Arkea Palace Study Materials

ARKEA PALACE Plate tectonics have played an important role in shaping the planet practically since its inception. Continents have been adrift since they originated, within a few hundred million years after the formation of the planet. This was revealed by the presence of 4billion-year-old Acasta gneiss, a metamorphosed granite, in Canada's Northwest Territory Slave Province. It is recommended that the formation of the crust is already well underway at this time. The Slave Provinces contained the oldest known terrestrial crust on Earth and represented the ancient core of the continent known as cratons. The discovery leaves little doubt that there were at least small patches of continental crust on Earth's surface during the first half billion years of its history. 3.8-billion-year-old metamorphosed marine sediments from the Isua Formation in a remote mountainous area of southwest Greenland (Figure 27) provide evidence of saltwater oceans. Earth's oldest sedimentary rocks also contain chemical fingerprints of complex cells that are 3.9 billion years old. Around this time, a swarm of debris left over from the creation of the solar system bombarded the Earth and Moon.The bombardment may have sent heat and organic compounds to Earth, triggering the rapid formation of primitive life. Or, the blow could wipe out life that existed in a colossal mass extinction. The continental crust is probably only about 10 percent of its current size and contains chunks of granite that float freely above Earth's watery surface. These ancient crustal slices called Archean terranes are older than 3.3 billion years and are found mainly in Canada, Greenland, southern Africa, and Western Australia.The 2.5-billion-year-old Kaapvaal, which comprises most of South Africa, is one of Earth's oldest cratons. A 3.5-billion-year-old slice of continental crust called the Pilbara Craton, over thousands of square miles in its area, lies northwest of Australia and has barely been disturbed for centuries. The area is also known for a famous formation called the Warrawoona Group that contains the world's oldest fossil cells, the earliest strong evidence of life on Earth. Chert is a solid and very hard sedimentary rock composed of microscopic silica grains. Archean chert deposits that are more than 2.5 billion years old suggest that most of the crust was under the sea during this time. Most Precambrian cherries are thought to be chemical deposits derived from silica-rich water in the deep ocean. The ocean contains an abundance of dissolved silica, which is leached from volcanic rocks flowing to the seafloor. Modern seawater lacks silica because organisms such as sponges and diatoms extract it to build their skeletons. When organisms die, their skeletons build up large deposits of diatomaceous earth. Some of the rocks date back 3.7 billion years, suggesting that a small continental crust was forming before this time or that the crust was recycled into the mantle. The slices of granite crust are combined into a stable body of underground rock called kraton (Fig. 28), where all other rocks are deposited. They consist of highly altered granite and metamorphosed marine sediments and lava flows. The rocks come from magma intrusions into primitive marine crust. Only three sites in the world, located in Canada, Australia, and Africa, contain rocks exposed to the surface during Earth's early history that have remained essentially unchanged throughout geologic time. Eventually, the crustal wedges began to slow down their erratic wanderings and merged into larger landmasses. Constant bumps and burrs of strong tectonic activity build up the crust inside and out. Continents continued to grow rapidly until the late Archean 2.5 billion years ago, when they occupied up to a quarter of the Earth's surface or about 80 percent of the continent's land today. During this time, plate tectonics began to operate extensively. Much of the world as we know it began to take shape. Ophiolites are the best evidence for ancient plate movement. They are part of the oceanic crust that exfoliates during plate collisions and is plastered on continents. Blueschists (Figure 29), which are metamorphosed rocks from subduction oceanic crust, are also pushed to the continents. This resulted in the linear formation of greenish volcanic rocks along with masses of light-colored granite and gneiss, which are common igneous and metamorphic rocks comprising most of the continents. In addition, many ophiolites also contain ore-bearing rocks, which are important mineral resources worldwide. Kratons are numerous. They range in size from about one-fifth of present-day North America to smaller than the state of Texas.Kratons are also highly mobile and move freely in the molten rock of the upper mantle called the asthenosphere. They are independent minicontinents that periodically collide and bounce off each other. The collision squeezed the front edge of the palace, forming small parallel mountains that were probably only a few hundred feet high. All the cratons eventually coalesced into one large landmass several thousand miles wide called a supercontinent, which existed as early as 3 billion years ago. At the points where the kratons collided, the mountains pushed up. The seams that joined the mainland are still visible today as the core of an ancient mountain range called orogen, from the Greek oros, meaning "mountain." The original craton was formed within the first 1.5 billion years of Earth's existence and only amounts to about one-tenth of today's landmass. The average rate of continental growth since then has probably been as much as one cubic mile per year. Constant rifts and patching of the interior along with sediments deposited along the continental margins eventually built the supercontinent outward. By the end of the Archean, it had almost equaled the total area of today's continent. Most of the land was covered by shallow seas until about 800 million years ago. After discussing the simple life forms of the Archean eon, the next chapter will focus on the more complex organisms and rock formations of the Proterozoiceon.