Era's of Earth's History

Intro:
       We all have a lot of questions about how the earth began. By the use of various technology and space probes, we have increased our understanding of the solar system.
       The earth is probably at least 4 1/2 billion years old. The oldest rocks that have been discovered are 4.3 billion years old. The age of rocks can be determined by measuring the number of radioisotopes in them. A radioisotope gives off radiation. For example, uranium gives off radiation and changes to lead over a specific time. Scientists can use this information to find the age of rocks by comparing amount of uranium and the amount of lead in it.
       Many scientists believe that the solar system developed from a nebula, which is a cloud of gas and dust particles. The sun may have been formed from the center of the nebula. As the nebula spiraled around the sun, it flattened and sections began to spin. Gas and dust collected near the centers, These collections slowly developed in to spiraling planets traveling around the sun.

Theories:
       In 1905, a theory was proposed by Thomas Chamberlain, called the planetesimal theory. This theory states that a rapidly moving star passed close to the sun and the gravity of the star pulled arms of gas from the sun. The gas cooled and formed solids called planetesimals which collected in the centers of small eddies and in turn formed planets.
       Another theory was proposed in 1919 by Sir James Jeans and Harold Jeffreys, called the tidal theory. This theory tells of hot gas pulled from the sun and the gas turning in to liquid balls. Each ball slowly cools and a hard crust is formed around each of them. This theory assumes that the earth was first a gas and then a liquid and finally a solid.
       In the 1930's, R. Lyttleton proposed another theory called the double star theory. Our galaxy contains many two star combinations called double stars. According to the theory, the sun and a companion star once formed a pair. The companion star exploded in to gas and was captured by the sun's gravity. The planets then developed from this cloud.
       In the 1940's and 1950's, more theories were developed on formation by condensation. These theories assume that a star exploded and the material from it escaped in to space. The sun formed from the nebula and small masses of dust condensed in to planets.

Primitive Earth:
       So what was earth like in the beginning? Scientists believe that it began as a waterless mass of rock. Heating of the interior of the earth caused chemicals to form, such as water and parts of the atmosphere. After millions of years, the water began to form in to oceans. The earth's earliest atmosphere contained hydrogen, methane, helium, and ammonia. It may have contained much carbon dioxide, but little free oxygen. The oxygen that we use now comes from plants, which is a result of photosynthesis.
       It is believed that the continents were once one large land mass, known as Pangaea. In theory, about 200 million years ago, the Pangaea began to break apart and split in to two, Gondwanaland and Laurasia. Gondwanaland proceeded to break apart and formed Africa, Antarctica, Australia, and South America as well as India. Laurasia split to form Eurasia and North America. This is the theory of plate tectonics. It explains continental drift and sea -floor spreading (formation of new oceanic crust). Plates move in three different ways, 1) away from each other; 2) towards one another, and 3) past each other. When plates collide, mountains are formed. Plates that slide past each other create a transform fault. The sliding motion can cause earthquakes, and the plates to shift.

History of the Earth:
       Most of our earth's history can be found by looking at rocks. Rocks have always been forming, eroding, and reforming. Sediment, eroded rock, accumulates in layers, called strata. Strata can hold valuable information on the earth's past, such as fossils and composition. One example of this is ripple marks on rocks. These can show geologists about the land area, and even which way current moved in a river.
       Geologists believe that rocks can provide clues to how the earth works today. They believe that the same laws of chemistry, physics and biology existed in the past. This is known as the principle of uniformitarianism.
       The earth's history is divided in to three stretches of time, called eons. These are Archean, Proterozoic, and Phanerozoic. The first two eons are known as Precambrian, The Phanerozoic is split in to 3 eras. Eras are divided into periods, and these are divided in to epochs.
       The Precambrian Time makes up the first 4 billion years of our planet's history. During the Archean Eon, the earth began. After millions of years, a rock crust formed. As the crust melted and hardened, continents began to form. The ocean and atmosphere were also formed. Archean rocks were made, such as schist and gneiss. They were formed from sandstone, shale, lava, and volcanic ash. Fossils found in these rocks are usually primitive bacteria, such as blue-green algae. In the Proterozoic eon fossils are found that are about 700 million years old. Some fossils include worms, jellyfish, corals, and other primitive invertebrates.
       The Paleozoic Era began about 570 million years ago.It included six periods. These are known as Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Land plants developed from the green algae during the Silurian period and flourished during the Devonian. There was a great variety of plants during the Carboniferous period, including a few that grew 100 feet long.
       The Mesozoic Era began some 240 million years ago. It consisted of three periods called Triassic, Jurassic, Cretaceous. Plants of this era include the gymnosperms and angiosperms. Organisms lived in the warm seas. Some of these organisms include algae and protozoans. Other animals that flourished were fish, amphibians, reptiles, sand dollars, snails, clams, and ammonites. During the late Triassic Period, the first warm blooded mammals appeared. The first birds appeared during the Jurassic Period.
       During the Triassic Period, Pangaea broke apart-forming the continents that we have today. During the Jurassic and Crustaceous periods, plates collided and generated large amounts of molten rock, which in turn formed Sierra Nevada. Mountain building occurred as well and the Rocky Mountains were created.
       The Cenozoic Era began 65 million years ago and has not ended yet. It covers the Tertiary period and Quaternary period. We divide the Tertiary period in to five epochs, Paleocene, Eocene, Oligocene, Miocene and Pliocene.The Quaternary period consists of the Pleiestocene Epoch and Halocene Epoch.
       During this era, a number of mountain ranges were formed. Some of these were the Alps, Andes, and Himalayas. Along with mountain ranges, some volcanoes were formed. The animals and plants that we are familiar with today, came in to existence during this period. In the Pliocene Epoch, the earliest human like fossils were found.

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