GC1T0RB Earthworks | Supercontinents (Earthcache) in Wisconsin, United States created by seldom|seen

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A cache by seldom|seen Message this owner

Hidden : 5/29/2009

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SUPERCONTINENTS

Pangaea, was the supercontinent that is theorized to have existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration.[1]

The name was first used by the German originator of the continental drift theory, Alfred Wegener, in the 1920 edition of his book The Origin of Continents and Oceans (Die Entstehung der Kontinente und Ozeane), in which a postulated supercontinent Pangaea played a key role.

The breakup and formation of supercontinents appears to have been cyclical through Earth's 4.6 billion year history. There may have been several others before Pangaea. The next-to-last one, Rodinia, formed about 1.1 billion years ago during the Proterozoic era, and lasted until 700-750 Ma. The exact configuration and geodynamic history of Rodinia are not nearly as well understood as for Pangaea.

Rendering of Pangean Supercontinent

When Rodinia broke up, it split into three pieces, the supercontinent of Proto-Laurasia and the supercontinent of Proto-Gondwana, and the smaller Congo craton. Proto-Laurasia and Proto-Gondwana were separated by the Proto-Tethys Ocean. Soon thereafter Proto-Laurasia itself split apart to form the continents of Laurentia, Siberia and Baltica. The rifting also spawned two new oceans, the Iapetus Ocean and Khanty Ocean. Baltica was situated east of Laurentia, and Siberia northeast of Laurentia.

Around 600 Ma, most of these masses came back together to form the supercontinent of Pannotia, which included large amounts of land near the poles and only a relatively small strip near the equator connecting the polar masses.

About 540 Ma, near the beginning of the Cambrian epoch, Pannotia in turn broke up, giving rise to the continents of Laurentia, Baltica, and the southern supercontinent of Gondwana.

In the Cambrian period the independent continent of Laurentia, which would become North America, sat on the equator, with three bordering oceans: the Panthalassic Ocean to the north and west, the Iapetus Ocean to the south and the Khanty Ocean to the east. In the Earliest Ordovician, around 480 Ma, the microcontinent of Avalonia, a landmass that would become the northeastern United States, Nova Scotia and England, broke free from Gondwana and began its journey to Laurentia.[2]

Euramerica's formation Appalachian orogeny.

Baltica, Laurentia, and Avalonia all came together by the end of the Ordovician to form a minor supercontinent called Euramerica or Laurussia, closing the Iapetus Ocean. The collision also resulted in the formation of the northern Appalachians. Siberia sat near Euramerica, with the Khanty Ocean between the two continents. While all this was happening, Gondwana drifted slowly towards the South Pole. This was the first step of the formation of Pangaea.[3]

The second step in the formation of Pangaea was the collision of Gondwana with Euramerica. By Silurian time, 440 Ma, Baltica had already collided with Laurentia to form Euramerica. Avalonia hadn't collided with Laurentia yet, and a seaway between them, a remnant of the Iapetus Ocean, was still shrinking as Avalonia slowly inched towards Laurentia.

Gondwana and Euramerica separation

It was during these periods of continental shifting and colliding that the rocks found in High Cliff were formed. Sea levels were high during the Ordovician period, and if you can imagine the lakeshore as high at the Red Bird trail you can get some sense of high much water once existed here. Those early oceans teamed with life, most of which utilized the high concentration of calcium carbonate in the water to create their shells, exoskeletons and bones. Thus after many millenia of calcite-based life cycles, the ocean floors were rich with lime deposition.

Ordovician Rocks

Ordovician rocks are chiefly sedimentary. Marine sediments that make up a large part of the Ordovician system consist chiefly of limestone, because of the restricted area and low elevation of solid land, which set limits to erosion and resulted in accumulation of calicite based sediments. This is also the cause of the dramatic erosion along the bluffs, since the soft sedimentary Ordovician limestone erodes easily away leaving the harder Silurian metamorphic stone behind. You can see evidence of both stone types along the bluffs.

Silurian Rocks

The continents in the Silurian period remained much as they had been in the preceding Ordovician period, with approximately the same areas being subject to flooding by shallow seas. The earth was relatively tectonically inactive during the Silurian. The Appalachian Mountains, which uplifted during the Ordovician, were being eroded. Large coral reefs and algae were abundant, indicating that warm, shallow seas predominated. In North America, the Silurian ended quietly; however, in the British Isles, Scandinavia, and France, as a result of the Caledonian disturbance, great mountains continued to be thrust up. Economic resources of the Silurian strata, besides salt, are iron ore (near Birmingham, Ala.) and quartz sandstone, used in glass manufacture. Dominating the life of the Silurian were marine invertebrates, including crinoids and cystoids, mollusks, and eurypterids, invertebrates related to crabs and insects. Members of the trilobite family were still numerous; primitive fishes increased in number. Also notable in the Silurian fauna were scorpions, possibly the first animals to live on land and take their oxygen from the air.

Coordinates will bring you to the Upper trail head of the Lime Kiln Trail. When you reach the bottom of the steps, proceed about 20 yards south to see the iconic stone. Explore the bluffs a little, looking for the 2 types of stone. You'll need to answer a couple questions about the rocks types.

Logging Requirements:
To complete this Earthcache, please provide answers to the following questions.

A. How long do you think the column of rock in the picture has been
standing this way, hundreds, thousands or millions of years?
B. Find a piece of Ordovician limestone and describe it's color and hardness.
C. Find a piece of Silurian sedimentary stone and describe it's color and hardness.

References
1. Plate Tectonics and Crustal Evolution, Third Ed., 1989, by Kent C. Condie, Pergamon Press
2. Stanley, Steven (1998). Earth System History. USA. pp. 355–359.
3. Stanley, Steven (1998). Earth System History. USA. pp. 386–392.
4. The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2007, Columbia University Press.

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