FLEXIÓN PURA

Species that rapidly evolved during the early Cambrian advanced significantly in the warm Ordovician seas. The warming was largely because the atmos-phere held as much as 16 times today’s carbon dioxide content, enough to heat the climate to tropical levels even though the Sun was 4 percent dimmer than at present.The average global temperature was about 18 degrees Celsius, some 8 degrees hotter than today. Corals, which require warm waters, began build-ing extensive carbonate reefs. In addition, the first fish appeared in the ocean.

The existence of freshwater jawless fish on the continents suggests that lakes and streams were inhabited by red and green algae.

Plants began to invade the land and extend to all parts of the world dur-ing the late Ordovician about 450 million years ago. The early land plants absorbed large quantities of atmospheric carbon dioxide. Rapid burial under anaerobic conditions deposited the organic carbon into the geologic column, where it converted into coal. Plants also aided the weathering process by leaching minerals from the rocks. Carbonate rocks, such as limestone deposited by shelly organisms from the Cambrian onward (Fig. 67), locked up massive amounts of carbon dioxide.

The withdrawal of substantial amounts of carbon dioxide from the

variations preceded changes in the extent of the more recent ice ages, suggest-ing that earlier glacial epochs might have been similarly affected.The variations of carbon dioxide levels might not be the sole cause of glaciation. However, when combined with other processes, such as variations in Earth’s orbital motions or a drop in solar radiation, they could become a strong influence.

Continental movements might also be responsible for the late Ordovi-cian glaciation. Magnetic orientations in rocks from many parts of the world indicate the positions of continents relative to the magnetic poles at various times in Earth history. Paleomagnetic studies in Africa revealed very curious findings, however.They placed North Africa directly over the South Pole dur-ing the Ordovician, which led to worldwide glaciation.

Additional evidence for such widespread glaciation came from another surprising location—the middle of the Sahara Desert. Geologists exploring for petroleum in the region stumbled upon a series of giant grooves cut into the underlying strata by glaciers. Rocks embedded at the base of glaciers scoured the landscape as the ice sheets moved back and forth. Other collaborating evi-dence suggests that thick sheets of ice blanketed the Sahara and included erratic boulders placed by moving ice and eskers, which are sinuous sand deposits from glacial outwash streams.

A major mountain building episode from the Cambrian to the Ordovi-cian deformed areas between all continents comprising the southern super-continent Gondwana, indicating their collision during this interval. The

Figure 67 Intensely

middle Paleozoic fern Glossopteris (Fig. 68), named from the Greek word meaning “featherlike,” and whose fossil leaf impressions actually look like feathers, is found in coal beds on the southern continents and India. However, the plant is suspiciously absent on the northern continents. This suggests the existence of two large continents, one located in the Southern Hemisphere and another in the Northern Hemisphere, separated by a large open sea.

Matches between mountains in Canada, Scotland, and Norway indicated their assembly into the northern supercontinent Laurasia during this time.

At the end of the Ordovician, glaciation reached its peak. Ice sheets radi-ated outward from a center in North Africa.Around 430 million years ago, the ice sheets largely disappeared.As Gondwana continued drifting southward, the farther south it went, the smaller the ice sheets became. When the center of the continent neared the South Pole, the winters in the interior became colder.Yet the land warmed sufficiently during the summer to melt the ice.

Meanwhile, the southern glaciated edge of Gondwana moved northward into warmer seas, and the glaciers soon departed.

Figure 68 Fossil Glossopteris leaves helped prove the theory of continent drift.

between about 570 and 480 million years ago suggests that this ancient sea-coast faced a wide, deep ocean.

The Iapetus stretched at least 1,000 miles across from east to west and bordered a much larger body of water to the south. It was dotted with vol-canic islands and resembled the present-day Pacific Ocean between Southeast Asia and Australia. The shallow waters of the nearshore environment of this ancient sea from the Cambrian to the mid-Ordovician, about 460 million years ago, contained abundant invertebrates, including trilobites, which accounted for about 70 percent of all species. Eventually, the trilobites faded, while mollusks and other invertebrates expanded throughout the seas.

The closing of this ancient ocean basin in the Ordovician as Baltica approached Laurentia signaled the formation of Laurasia. Large-scale mountain building followed the closing of the Iapetus when the continents flanking the sea collided, pushing up mountains in northern Europe and North America, includ-ing those that evolved into the Appalachians. The spate of mountain buildinclud-ing might have triggered a burst of species diversity.The largest of the increases was the Ordovician radiation of marine species around 450 million years ago.

Figure 69 About 500 million years ago, the continents surrounded an ancient sea called the Iapetus.

NORTH AMERICA

EUROPE

GONDWANA

Iapetus Sea

The foreland basins filled with thick sediments eroded from nearby moun-tains. Erosion from the mountains might have pumped nutrients into the sea, fueling booms in marine plankton, thereby increasing the food supply for higher creatures.Therefore, the number of genera of mollusks, brachiopods, and trilo-bites (Fig. 70) dramatically increased, because organisms with abundant food are more likely to thrive and diversify into different species.

Island arcs lying between the two colliding landmasses were scooped up and plastered against continental edges as the two continents collided. The oceanic crustal plate carrying the islands dived under Baltica in a process known as subduction.The subduction rafted the islands into collision with the

Figure 70 Fossil brachiopods and trilobites from the Bonanza King Formation,Trail Canyon, Death Valley National Monument, Inyo County, California.

