Earth

Earth Structure

The cosmic period of development of Earth and other objects in the solar system, which began with accretion of matter 4.6 billion years ago, ended. Heavy bombardment ended as well. Pieces of unused building material ceased to fall on planets and their moons. The surfaces of celestial bodies slowly cooled down. Approximately 4.0 billion years ago the cosmic period of planetary evolution, including our planet, ends. Earth has an atmospheric coat, which protects it from the harmful effects of different types of radiation and from impacts of objects on its surface. Traces of perishing meteors in the atmosphere from time to time beautifully animate the seemingly calm night sky. The Earth's internal structure was formed from the beginning into several overlaping zones: the inner core, the outer core, the lower mantle, the upper mantle, and the crust. Part of the crust, which reaches a depth of 70 - 80 km, are so called lithospheric plates. They are not connected, they move freely. Where they support continents, they are more massive, under the oceans they become thiner. Warm current in the depths of the Earth moves the plates and that changes the shape and position of the land and oceans. The area where the old crust descends to a depth below the young crust is called the subduction zone. At its edge volcanic activity increases. When two plates collide, we talk about collision zones. And thirdly: rifting is a movement in which plates move away from each other and abyssal rocks rise to the surface. Under the crust there are two layers of mantle - the upper ends at a depth of 600 km and the lower at a depth of 2900 km. The temperature and pressure are already so high that the rocks here are melting. The outer core is liquid up to a depth of 4980 kilometers and the inner core from iron and nickel is solid. It ends in the center of the Earth, at a depth of 6370 km below the planet's surface. The inner core rotates at a different speed than the other zones. This creates a dynamo effect, generating an electric current and a magnetic field. The magnetic field surrounds the Earth and deflects dangerous proton solar wind off the planet. The Earth is 150 million kilometers away from the Sun, orbits it once a year, one day on it lasts 24 hours, lies in a habitable zone suitable for life, and is waiting for its inhabitants.

About Chronological Order

Already in the nineteenth century fossil collectors began to systematically study not only the found fossils, but also the layers of the earth's crust in which they were found. Soon showed up that the layers could be sorted from the oldest to the youngest. Considering the evolutionary changes of the found fossilized animals and plants, changes in climate, mountain-building processes and many other factors, geologists put each period into a chronological order. That was refined and completed by modern analytic methods of today with a new knowledge of natural science and climatology. Now our today‘s view of the chronological order of geological periods of our planet Earth opens to us as follows:

Pleistocene was affected by strong temperature changes. It is a period of ice ages, glacials and interglacials, interglacial ages. Four are recognized: Günz, Günz-Mindel, Mindel, Mindel-Ries, Ries, Ries-Würm, Würm.

(Regression - sea level drop, Transgression - sea level rise, # - sudden event, anoxia - oxygen depletion, arid - dry climate, glacials - ice and interglacial ages).

The time schedule is only approximate. The borders in the time schedule apply only generally. It is based on the most significant changes within the given period, changes in the structure of sediments and changes of the fossils enclosed in them.

How the Earth Developed

All the development of animals and plants happens slowly and takes long time. Geologists therefore expanded the time schedule with lots of fine data serving mainly to professionals. A very simplified view on movements of continents, mountain-building processes, and climate changes is as follows: In the Proterozoic 1,000 Ma years ago supercontinent Rodine was formed. It was divided into smaller parts - Laurence, Gondwan, Siberia and Baltic about 650 Ma years ago. Between Silurian and Devonian Laurence and Baltic was connected. In the Carboniferous Gondwana freezes, it descended to the southern polar areas due to a movement of continental plates. In the Permian smaller parts of the land connect into a larger whole - Pangea, which falls apart again during the Mesozoic. In the Cenozoic continents get approximately present form. Their edges are still flooded by the sea. The mountain-building movements were also mapped by dozens of detailed studies, we only need a simplified view of the main events. In the Proterozoic Cadomian orogeny took place. A peripheral area of Gondwana was affected. During it the last outflow of fundamental crystalline rocks in Europe happened. A collision of Laurence, Baltic and Avalonia continents gave rise to Caledonian orogeny between Ordovic and Silur. Today's Canada, Greenland, the British Isles (except southern England), Spitzbergs and Scandinavian Mountains were affected. In the Devon and mainly during the Carboniferous, Hercynian orogeny occured. It has two branches: Variscian and Armorican. Armorican affected Brittany, Normandy and surrounding areas, Variscian created mountains of Central Europe, including massifs of the Czech Republic. A sedimentation of limestone deposits and a general formation of karst phenomenas is also dated to the Devonian period. Solidified sediments of Carboniferous – Permian lake basins and swamps turned over millions of years into a high-quality hard coal. Great brown coal basins were formed only in the Cenozoic. From the Carboniferous period are also petrified equisetums and lycopodiums – auracarias. On the border of the Cretaceous and Cenozoic a large asteroid landed in a place where the Gulf of Mexico is now. After this catastrophe a period of large extinction occured and our planet lost many animal and plant species, including dinosaurs. The impact of a small cosmic body into surroundings of today’s Wuppertal caused an eruption of the local molten rocks. Pieces of these rocks fell like small green drops to the area of Vodňany and České Budějovice. They are the world-famous moldavites, 14.8 - 15.2 Ma old.

Brief History of the Earth

All life was born in the sea. Ideas about the evolution of living organisms are subject to direct comparisons and evolutionary /development/ studies. Most importantly found fossils provided evidence of the chronological order of this evolution. In a very simplified form, it looks like this: 3,850 Ma ago were found organic molecules in rocks - building materials of higher protein structures. And about 3,500 Ma ago appeared so far anucleate but already highly organized fossils. The seas of the Proterozoic are dominated by bacterias, algae, stromatolites and the first soft multicellular creatures. They only left traces of crawling and excrements in rocks - ichnofossils.