Apple shares plunged Thursday after the company reported quarterly results that point to growth slowing after five blowout years.

The stock was down $53.55 or 10 percent, at $460.46 in midday trading.

Apple Inc. is still the world's most valuable company, a position it's held for more than a year. It's now worth just 3.5 percent more than No. 2 Exxon Mobil Corp, however.

Late Wednesday, Apple reported October-December earnings that were flat compared with the year before. Sales grew 18 percent from the year before, but the startup of production lines for multiple new products like the iPhone 5 and iPad Mini held back profits.

Of more concern to investors is Apple sales growth forecast for the current quarter of around 7 percent —far from the 50-percent-plus rate it's often hit in recent years. Apple usually lowballs its forecasts, but Chief Financial Officer Peter Oppenheimer indicated that it's moving toward providing more realistic figures from now on.

Analysts believe Apple is unable to fully capitalize on the global smartphone boom with just one new —and very expensive— phone model every year. There has been speculation among company-watchers that Apple may launch a cheaper iPhone, or a premium model with a bigger screen, but CEO Tim Cook put a damper on those rumors in his comments on a conference call following the report.