban fishing or boating.

The newly protected area will enlarge the Gulf of the Farallones and Cordell Bank national marine sanctuaries by 2,771 square miles, more than doubling their size, and will extend from Bodega Bay near the Marin County-Sonoma County border north to Point Arena in Mendocino County.

The area is one of the West’s most scenic coastal landscapes, famous for its steep cliffs, rugged wind-swept bluffs and long sandy beaches. In the late 1970s and early 1980s, oil companies showed interest in sinking new rigs off the area, which includes the communities of Jenner, Sea Ranch and Gualala, along with Fort Ross, a former Russian fur-trading outpost dating back to 1812.

“The waters off the Northern California coast are incredibly nutrient-rich and drive the entire natural system and, for almost a decade, local communities have been petitioning their elected officials to expand sanctuary protection to these areas,” said Daniel J. Basta, director of the NOAA’s Office of National Marine Sanctuaries.

In recent months, Woolsey, who is retiring from Congress on Jan. 3, urged President Barack Obama to use his executive authority to create a new national monument along the scenic Sonoma coast. Obama, however, stopped short of creating a monument, opting instead to use the NOAA administrative process, which triggers public hearings in Northern California early next year, along with detailed environmental studies. It is expected to take up to two years to finalize.

The reason Obama chose the slower process: President George W. Bush used executive authority in 2006 to create a marine monument in the remote Northern Hawaiian islands, upsetting some Gulf of Mexico senators who were concerned the authority might one day be used by presidents to ban oil drilling there, an administration source said Thursday. As a result, NOAA Administrator Jane Lubchenco said during her confirmation hearings several years ago that the Obama administration would not create national monuments in the oceans, but instead use existing NOAA rules.

“This is one of the crowning achievements of the coastal protection movement in California,” said Richard Charter, a senior fellow with the Ocean Foundation in Washington, D.C. “This is a permanent ban on offshore drilling, forever, at a time when Congress has not been particularly interested in conservation.”

Woolsey has tried to pass bills in Congress since 2004 with the same goal. But her most recent effort, HR192, has been blocked by House Republican leaders who oppose new limits on oil and gas production.

NOAA has the authority, without a vote of Congress, to enlarge sanctuary boundaries. Efforts are already under way by NOAA, for example, to expand the Monterey Bay National Marine Sanctuary to include a small section of water in front of the Golden Gate Bridge that was left out of the original Monterey sanctuary designation in 1992.

Although technically the Obama administration could change its mind after the public hearings, that is highly unlikely. The expanded boundaries are supported by Gov. Jerry Brown and a large number of the state’s congressional representatives. And NOAA has never reversed course after starting a sanctuary expansion and decided not to proceed.

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/paulrogerssjmn.