Spurred by the sudden fall of a chunk of Vermont marble from a column, the National Park Service has decided to close parts of the Jefferson Memorial for at least a year so contractors can undertake emergency studies and repairs of the 47-year-old structure.

The problem underscores the conclusions of two studies released in April that found that the Lincoln and Jefferson memorials needed at least $12 million in tests and repairs because of budding structural flaws and marble erosion caused by acid rain, insects, jet fuel and other factors.

Park Service officials said yesterday that tourists have been barred since Tuesday from the circular colonnade around the outside of the Jefferson. Inside the main chamber, they are now kept 10 feet from the walls by fencing covered in plastic sheets.

In addition, the interior walls of the memorial, bearing inscriptions of some of Jefferson's most famous writings, will soon be obscured by scaffolding that must be erected so contractors can begin inspecting marble for flaws and deterioration.

The problem began in May when a piece of marble 20 inches long and weighing about 20 pounds fell one night from a carved scroll atop one of the memorial's Doric-style, 42-foot columns in the colonnade, said Vikki McGraw, assistant superintendent for Park Service's National Capital Parks-Central.

Park Service officials said, however, that they did not know the situation was serious until this week, when contractors began examining the scrolls of the 53 other columns. During that inspection, a similar piece of marble broke off another scroll when a worker bumped it with a movable scaffold. He was injured slightly by the falling marble, Park Service officials said.

To prevent similar occurrences, workers then deliberately removed portions of half a dozen other scrolls this week after concluding that the stone appeared weakened, said McGraw. These pieces will be reattached or replaced, officials said. The decision was also made to close parts of the memorial to the public.

As they did when the studies were released in April, Park Service officials said yesterday that the Jefferson Memorial was in good condition overall. But they said the falling marble prompted them to accelerate tests they had planned to make in 1991 in response to the April studies' warnings.

"This has changed our opinion a little bit," Anthony S. Donald, a Park Service architect, said yesterday at the Jefferson. "We have a lot more going on in relation to stone issues than we originally thought."

Calling the falling chunks "an early warning signal," McGraw said, "it tells us we need some immediate study."

Because all the pieces that either fell or were removed came from about the same place on the decorative scrolls, the problem could be a flaw peculiar to their design, McGraw said. But that flaw could be exacerbated by other factors.

"We're waiting for the experts to give us more definite information," McGraw said, "but they're looking at vibrations from National, the proximity to {Interstate} 395, weathering."

By Tuesday, scaffolding should be erected along the northern walls of the circular inner chamber so that architects, engineers and geologists can begin a stone-by-stone check, Park Service officials said. Eventually scaffolding will ring the inner chamber, leaving only the dome and the 19-foot bronze statue of Thomas Jefferson unobscured.

The dome itself, as well as the memorial's exterior walls, will also be checked in a process that will take a year and lead to recommendations on correcting any problems found, the Park Service said in a statement. McGraw said the initial analysis might cost up to $40,000, but she had no estimate for the cost of any repairs.

Tourists applauded the work yesterday, even though scaffolding and fencing will fill many photographs in the coming months. "That'll be sad for a while, but in the long run people will benefit," said Phil Steillian, who was visiting from Santa Cruz, Calif.

"Most anybody would like a free and clean and pristine picture, but if you look at it in the long run, it's good for the nation's capital," said Jim Kennedy, who had his family in tow from Xenia, Ohio.

And Susan Butler of Redondo Beach, Calif., said of the scaffolding, "Shoot around it . . . . I don't think you can be selfish about saving a monument."