The first-order features on the Arctic Sheet of the Geologic Map of the Circum-Pacific Region are its continents and ocean basins. Traditionally, geologists considered these features to be primeval components of the surface of the Earth, but Alfred Wegener, beginning in 1912, championed the idea that the continents move about on the surface. After that, the theory of plate tectonics added that seafloor spreading continuously creates new ocean floor--as at the Juan de Fuca Ridge--and subduction (where one plate descends beneath another) continuously destroys it--as at the Aleutian Trench.
We now know that some rocks on the continents are as old as 4 billion years (Archean), but the world's seafloor is nowhere older than about 180 million years (Jurassic). Details of the age of the seafloor for this map sheet appear on a companion map, the Arctic Sheet of the Plate-Tectonic Map of the Circum-Pacific Region. The oldest seafloor on the Arctic Sheet is 153 million years, near Makarov Seamount off Japan, and the youngest is still forming, along the Mid-Arctic (Nansen), Mid-Atlantic (Reykjanes), and Juan de Fuca Ridges.
Interior areas of the continents are marked by cratons--ancient flat stable areas. Archean and Proterozoic rocks (red and brown on the map) crop out on shields at the centers of the cratons, and broad little-folded overlying bands of Paleozoic rocks (purple and blue) lie on platforms around the shields.
The old Archean rocks of the cratons contain complex geologic structures. They once had the rough topography and varied crustal thickness of present-day mountain regions. The mountains had deep, low-density crustal roots that rose up by isostacy--a process similar to floating. Over a period of about 1 billion years, repeated uplift of the deeply rooted areas, followed by erosion of the resulting mountain highlands, gradually led to a uniform crustal thickness for the cratons. After that, no further impetus for differential uplift existed, and the cratons have remained low and stable.
The continents, even including the cratons, are made up of tectonic terranes--fault-bounded bodies of rock that have geologic histories different from those of adjacent terranes. The cratons are the oldest tectonic terranes, and on this map they consist of the Laurentia Craton (North America and Greenland), the Baltica Craton (Scandinavia), the Siberia Craton, the North China Craton, and the South China Craton (including Korea).
Some of the oldest rocks on the surface of the Earth crop out within the Arctic Sheet. Volcanic conglomerate at Isua, Greenland, is 3.81 billion years old; and gneiss (originally granite) at Granite Falls, Minnesota, is 3.68 billion years old.
These localities lie on parts of a stable shield. Until a breakup about 700 million years ago, that shield was continuous with a locality that contains the Earth's presently oldest known materials, reworked zircon crystals of 4.2 billion years ago in metamorphosed sandstone near Mt. Narryer, Australia.
Most geologists believe that the Earth formed about 4.5 billion years ago, based on the radiometric dating of meteorites assumed to be the same age. A more precise age combines the isotopic ratios of old lead in meteorites with those of bedded lead ores on the Earth that range in age from 2.6 to 3.5 billion years. The samples contain both initial lead and uranium-derived lead, and their differing known ages of formation permit solving for the age of the Earth and the other bodies in the Solar System, giving a result of 4.54 billion years.
Crustal rocks began forming soon after the origin of the Earth, but the first cratons required about 1 billion years to stabilize. Therefore, the oldest rocks on this map, which formed when the Earth was less than 500 million years old, were not yet stable and were subject to uplift and erosion like those of young mountain belts. They formed at island arcs, and only later did continental collisions at subduction zones gradually assemble them into continents.
True cratons became possible about 3.5 billion years ago. We are not certain where they lay during the Archean and the Early Proterozoic. Rogers pointed out evidence that about 3 billion years ago Laurentia and Australia may have constituted a combined continent, which he named Ur. The situation is clearer near the end of the Proterozoic, when most of the Earth's cratons had assembled into a supercontinent called Rodinia.
Continental rifting began to fragment Rodinia about 700 million years ago, truncating the Laurentia Craton against a newly formed Pacific Basin. Australia and Antarctica likely constitute the southern part of the other half of that craton. On the basis of a similar early Paleozoic fauna, the Siberia Craton probably rested against western Canada. The Baltica Craton was next to Laurentia at a closed-up North Atlantic. The position of the two China cratons is less certain, however, and they may have been independent of Rodinia.