(Photo by C. B. Hunt, courtesy USGS)

continent and deposited the formerly submerged rocks onto the present west coast of Norway. Slices of land called terranes residing in western Europe migrated into the Iapetus from ancient Africa. In the same manner, slivers of crust from Asia traveled across the ancient Pacific Ocean called the Pantha-lassa, Greek for “universal sea,” to form much of western North America.

A large portion of the Alaskan panhandle, known as the Alexander Ter-rane, began its existence as part of eastern Australia some 500 million years ago. About 375 million years ago, it broke free from Australia, traversed the proto–Pacific Ocean, stopped briefly at the coast of Peru, slid past California, and rammed into the upper North American continent around 100 million years ago.The entire state of Alaska is an agglomeration of terranes that were pieces of ancient oceanic crust.Terranes are well exposed in the Brooks Range (Fig. 71), a major east-west–trending mountain belt that makes up the spine of northern Alaska. Basaltic seamounts that accreted to the margin of North America traveled halfway across the ocean that preceded the Pacific.

Terranes (Fig. 72) are fault-bounded blocks. They range in size from small crustal fragments to subcontinents, with geologic histories markedly dif-ferent from those of neighboring blocks and of adjoining continental masses.

Figure 71 Steeply dipping Paleozoic rocks of the Brooks Range, Anaktuvuk district, Northern Alaska.

(Photo by J. C. Reed, courtesy U.S. Navy and USGS)

(Note: Do not confuse the term terrane with the word terrain, which means landform.) Terranes are usually bounded by faults and are distinct from their geologic surroundings.The boundary between two or more terranes is called a suture zone. The composition of terranes generally resembles that of an oceanic island or plateau. Others are composed of a consolidated conglomer-ate of pebbles, sand, and silt that accumulconglomer-ated in an ocean basin between col-liding crustal fragments.

Terranes, which are as much as 1 billion years old, are dated by analyz-ing entrained fossil radiolarians (Fig. 73), marine protozoans that lived in deep water and were abundant from about 500 million to 160 million years ago.

Different species also defined specific regions of the ocean where the terranes originated. Many terranes traveled great distances before finally adhering to a continental margin. Some North American terranes have a western Pacific origin and were displaced thousands of miles to the east.

From the Cambrian to the end of the Paleozoic, the western edge of North America ended near present-day Salt Lake City. Over the last 200 mil-lion years, North America has expanded by more than 25 percent during a major pulse of crustal growth. Much of western North America was assem-bled from oceanic island arcs and other crustal debris skimmed off the Pacific plate as the North American plate headed westward after the breakup of the supercontinent Pangaea.

Terranes exist in a variety of shapes and sizes, ranging from small slivers to subcontinents such as India, itself a single great terrane. Most terranes are elongated bodies that deform when colliding with a continent. Terranes

cre-Figure 72 Distribution of 2-billion-year-old terranes.

ated on an oceanic plate retain their shapes until they collide and accrete to a continent. They are then subjected to crustal movements that modify their overall dimensions.The assemblage of terranes in China is being stretched and displaced in an east-west direction due to the continuing squeeze India is exerting onto southern Asia as it raises the Himalayas.

Granulite terranes are high-temperature metamorphic belts formed in the deeper parts of continental rifts. They also comprise the roots of moun-tain belts created by continental collision, such as the Alps and Himalayas.

North of the Himalayas is a belt of ophiolites, which marks the boundary between the sutured continents.Terrane boundaries are commonly marked by Figure 73 Upper

Devonian radiolarians from the Kandik Basin, Yukon Region, Alaska.

(Photo by D. L. Jones, courtesy USGS)

ophiolite belts, consisting of marine sedimentary rocks, pillow basalts, sheeted dike complexes, gabbros, and peridotites.

Terranes also played a major role in the creation of mountain chains along convergent continental margins. For example, the Andes appeared to have been raised by the accretion of oceanic plateaus along the continental margin of South America.Along the mountain ranges in western North America, the terranes are elongated bodies due to the slicing of the crust by a network of northwest-trend-ing faults. One of these is the San Andreas Fault in California, which has under-gone some 200 miles of displacement in the last 25 million years.

Around 500 million years ago, North America was a lost continent.

South America, Africa, Australia, Antarctica, and India had assembled into the supercontinent Gondwana. However, North America and a few smaller con-tinental fragments were drifting freely on their own. At this time, North America was situated a few thousand miles off the western coast of South America, placing it on the western side of Gondwana.About 750 million years ago, North America lay at the core of an earlier supercontinent called Rodinia, when Australia and Antarctica bordered the west coast of the North American continent.

North and South America apparently abutted one another at the begin-ning of the Ordovician (Fig. 74), placing what would be present-day Wash-ington, D.C., close to Lima, Peru.A limestone formation in Argentina contains

Figure 74 North America (top) abutted South America (center) during the early Ordovician.

NORTH AMERICA

a distinct trilobite species typical of North America but not of South Amer-ica. The fossil evidence suggests that the two continents collided about 450 million years ago, creating an ancestral Appalachian range along eastern North America and western South America long before the present Andes formed.

Later, the continents rifted apart, transferring a slice of land containing trilo-bite fauna from North America to South America.