Hawaii Sand Beach Composition(1)

                                        Hawaii beach sand composition

Most beaches in the world are made up of the quartzmineral; others contain small rocks, coral or shell fragments, iron, smallgemstones and other minerals. The Hawaiian islands are of volcanic origin,formed by undersea volcanoes. Since Hawaii does not have a continental sourceof quartz, the popular white beaches here are composed of the carbonate shellsof marine organisms.

The Hawaiian islands are surrounded by an extensive coral reef, whichcontinuously grows as well as breaks down. The coral reef is built by marineanimals, such as corals, and plants, such as algae, that live on the reefs.

Corals are the main contributor to a reef's growth. A coral is a tiny animalreferred to as a polyp.It has a hard shell that is composed of calcium carbonate, which it extractsfrom the substances of the ocean. A collection of matured hard coral is calleda coral colony. Coralline plants, or calcareous algae, also contribute to a coral reef's growth. This algae grows on the coralheads and also uses calcium carbonate in the reef building process. Many otherspecies, such as sponges and other algae, live on the coral reef that are partof this ecosystem.

The reef breaks down into sand by two main factors – mechanical erosionand bioerosion. These two processes create some of the sand for Hawaii'sbeaches. Mechanical erosion is mainly waves and currents that erode the reefand break it down. Bioerosion is the breaking down of the reef structure intosediment by various marine animals.

One such marine species that significantly contributes to bioerosion isthe parrotfish. This fish species is named for their dentition. They have many teeth that form aparrot-like beak with which they scrape off algae and a variety of reeforganisms and pieces of coral from the reef. They play an important role inmaking the coral reef grow because the algae they feed on would otherwisesmother the coral. During the feeding process they grind up coral rock andingest it. After the rock is digested, they excrete it as sand. One parrotfish can produceabout 90 kg of sand per year.

Vocabulary

polyp n.息肉

coralline adj.珊瑚线

calcareous adj.钙质的

dentition n.牙列

excrete v.排泄