Ask Mr Brain...all will be explained

PUBLISHED : Monday, 23 April, 2001, 12:00am
UPDATED : Monday, 23 April, 2001, 12:00am

Is it true that Britain is sinking in the south and rising in the north?

It is true - it is the result of a process known as isostatic rebound. Since the last Ice Age, a huge burden of ice has been removed from the north of Britain. As the Earth's crust is not rigid but slightly elastic, it gradually responds to the addition of removal of weight above it by sinking or rising.

This adjustment takes thousands of years. Scandinavia and Scotland were under more than 300 metres of ice during the ice ages, and uplift is fastest in the northern Baltic. In Britain, the process is fastest in northeastern Scotland, where raised beaches exist several metres above the present sea level.

The south of Britain is sinking because of several reasons. First, the burden of Scottish ice pushed up the crust in surrounding areas which were ice-free, just as pressing down one part of a water bed makes adjacent areas of it rise. The process is now in reverse. The once-raised regions of southern England and the southern Baltic are now sinking.

Secondly, sea level is rising globally. Before, it rose rapidly as the ice sheets over places like Scotland melted. Now the melting of glaciers because of global warming sends more water into the oceans. As the oceans grow warmer, thermal expansion raises sea level.

The south of Britain has a sinking crust and a rising sea level. Without rising sea levels, the line between the sinking and rising parts of Britain would run between Wales and Yorkshire. Because of the rising sea level, the line is further north, near the border of England and Scotland.

How can salmon live in both salt and fresh water?

Salmon, as well as eels, flounders and some types of bass are called anadromous fish. It means that in fresh water, they are able to concentrate salts into their bodies, while in the sea, they are able to expel excess salts.

The chemical reactions to either get rid of or store salts take place in the gills. Chloride cells in the gills produce an enzyme that enables marine fish to get rid of excess salt in the blood which builds up when they drink sea water.

The enzyme collects sodium ions in the blood which are then pumped out of the gills. The kidneys also filter out ions.

While in fresh water salmon concentrate salts to compensate for the low-salinity environment. They produce very dilute, and large amounts of urine to rid themselves of excess water. At the same time, in a reverse process the chemical reactions at the gills serve to store ions.

Salmon start their life in fresh water and return to the river when they hatch. They spend a relatively short time in fresh water. The majority of their life is spent in the sea. Some species spend longer than others in fresh water.