The Ocean 9 



addition of juvenile water from the interior of the Earth by volcanic and thermal 

 activity. 



According to Penck, about 12 km^ of solid material are carried into the sea annually 

 and this would raise the level of the sea by about 33 mm in a thousand years. This 

 nomic movement will continue as long as there is land that can be eroded. When this 

 final state of erosion has been reached the sea-level will have risen about 250 m 

 higher than it is at the present time. The juvenile increase in the level of the sea amounts, 

 according to Penck, to not more than about 2-8 mm in 1000 years or barely one- 

 twelfth of the nomic. It will continue as long as volcanic activity on the Earth persists. 



A faster change than either of these eustatic movements is that due to the melting 

 of glaciers. During the ice ages there was approximately 40 miUion km^ more ice 

 covering the land than there is at the present time. This melted during a period of 

 10,000 to 20,000 years and raised the surface of the sea by 100 m or by 5-10 m in 

 1000 years (Ramsay, 1939; Penck, 1933). Melting of the present-day ice of glaciers 

 covering the land (22-2 million km^) would raise the sea-level by 55 m. The level of 

 the ocean varied during the ice ages over a maximum range of 155 m. 



The movements of the solid crust of the Earth may be of either tectonic or volcanic 

 origin or they may be due to isostatic elevation or subsidence of single parts of the 

 crust. The first may be accompanied by considerable local changes in a short time. 



Chart datum. Sea charts showing depths at different places give a picture of the 

 topography of the sea bottom. These depths are not calculated from sea-level (as a 

 reference level) but from a so-called chart datum. This has been done for purely 

 practical reasons concerned with navigation. Chart datum on English and German 

 charts is that of mean low-water springs; on French charts it is the level of the local 



I Nash Point I 

 



Portishead 



5 10 15 



Fig. 5. Mean sea level and chart datum in the main shipping route in the Bristol Channel. 



Dungeness g^-^ ^^^ 



\ Le Colbart 



\ J, 







5 10 15 Sm 



Fig. 6. Mean sea level and chart datum in the straits of Dover. 



