You have asked a couple of great questions. And Charles Darwin himself was one of the first people to try to answer them!

Coral reefs form in the warm shallow water of the tropics, all over the world. The animals that make coral reefs, the corals themselves, can only survive in water that is warmer than about 70 degrees all year. Since the ocean gets much colder than that here, corals can't live in California. But, the water in many other places (like Hawaii) is warm enough and it is in these places where coral reefs form.

Coral reefs are made up of millions and millions of individual corals, who live in colonies. Each individual makes a small external skeleton of calcium carbonate, the same thing that clam shells are made from.

Each individual skeleton is attached to all the others, and so while each animal doesn't make much, the entire structure can be huge. Imagine looking at a single brick (like an individual coral) and comparing it to a huge brick building (the coral reef).

There are lots of different kinds of corals that make up a coral reef, and each kind grows at different speeds. Some grow very slowly (less than 2 mm per year, or 1/10th of an inch), but others can grow much faster, up to 10 cm per year (or 4 inches). Since the reefs are made up of all of these different kinds, the entire reef will grow pretty slowly, usually around 1 cm per year (a little less than 1/2 inch). It seems to me like coral reefs form at a pretty slow rate, but it really depends what you are comparing it to!

The corals grow throughout the year, but they seem to grow best when the water is warm and clear. They probably grow best during the summer, whenever it is summer where they are growing. So coral reefs are continuous growing, forming and changing. But since they grow so slowly, it is really hard for us to watch this process. Still, scientists estimate that some coral reefs may have been growing continuously for over a million years!

Coral reefs are built from tiny creatures (smaller than the size of a quarter) called polyps. These soft-bodied polyps remove calcium carbonate from ocean water and use it to in their hard outer skeletons.

Polyps live in colonies and when one dies, its skeleton remains for other polyps to live on. Algae then deposit limestone to cement polyp skeletons into place. This process of living coral polyps building on top of the skeletons of their predecessors slowly builds up the reef at a rate of less than 5 inches per year. To start, you need a few polyps to begin growing in the same area; forming a colony on the same rock would be a good start.

Warmth and light are also necessary for a coral reef. Polyps can't survive in water much colder than 61 F. Shallow waters are good places for coral reefs to grow because sunlight can penetrate to the reef, allowing the algae and other plant life on the reef to undergo photosynthesis. With these requirements, most reefs are found in tropical waters.

The Great Barrier Reef in Australia is the most famous coral reef, having taken over 600,000 years to reach its present size. However, others are found off the coasts of Florida, Brazil, Sri Lanka, and Madagascar, to name just a few.

Corals start their life as a free-swimming young (planula larvae) that are carried by ocean currents. The larvae will drift with the current until it finds a hard bottom to attach itself. Once the larvae attaches to the bottom it quickly changes into a polyp.

Corals secrete new skeletal material at different rates, depending on seasonal and yearly differences in the earth's climate. This variation in growth results in a banding pattern that can be seen in coral cores, much like the bands on trees, and allows scientists to age corals and determine their growth rates. From these rings scientists have found that corals grow very slowly, at a rate of only 1 to 10 cm (.4 to 4 inches) per year, and that some coral colonies are hundreds to thousands of years old.

Coral reefs form where there is (1) enough sunlight, (2) warm temperatures, and (3) favorable chemical conditions.

Usually, this is on the coasts of tropical land-masses or around eroding tropical volcanic islands.

Corals grow continually, but the fossil record of coral growth implies much slower growth rates over periods of geologic time than we observe over human time. It is likely that such growth is sporadic, slower at times and faster at others.