Tightly folded rocks, marked on the map by narrow linear bands of the geologic-age colors, generally occur along lines of continental collision. Old lines of continental rifting (passive continental margins) commonly underlie these foldbelts, either on both sides, or on one side next to an accreted island arc.
Continental rifting and continental collision both take appreciable time to complete. Rifting must work its way slowly down through 40 km of continental crust before basaltic crust begins to form along the axis. During this 20 to 40 million year process, the continental crust thins and slides down toward the rift axis. Early sedimentary basins follow such rifts, then the flanks rise owing to hot mantle material near the surface, followed by renewed subsidence as the system cools. After that, sedimentation is continuous along the boundaries between the continents and the new ocean basin.
The consolidation along a line of continental collision also takes 20 to 40 million years. Sedimentary layers that had formerly draped across the margins become part of the foldbelt, along with overlying deposits formed during the collision process.
Abundant granite and granite gneiss in the Archean suggest an early origin for the volcanic-arc process of crystal settling that separates light-colored granitic rocks from dark-colored rocks derived from the Earth's mantle. The granitic bodies lie between greenstone bodies that were originally basaltic lavas erupted at seafloor-spreading axes. The greenstone was squeezed between colliding granite-bearing volcanic arcs at subduction zones.
Igneous rocks of all ages on the Arctic Sheet provide a clue to how one important process in the Earth works. The composition of extrusive (volcanic) rocks along former volcanic arcs is systematically displaced toward darker-colored types than the intrusive (plutonic) rocks formed at depth along such arcs. To both the extrusive and intrusive igneous rocks, we applied a three-fold classification: (1) felsic (light colored), (2) intermediate, and (3) mafic (dark colored). On the map, intermediate extrusive rocks (andesites) are the most abundant type, whereas intermediate intrusive rocks (diorites) do not show up at all.
To some extent, this effect comes from the fact that coarse-grained rocks look lighter colored than fine-grained rocks of the same chemical composition. Diorite does of course crop out in the area of the map sheet, but in bodies so small that at the scale of the map we necessarily grouped it with more extensive nearby felsic (granite) or mafic (gabbro) intrusive rocks.
The crystal-settling process at volcanic arcs seems to allow diorite magma to pass up to volcanoes to produce andesite, but it does not preserve the diorite when the intrusive rock crystallizes at depth. Heavy dark minerals crystallize on the walls of the magma chamber. When they reach large size, they break away and settle through the magma to produce gabbro on the floor of the chamber. All the while, volcanoes erupt intermediate-composition magma to the surface as andesite. When the magma chamber does fully crystallize, light-colored felsic intrusive rocks remain. Therefore, either the underlying gabbro or the granite can appear on the map, but the diorite is missing.
The youngest Archean rocks contain extensive beds of banded iron formation (chiefly 2.7 to 2.6 billion years old). They seem to have formed when oxygen produced by photosynthetic bacteria became sufficiently concentrated to precipitate dissolved iron from the ocean. Proterozoic rocks (2.5 to 0.5 billion years old) constitute the remainder of the shield areas on the Arctic Sheet, and they underlie extensive little-deformed sediment cover on the continental platforms. During the 2 billion years of the Proterozoic, plate-tectonic processes seem to have operated at rates comparable to those of earlier and later eras.
Late in the Proterozoic, about 800 million years ago, the Rodinia Supercontinent comprised most of the world's cratons. As noted above, Australia and Antarctica (along with India on the far side) pulled away from Rodinia about 700 million years ago to produce the west coast of North America. About 600 million years ago, Australia, Antarctica, and India began to close with Africa and South America to produce the large Southern Hemisphere continent of Gondwana. The South America edge of Gondwana broke away from what remained of Rodinia to produce the east coast of North America.
During the interval from 600 to 500 million years ago, at the dawn of the Paleozoic Era, the remaining parts of Rodinia (North America, Siberia, and Baltica) moved from south polar regions to equatorial regions, and then these three cratons separated widely from each other. Multicellular organisms had originated immediately before this time interval (at about 670 million years ago), and by the end of it, the warm intervening Iapetus Ocean of the Cambrian Period had spawned most of the phyla of organisms that live today.
