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Considering all the great cataclysms in the Earth throughout its long history, it is remarkable how life has managed to survive to the present. Over ninety-nine percent of all species that have inhabited the planet from the beginning have become extinct, so that those living today represent only a tiny fraction of the total. Billions of plants and animals have existed in the geologic past. Most lived during the last six hundred million years. A period of phenomenal evolution of species as well as tragic mass extinctions. The common denominator in all mass extinctions is that biological systems were in severe stress due to rapid and extreme changes in the environment. The ultimate environmental hazard is mass destruction by the impact of a meteorite, asteroid, or comet on the Earth. Over one hundred large meteorite craters have been located throughout the world, some which might be associated with mass extinctions of species. The impact of asteroids melted the Earth's crust by impact friction, and blasted apart half the crust to form huge impact basins.
When a meteorite goes into the Earth's surface, it generates a shock wave with a pressure that travels down into the rock and reflects back up into the meteorite. As the meteorite burrows into the ground, it forces the rock aside, flattening itself in the process. It is then deflected and ejected from the crater, followed by a spray of shock-melted meteorite and melted and vaporized rock that shoots out at tremendous velocities. The finer material lofts high into the atmosphere, while the coarse debris falls back around the perimeter of the crater, forming a high rim. Craters of extremely large meteorite impacts might temporarily reach depths of twenty miles or more, and a hot mantle below. The exposure of the mantle would cause a gigantic volcanic explosion, releasing more material into the atmosphere than the meteorite itself.
When a large extraterrestrial body, such as an asteroid or comet, slams into the Earth, the impact can produce thick dust clouds, huge waves, extremely toxic gases, and strong acid rains. If a large meteorite landed in the ocean, it would instantly evaporate massive quantities of seawater, saturating the atmosphere with clouds of steam. This would raise the density of the atmosphere and increase opacity, making it impossible for sunlight to penetrate. Solar radiation would heat the darkened sediment layers of the atmosphere and cause a thermal imbalance that could seriously alter weather patterns, turning much of the land into a desert.
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Most extinctions of the past appear to have coincided with major planetary changes, brought on by tectonic forces and lowering sea levels. Global cooling caused many extinctions as climate is possibly the most important factor influencing species diversity. Many mass extinctions coincided with periods of glaciation. Not all climate cooling resulted in glaciation, nor did all extinctions follow a drop in sea level cause by growing glaciers. At least thirty meteorites between the sizes of baseballs and soccer balls hit Earth each year, but collisions with large meteorites are rare.
At the start of the Mesozoic Era (50 mya), the few surviving organisms found themselves in a relatively empty world. As Pangaea slowly separated into individual continents, the glaciers melted, and the ocean rose and re-flooded the continental shelves, forming huge, shallow inland seas. During the Mesozoic, Earth's biota changed immensely. At one point, the Earth's biota was relatively homogeneous. Distinct terrestrial floras and faunas evolved on each continent. We have often thought of the Mesozoic Era as being very warm. There was a very large seasonal temperature contrast at high southern latitudes. Most of this large temperature difference is because the continents were all still joined at this time. There was no evidence of glacial conditions. An important observation is that mammals first arise during the Mesozoic. This is important because mammals are endothermic, which enables them to adjust their body temperatures with their environment. It is known that the early and middle parts of the Mesozoic Era were very dry over large portions of the continents. The localization that began during the Mesozoic continues to influence the geography of life today. The Mesozoic is divided into three time periods the Triassic (45-08 mya), the Jurassic (08-146 mya), and the Cretaceous (146-65 mya). During this time, there have been two known mass extinctions, and two minor extinctions.
At the beginning of the Triassic period, the land was still bonded together in one huge continent known as Pangaea. Pangaea started to break apart in the mid-Triassic forming Gonduana in the south, and Laurasia in the north. While Pangaea was breaking apart, mountains were forming due to subduction of the ocean plates beneath the continental plates. As mountains were forming, the continents began to split by the spreading rift. Pangaea influenced the climate of the Triassic Era. Its position was close to the equator, and the activity all around caused the continents to break up. The continents were at high elevation compared to the sea level. Due to the low sea level, flooding of the continents did not occur. There were no polar ice caps. The sea level rose as the rift grew between some continents, flooding the others. Due to this, the rest of the world was left high and dry.
During this period, many invertebrate lineages became more diverse. On land, conifers and seed ferns became the dominant trees. The first frogs and turtles appeared. Reptiles began, which gave rise to dinosaurs, crocodilians, and birds. These creatures and their relatives ruled the land for over one hundred fifty million years. Starting out as small lizard-like animals, and forming into huge beasts that could walk bipedal. The more primitive form seem to have been cold-blooded, but researchers have come to find that some late descendants had warm blood. Near the end of the Triassic period, about two hundred million years ago, large families of terrestrial animals began dying off. The extinction occurred over a period of less than a million years, and was responsible for killing off nearly half the reptile families. Called "The Triassic Crisis", it was said to have eliminated tropical reef corals, possibly due to a colder climate. Although it has been viewed as one of the less significant extinctions, studies have shown that almost a quarter of all families became extinct in the late period. Why they went extinct is not known, but a meteor impact is suspected. The extinction forever changed the life on Earth, and made the way for the rise of the dinosaurs.
