The Outbreak of Panoa

THE OUTBREAK OF PANOA Throughout Earth's history, continents seem to have experienced cycles of collisions and rifts. Blocks of smaller continents collide and merge into larger continents. Millions of years later, the continents broke apart, and the abysses filled with seawater to form new oceans. The regions that currently border the Pacific Basin do not seem to have collided with each other. The Pacific Ocean is a remnant of an ancient sea called Panthalassa, which narrowed and widened in response to the splitting, spreading, and continental reconvergence in the area occupied by the present-day Atlantic Ocean. Some oceans have repeatedly opened and closed around the Atlantic Basin, while one ocean has existed continuously in the Pacific Basin area.The Pacific Plate is barely larger than the United States after the outbreak of Pangea in the early Jurassic about 180 million years ago. The rest of the seafloor consists of other, unknown plates that disappear as the Pacific plate grows. As a result, no oceanic crust is older than the Jurassic age. Jurassic is a tumultuous time. Pangea split into today's continents, leaving a gaping fissure filled with seawater to become the Atlantic Ocean. Sea levels and climate change dramatically as continents move away from each other. Yet life underwent this tremendous geological upheaval over millions of years with little effect. When Pangaea began to separate into its current continent (Figure 155), a major rift developed in the Caribbean at this time. It sliced north through the continental crust connecting North America, northwest Africa, and Eurasia and began to open up the Ocean.The Atlantic process took several million years along a zone hundreds of miles wide. The breakup of North America and Eurasia probably resulted from upwelling basaltic magma weakening the continental crust. Many flood basalts exist near continental boundaries, where rifts separate continents today. Episodes of flood basalt volcanism are relatively short-lived events, with major phases generally lasting less than 3 million years. India, located between Africa and Antarctica, drifted away from Gondwana.Antarctica, still attached to Australia, swung away from Africa towards the southeast, forming the proto-Indian Ocean.The rift separating the continents was breached and flooded with sea water, forming the baby North Atlantic Ocean. Many spreading ridges of the Atlantic seafloor remain above sea level, creating a series of stepping stones for animal migration between the Eastern and Western hemispheres. About 125 million years ago, the North Atlantic baby gained a depth of about 2.5 miles and was cleaved by an active mid-ocean ridge system that produced new oceanic crust. At about the same time, the South Atlantic began to form, opening like a zipper from south to north. The rift propagates north by several inches per year, proportional to the degree of separation between the plates. The entire process of opening the South Atlantic was completed in just about 5 million years. By 80 million years ago, the North Atlantic had become a fully developed ocean. About 20 million years later, the MidAtlantic rift expanded into the Arctic Basin, separating Greenland from Europe. After the separation, the continents traveled in bursts rather than drifting apart at a constant speed. The rate of seafloor distribution in the Atlantic is matched by plate subduction in the Pacific, where one plate dives under another, forming deep trenches. This is why the oceanic crust of the Pacific Basin dates back no further than the beginning of the Jurassic.A high level of geological activity around the rim of the Pacific Basin produced almost all of the Pacific-facing mountains and island arcs along its perimeter. Much of western North America gathers from an arc of islands and other crustal debris slides off the Pacific plate as the North American plate continues to head westward. Northern California is a pile of crustal fragments that gathered in about 200 million years ago. A nearly complete piece of oceanic crust, the kind pushed to the continent by drifting plates, sits in the middle of Wyoming. The Nevada Orogeny produced Sierra Nevada in California (Fig. 156) during the mid to late Jurassic. The rupture of Pangea suppressed the ocean basin, causing sea level rise and sea encroachment inland. In addition, increased volcanism flooded the continental crust with a large amount of basalt. Increased volcanic activity also increased atmospheric carbon dioxide content, resulting in a strong greenhouse effect that led to a warmer Mesozoic climate. The separation and spread of continents may have also contributed to the extinction of many dinosaur species. Continental drift alters global climate patterns and brings unstable weather conditions to many parts of the world. Massive lava flows from perhaps the most volcanically active period since Earth's early days may have dealt a major blow to the planet's climate and ecological stability.