How the Valleys were formed
The Earth’s crust is very thin, extending to depths of 30 to 65 km under
the continents, and only 5 to 15 km under the oceans
hidden deep beneath the mantle at a depth of 2,885 km is the outer edge of the Earth’s iron-nickel core – the peach pit if you will! But the similarities end there. The inner core may be solid, but the outer part is hot metallic liquid – the only liquid layer in the planet.
The uppermost mantle, along with the thin crust, defines a fairly rigid layer known as the lithosphere. The lithosphere ranges from 10 to 250 km thick and is broken into fragments, called plates, which move around the Earth like pieces of a giant jigsaw puzzle.
Below the lithosphere, enormous heat creates convection cells in the deeper mantle. The stiff lithospheric plates are carried along by these currents of hot flowing rock, much like a conveyor belt. The movement is slow – not much faster than your fingernails grow – but over geologic time, the effect is very dramatic. As the plates move, they carry with them the parts that poke above the water – the continents and islands – and in so doing, constantly reshape the geography of our planet!
This activity, known as plate tectonics, has not been steady over time. Some periods were calmer; others, like the Devonian, much more intense. Where plates moved apart, oceans formed, and where they pushed against each other, continents collided and the towering majestic peaks of new mountain chains emerged from the disappearing Devonian seas...
Over the past centuries the names of famous divisions in the geologic timescale began to emerge – l the Cambrian, Silurian, Carboniferous, Jurassic and Cretaceous periods – and these were named after historical or contemporary regions where the rocks of that period are plentiful (Cambria and Jura, for example), or where the rocks reflect the dominant characteristics of the time (“Age of Coal”, “Age of Chalk”, etc.). The geological timescale was finally, albeit slowly, taking shape.
he Devonian Earth was nothing like the planet we know. Not only were the living creatures unlike those of today, and the climate dramatically different, but even seen from space, the planet would hardly have been recognizable.
The Paleozoic Era ended with the largest mass extinction in Earth's history, the Permian–Triassic extinction event. The effects of this catastrophe were so devastating that it took life on land 30 million years into the Mesozoic to recover.
The Paleozoic was a time of dramatic geological, climatic, and evolutionary change. There were periods of immense changes and these were known as the Cambrian Period which witnessed the most rapid and widespread diversification of life in Earth's history, known as the Cambrian explosion, in which most modern phyla first appeared. Fish, arthropods, amphibians and reptiles all evolved during the Paleozoic. Life began in the ocean but eventually transitioned onto land, and by the late Paleozoic, it was dominated by various forms of organisms.
Great forests of primitive plants covered the continents, many of which formed the coal beds of Europe and eastern North America. Towards the end of the era, large, sophisticated reptiles were dominant and the first modern plants (conifers) appeared.
, and is subdivided into six geologic periods (from oldest to youngest): the Cambrian (545mya) , Ordovician, (490mya) Silurian (434mya) Devonian, (410 mya)Carboniferous (354mya) and Permian(298 mys) Triassic (251 Mya)
studies and evidence of glaciers indicate that central Africa was most likely in the polar regions during the early Paleozoic. During the early Paleozoic, the huge continent Gondwanaland had either formed or was forming and a subduction plate uplifted eastern Australia.
Throughout the early Paleozoic, the Earth's landmass was broken up into a substantial number of continents. Towards the end of the era, the continents gathered together into a supercontinent called Pangaea, which included most of the Earth's land area.
Geologically, the Paleozoic starts shortly after the breakup of an early supercontinent called Pannotia
periods were warm greenhouse periods, with the highest sea levels of the Paleozoic (200 m above today's);
The early Cambrian climate (540mya) was probably moderate at first, becoming warmer over the course of the Cambrian, as the second-greatest sustained sea level rise got underway. However, as if to offset this trend, Gondwana moved south with considerable speed, so that, in Ordovician time, most of West Gondwana (Africa and South America) lay directly over the South Pole
The late Paleozoic began with a spike in atmospheric oxygen, while carbon dioxide plummeted to unheard-of lows. This destabilized the climate and led to one, and perhaps two, ice ages during the Carboniferous. These were far more severe than the brief Late Ordovician Ice; but, this time, the effects on world biota were inconsequential. By the Cisuralian, both oxygen and carbon dioxide had recovered to more normal levels. On the other hand, the assembly of Pangea created huge arid inland areas subject to temperature extremes. The Lopingian is associated with falling sea levels, increased carbon dioxide and general climatic deterioration, culminating in the devastation of the Permian extinction. It took 30 million years for the earth to recover.
Devonsian era foliage. (410 mya) An artist's impression of early land plants
the Devonian Earth was nothing like the planet we know. Not only were the living creatures unlike those of today, and the climate dramatically different, but even seen from space, the planet would hardly have been recognizable.
Towering lycopsid rainforests dominated the tropical belt of Euramerica. Climate change caused the Carboniferous Rainforest Collapse which fragmented this habitat, diminishing the diversity of plant life in the late Carboniferous and Permian.