Lake Vänern

Hjortens_Udde,_lake_Vänern_Sweden,_2003-04On a map of Sweden, Lake Vänern would take a big fresh bite out of the country’s middle. About 2,150 square miles (5,600 km2) in extent, it is 87 miles long (140 km) and decorated with a large archipelago
of islands called Millesvik. The lake’s average depth is 88 feet (27 m), its deepest point 347 feet (106 m) down. Vänern is the fourth largest lake in all of Europe and the largest between the Ural Mountains and the Atlantic Ocean. The 28th-largest lake in the world, it stretches across most of Sweden’s east-west dimension. The lake’s southern tip lies near the city of Göteborg, where Volvo cars are made for the worldwide market. Its eastern shore is just a couple of hours’ drive west of Stockholm, the capital city on the opposite side of
the country. And its northern shore is close to Sweden’s border with Norway.
About a dozen larger and smaller towns touch its edges.
This beautiful lake lies in a “lake district” rich in lakes and forests and laced with small streams and rivers. Europe’s lakes lie mostly in a wide, roughly east-west, zone, many tens and tens of thousands of them in countries from Britain to Finland. Though Vänern has an ample companion lake, Vättern, most of the lakes that dot these European latitudes are smaller.
We see a similar phenomenon in the United States, in the northern tier of states from Maine through Minnesota, where the Great Lakes are accompanied by many tens of thousands of smaller lakes. And Canada has a similar zone, an even larger terrain of lakes and forests. Why lakes are often found in districts is a geological question that this chapter shall address.
In a typical lake district resources are generally exploited along two main dimensions. The first is timber, used by the wood products and paper products industries. The second is through the creation and use of hydroelectric power, which harnesses the flow of the many rivers. More than 100 rivers here at Lake Vänern are harnessed to make ctricity.
The primary reason for this blue “polka-dotting” of the planet, the creation of lake districts, is the glaciers. The glaciation will be discussed, but first, Lake Vänern’s even longer ago origins will be described.
Origins
Lake Vänern lies in a stable stretch of the Eurasian plate, one of the broad rocky slabs of the Earth’s crust that carry the continents. The area is called the Scandinavian shield. (For a map of all the Earth’s shields, or broad central and stable areas, see chapter 3: Lake Superior.) Shield areas are now geologically quiet, with no tectonic ctivity taking place. Because of this no one need fear an earthquake, nor admire a distant volcano, at Lake Vänern.
C041559When one goes further back in time, however, more action was taking place. In the Precambrian epoch (4 billion to 600 million years ago), a period of emphatic tectonic activity occurred near here, though not at what would become Lake Vänern. Areas both north and northeast of the lake and south of it show evidence of ancient crustal collision, land deformation, mountain building and mountain erosion, thickening of the crust, and rotation and rumpling of the terrain. This happened here, not only in the Precambrian but also in the period from about 1.675 million to 975,000 years ago. This later date, well after the Precambrian epoch ended, was probably the last major reworking of the land in the area. In between these two periods, during the Cretaceous period (135 to 70 million years ago), the land that is now under Lake Vänern’s water was once part of the ancient Tethys Sea. At this time mainland Scandinavia and Greenland were still attached. Land-action near continental shields is defined as orogenic activity. Orogenic belts, also called mobile belts are the impact or collision zones at the edges of the stable ontinental sections or shields. In these belts the land is compressed, squeezed, pressed down, lifted up, folded, and thus “recycled” by tectonic activity, often in the form of ancient earthquakes and volcanoes. An orogenic phase is short in geological terms, usually lasting “only” tens of millions of years.
After that phase ended here, and the period of the Tethys Sea passed, the land that would later hold Lake Vänern settled down, looking and acting like part of the shield, as it does today. Even its narrow peninsulas, such as the one shown in the color insert on page C-5, are stable.
Glacial Origins
Lake Vänern as we see it now is “young.” Well beyond and after the original tectonic molding, the land required a glacier to make a lake. A classic glacial lake, it is a typical one across the temperate zone of the planet.
Most of the lakes of the world are actually found in this zone and are anywhere from about 18,000 to about 9,000 years old. Their basins were molded into final form by the glacial ice sheet as it advanced, then filled with water as it retreated. The glacial ice in more northerly places, such as the Lake Vänern area, took longer to melt than it id farther south.
80 G Lakes This lake is only about 9,000 years old. The glaciers of the world, even today, are great white bulldozers. Each year they push and shove around approximately 4.3 billion tons of rock and soil. Just imagine how much scraping and crunching was done by the immense glaciers of the past as they advanced or retreated and grew eavier or lighter, depending upon local snowfall. They made tens and tens of thousands of lakes in the lake districts of the world, smaller ones from single large blobs of ice. Near Lake Vänern, though across the border in Norway, laciation has not entirely vanished. Valley glaciers still exist at high elevations, a few white dots too minuscule to appear on most maps. Elsewhere in Scandinavia, Iceland’s Vatnajökull glacier (the largest in Europe) is still vigorous. Any summer day visitors can walk right up to one of its lobes (like a snout or paw) and see plenty of black lava rock, sand, and other sediments on its ice surface. This material both coats the ice and curls up in front of it, ready to be shoved forward, probably next winter. The glacier looks as though it has a dirty paw, though it is ore like the edge of a giant moving van.
We still live in the Ice Age, but in a warm “interglacial” period. The last true heyday of the great ice was about 20,000 to 18,000 years ago. After that the melting became quite steady. What looks like a typical, ordinary world to us—glaciers and ice mostly at both poles—is only a planet snapshot,our one small, precious, moment of geological history.