SLIDESHOW OF DIAGRAMS WITH EXPLANATIONS OF HOW LANDFORMS ARE CREATED
HOW AND WHERE DESERTS FORM
SOURCE:HTTP://WWW.CARTOGRAPHERSGUILD.COM/REGIONAL-WORLD-MAPPING/6464-WHY-AUSTRALIA-MOSTLY-DESERT.HTML
The main condition necessary for creating a desert is to have significantly greater evaporation than rainfall. For example, in the Mojave desert there is about 5 inches a year of rainfall, but about 300 inches a year of evaporation. Oftentimes the rainfall that does occur happens over the span of a few hours, making for heavy flooding.
Situations that cause greater evaporation than rainfall:
(a) being under the descending leg of a Hadley cell (see Hadley Circulation in your friendly neighborhood search engine). These areas fall roughly around 35 degrees north and south latitude here on the home planet. Just about where Australia sits in the south (and the Sahara in the north).
(b) being downwind of mountains. Mountains force air masses up, which makes them colder, colder air masses can hold less water so it drops as precipitation on the mountains. As the air comes down the backside of those mountains it warms again and the relative humidity drops to much lower than it was when it started up the mountains. In the case of the northern hemisphere deserts will be to the east of mountains due to Coriolis forces (forces due to the rotation of the spherical earth); in the southern hemisphere they will be on the west sides of mountains. In Australia, there is a lovely coastal range on the eastern edge of the continent with grassland grading to desert downwind of it.
(c) being far away from sources of moisture such as oceans. Rainfall happens. The more it happens the drier the air gets. If it happens long enough you don't get no rain no more and it's very dry. For example, the Gobi desert is very far downind from the nearest ocean. It's also a bit behind the Himalayas, but it's more a distance thing than rain shadow. Much of Australia is relatively far from oceans.
(d) being behind a cold offshort current. As the moist air masses pass over the current they get much colder, triggering the formation of fog and rain. This moisture is then unavailable for watering the desert areas. The Namib desert in Africa and the Atacama desert in South America have this condition (as well as being under a Hadley drop leg and having a rain shadow in the case of the Atacama).
The main condition necessary for creating a desert is to have significantly greater evaporation than rainfall. For example, in the Mojave desert there is about 5 inches a year of rainfall, but about 300 inches a year of evaporation. Oftentimes the rainfall that does occur happens over the span of a few hours, making for heavy flooding.
Situations that cause greater evaporation than rainfall:
(a) being under the descending leg of a Hadley cell (see Hadley Circulation in your friendly neighborhood search engine). These areas fall roughly around 35 degrees north and south latitude here on the home planet. Just about where Australia sits in the south (and the Sahara in the north).
(b) being downwind of mountains. Mountains force air masses up, which makes them colder, colder air masses can hold less water so it drops as precipitation on the mountains. As the air comes down the backside of those mountains it warms again and the relative humidity drops to much lower than it was when it started up the mountains. In the case of the northern hemisphere deserts will be to the east of mountains due to Coriolis forces (forces due to the rotation of the spherical earth); in the southern hemisphere they will be on the west sides of mountains. In Australia, there is a lovely coastal range on the eastern edge of the continent with grassland grading to desert downwind of it.
(c) being far away from sources of moisture such as oceans. Rainfall happens. The more it happens the drier the air gets. If it happens long enough you don't get no rain no more and it's very dry. For example, the Gobi desert is very far downind from the nearest ocean. It's also a bit behind the Himalayas, but it's more a distance thing than rain shadow. Much of Australia is relatively far from oceans.
(d) being behind a cold offshort current. As the moist air masses pass over the current they get much colder, triggering the formation of fog and rain. This moisture is then unavailable for watering the desert areas. The Namib desert in Africa and the Atacama desert in South America have this condition (as well as being under a Hadley drop leg and having a rain shadow in the case of the Atacama).
THE MID-ATLANTIC RIDGE
The mid ocean ridge systems are the largest geological features on the planet. The Mid-Atlantic Ridge (MAR) is a mostly underwater mountain range in the Atlantic Ocean that runs from 87°N -about 333km south of the North Pole- to subantarctic Bourvet island at 54°S. The MAR is about 3 km in height above the ocean floor and 1000 to 1500 km wide, has numerous transform faults and an axial rift valley along its length.
The ridge was discovered in the 1950s. Its discovery led to the theory of seafloor spreading and general acceptance of Wegener's theory of continental drift. The MAR separates the North American Plate from the Eurasian Plate in the North Atlantic, and the South American Plate from the African Plate in the South Atlantic. These plates are still moving apart, so the Atlantic is growing at the ridge, at a rate of about 2.5 cm per year in an east-west direction. Most of the ridge system is under water but forms land as a set of volcanic islands of varying size that run the length of the Atlantic Ocean. These islands are:
Information and image obtained from: http://whc.unesco.org/en/activities/504/ |
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