During the Paleozoic Era, from 544 to 240 million years ago, abundant marine fossils help record a succession of plate-tectonic events on the Arctic Sheet. Individual closures between continent-sized terranes took as much as 100 million years, and the mountain-building times commonly differed along the length of a suture. The first major episode of deformation, from 500 to 400 million years ago, occurred when Baltica and adjoining terranes closed against Greenland and eastern Canada (Caledonian/Taconic Orogeny). This was followed by the closure of Gondwana (Africa, South America) against North America and Baltica roughly 300 million years ago (Hercynian/Variscan/Allegheny Orogeny). At about this time also, Siberia closed against Baltica to create the foldbelt along the Ural Mountains. By the end of the Paleozoic Era (Permian Period), these collisions had created the new supercontinent of Pangea, which again brought together most of the world's cratons. Pangea shared the globe with the Panthalassa Ocean.
The Pangea Supercontinent extended from pole to pole. Glaciation was widespread, and circulation in the giant Panthalassa Ocean may have been less conducive to the maintenance of life than were the dispersed continents and oceans of former times. In any case, a grave biotic crisis took place at the end of the Permian Period, and three quarters of the Earth's life forms became extinct.
During the Mesozoic Era (blue green and green on the map), from 240 to 65 million years ago, continents of more ordinary size redeployed in the ocean basins, and smaller terranes consolidated with them. Also, the Siberian Traps erupted on the west side of the Siberia Craton. These flood basalts, from a mantle hotspot beneath the Eurasia Plate, cover about 600,000 square kilometers to an average thickness of 2 km. Plate movements did not cause them. Instead they likely came from a mantle plume rising from a reheated pile of sunken subduction slabs above the liquid outer core of the Earth.
Continental thinning across the central Atlantic Ocean and through the Gulf of Mexico and Mediterranean Sea began in the Triassic, as marked by half grabens containing Triassic red beds of the Newark Group in the eastern United States and Canada. During the Jurassic (about 160 million years ago) basaltic oceanic crust emerged along this rift, which split Pangea through the Gulf and Mediterranean from the Pacific to the Tethys (Asian) side of Panthalassa. At the same time, major augmentation took place at the east and south sides of Eurasia. During the Jurassic, Kolymia closed against the east side of the Siberia Craton at the Verkhoyansk Foldbelt, and during the Cretaceous, Chukotskia closed against Kolymia to complete the east tip of Eurasia. Arc-type volcanic and granitic rocks mark the subduction that preceded such collisions, and whichever side of the suture they lie on indicates the direction of dip of the subduction zone.
The North and South China Cratons had amalgamated in an oceanic setting during the Permian. They accreted to the south side of Eurasia during the Cretaceous, and subduction from the Pacific Basin then immediately began along the margin at Japan. North America moved west as rifting of the Atlantic began, and it collided with the Sonomia island-arc terrane (Nevada) during the Triassic. The Stikinia Terrane arrived at the west coast of North America during the Jurassic, and Wrangellia during the Cretaceous. Also during the Cretaceous, continental rifting extended northward and southward from the previously opened central Atlantic to create the margins of the modern Atlantic Ocean.
Oceanic plates on both sides of the Pacific, now well recorded on the seafloor by never-subducted Mesozoic and Cenozoic oceanic crust, moved northward and caused northward dispersion of coastal terranes along major strike-slip faults such as the Tanlu in China and precursors of the San Andreas in the United States. The Northern Alaska Terrane swung quickly from the Arctic Islands of Canada toward the Pacific Basin to open up the Canada Basin of the Arctic Ocean.
During the Cenozoic Era (orange and yellow on the map), from 65 to 0 million years ago, northward terrane dispersion continued along both sides of the Pacific Basin. Conspicuous effects in North America include extension at the Basin and Range and opening of the Gulf of California. Seafloor spreading continued at the Mid-Atlantic and Mid-Arctic Ridges, and that continuous spreading axis, which spans half the globe, narrows to a feather edge near the north coast of Siberia. India swept up and amalgamated island arcs off the south coast of Eurasia, and then the combined assembly crashed into the continent to form the Himalaya and the Tibetan Plateau. This collision continues to push China northeastward, where it has opened up Lake Baikal, reversed the displacement on the Tanlu Fault, and opened the Sea of Japan. Because the Africa Plate is virtually motionless, the Mid-Atlantic Ridge has moved sideways toward the west over the Earth's mantle, so that eruptions that mark the trace of the Iceland Hotspot, which is fixed in the mantle, cross Greenland and finally end today on the ridge.