MIAMI (Reuters) - Tropical Storm Ike, the ninth of a busy Atlantic hurricane season, formed on Monday midway between Africa and the Caribbean and was expected to grow rapidly into a hurricane that could threaten the United States or the Caribbean.

Ike was churning across the Atlantic on the heels of Hurricane Gustav, which pounded New Orleans on Monday as it came ashore on the U.S. Gulf Coast, and Hurricane Hanna, which strengthened as it neared the south-eastern Bahamas islands.

The peak of the six-month Atlantic hurricane season usually occurs around September 10, and an average season spawns 10 tropical storms. Six of those strengthen into hurricanes.

Ike's formation, and the possibility of another tropical depression developing in its wake in the coming days, means the storm activity this year is well above normal, bad news for U.S. oil and natural gas production in the Gulf of Mexico and for the millions living in the Caribbean and on U.S. coasts.