|Iran knows the value of its geography... too bad its map has the old borders of Yemen and one Sudan|
The strait at the narrowest are thirty-five miles wide at the point where Iran faces Oman's exclave off the United Arab Emirates. This narrow geography would allow a small fleet to wreck havoc on oil shipping convoys.
This threat, if acted upon, would cause a serious disruption in the world's oil supply and gasoline prices as well as being a cause of war/theater of war. Oil from Iraq, Kuwait, Iran, Saudi Arabia, Bahrain, Qatar, and the United Arab Emirates go through the strait to markets in China, Europe, and North America. Ships transport around 20% of the world's oil supply through the strait.
There are reasons to believe Iran will not do this. The primary one is Iran needs its oil money to survive. To close down the strait or to make them a war zone will cost them much of their budget and throw their own economy into a depression. Also, it would turn a war against American and/or Israel into a war against many of Iran's Arab neighbors. The United States said that it would reopen the strait by force if need be. It would not be the first time the United States fought the Islamic Republic of Iran over its actions in hurting the oil trade: Operation Praying Mantis saw the United States sink several Iranian naval ships over Iran's mining Persian Gulf convoy routes.
Saudi Arabia has also vowed to increase oil production if the strait are closed to reduce the economic damage. However, the shear shock of the closing of the strait would not be fully covered by more oil pumping. If the strait were to be closed then the world's economy would suffer for some moderate period of time.
The Strait of Hormuz are a chock point and Iran's ultimate trump card in terms of geography. While the potential of its damaging effect are well known, only time will tell if the strait are actually closed.