The mass extinction at the end of the Triassic was followed by another period of evolutionary changes during the Jurassic period. Bony fishes began to culminate in their dominance of the oceans. Salamanders and lizards first appeared. Flying reptiles evolved, and dinosaurs evolved into bipedal predators and quadrupedal herbivores. Several groups of mammals first appeared at this time. Plant-eating dinosaurs roaming the Earth, fed on ferns, cycads, and bennettitaleans. Dinosaurs dominated the land fauna during this period. The largest dinosaurs of all time were the sauropods, Diplodocus, Brachiosaurus, and Abatosaurus. Triceratops, and Tyrannosauraus Rex did not evolve until after the Jurassic was over. But there was more to life than dinosaurs. In the seas, the fishlike Ichthyosaurs shared the water with Plesiosaurs, crocodiles, and modern looking sharks and rays. Also, prominent in the ocean were cephalopods and ammonites, which were close relatives of the squid.
The existence of Pangaea for one hundred million years, overlying a large part of the Earth's mantle, suppressed to escape of heat from the mantle to surface, acting as a huge blanket. The mantle gradually heated up. This may have forced the continent apart, splitting the northern part form the southern part forming two continents. At the same time, climate stays very warm, not hot and humid as the Triassic period.
Two extinction events are speculated to have occurred in the Jurassic. The first extinction eliminated more than eighty percent of marine species, along with shallow water species. The second extinction occurred at the end of the Jurassic. Ammonoids, reptiles, and bivalves were severely affected. Dinosaurs were also affected as Stegosaurs and most types of sauropods did not survive into the Cretaceous period. The cause of these extinctions has never been solved, while other theories suggest volcanic activity, climate change, environmental pollution, or even cosmic radiation as causes.
The Cretaceous period is the last period of the Mesozoic Era, also ending with the extinction of dinosaurs. By the beginning of the Cretaceous, Pangaea was already rifting apart, and by the mid-Cretaceous, it had split into several continents. Seasons began to grow more noticeable as the global climate became cooler. Flowering plants and forests made up a huge percentage of the environment. Sea levels were high, and Earth was warm and humid.
No great extinction or burst of diversity separated the Cretaceous from the Jurassic period. The only difference was the Cretaceous saw the first appearance of many new life forms. Flowers, and forests joined the environment. At the same time, many modern groups of insects were beginning to diversify. Aphids, grasshoppers, and gall wasps appear, as well as termites and ants in the later part of the period. Cretaceous also saw the first radiation of the diatoms in the oceans. By the end of the period, many groups of mammals had evolved, but these mammals were generally small.
At the end of the Cretaceous period, sixty-five million years ago, an asteroid hit Earth in the Yucatan Peninsula, Mexico, forming what is today called the Chicxulub impact crater. It has seen estimated that half of the world's species went extinct. All marine reptiles, except the smallest of turtles became extinct. Ninety percent of all plankton species that lived in the surface waters of the ocean died out. Afterward, mammals began to diversify rapidly, sometimes into unusual forms. The most prominent to us is the disappearance of the dinosaurs. However, flowering plants, which now dominate the terrestrial environment, suffered severely in the extinction event. The most famous, if not the largest, of all mass extinctions marks the end of the Cretaceous period. This extinction marks the end of the Cretaceous and of the Mesozoic Era.
Ten or more major meteorites have impacted over the last six hundred million years, some of which coincide with mass extinctions. There were four extinctions alone in the Mesozoic Era. As I was reading on information, I came across something very interesting. The closest flyby of a large asteroid in recorded history occurred on March , 18, when asteroid 18 FC came within 40,000 miles of Earth. If the asteroid would have collided with the Earth, it would have been catastrophic. Due to the Earth's large size, the planet's gravitational pull would have accelerated the asteroid during its final approach. If a collision had occurred, the asteroid would have produced a crater five to ten miles wide. Astronomers did not detect the asteroid until it was already moving away from the Earth. The astronomers failed to notice the approach of the asteroid because it came from a sunward direction. Thousands of asteroids are capable of crossing the Earth's orbit for a close encounter. In December 8, 1, a large asteroid called Toutatis, flew within . miles of Earth. Almost all close encounters took astronomers by surprise, and not a single one was anticipated. Asteroids, meteorites, and comets have all come in contact with the Earth, causing the extinction of life. It happened before, and it will happen again. The only question and concern that I have, is when the next time will be.
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