The name means: "before the Cambrian period. It has been divided into three eras: the Hadean, the Archean and the Proterozoic. Source: Unknown. The Precambrian Era comprises all of geologic time prior to million years ago. The Precambrian was originally defined as the era that predated the emergence of life in the Cambrian Period. It is now known, however, that life on Earth began by the early Archean and that fossilized organisms became more and more abundant throughout Precambrian time.
The two major subdivisions of the last part of the Precambrian are the Archean oldest and the Proterozoic. Rocks younger than Ma are considered part of the Phanerozoic. Below is another type of breakdown of Precambrian time. Hadean Hadean "Hades-like" Era. This era began with the formation of the earth from dust and gas orbiting the Sun about 4. During this era the surface of the Earth was like popular visions about Hades: oceans of liquid rock, boiling sulfur, and impact craters everywhere!
Volcanoes blast off all over the place, and the rain of rocks and asteroids from space never ends. It's hard to take a step without falling in a pool of lava or getting hit by a meteor! The air is hot, thick, steamy, and full of dust and crud.
But you can't breathe it anyway: it's made of nothing but carbon dioxide and water vapor, with traces of nitrogen and smelly sulfur compounds! Any rocks that do form from cooling lavas are quickly buried under new lava flows or blasted to bits by yet another impact. Some people think that an asteroid as large as the planet Mars hit the Earth near the beginning of the Hadean era, completely smashing and melting the Earth and forming the Moon as part of the "splash!
No one has found any rocks on earth from this era. Only meteorites from space and moon rocks are this old. If any life formed on earth during this era, it was probably destroyed. This era begins about a billion years after the formation of the earth, and things have changed a lot!
Mostly everything has cooled down. Most of the water vapor that was in the air has cooled and condensed to form a global ocean. Even most of the carbon dioxide is gone, having been chemically changed into limestone and deposited at the bottom of the ocean. The air is now mostly nitrogen, and the sky is filled with normal clouds and rain. The lava is also mostly cooled to form the ocean floor. The interior of the earth is still quite hot and active, as shown by the many erupting volcanoes.
The volcanoes form lots of small islands in long chains. The islands are the only land surface. The continents have not formed yet.
The islands are carried over the surface of the earth by the movement of rock deep in the earth's interior. This movement results from the loss of heat from the deep interior and is called plate tectonics.
Occasionally the small islands collide with each other to form larger islands. Eventually these larger islands will collide to form the cores of the continents we know today. Thank goodness those pesky asteroids and meteorites are now mostly gone, so impact craters form only occasionally.
What about life? If you look closely, you will see evidence of blue-green algae actually simple bacteria floating in the ocean. That's all there is! Just single-celled bacteria in the ocean. There is as yet no life on land. Life began in the ocean near the beginning of this era. The oldest known fossils - the remains of different types of bacteria - are in archean rocks about 3. The igneous and metamorphic rocks beneath the younger sedimentary rocks were assigned arbitrarily to the Precambrian, without knowledge of the vast length of time they represented.
The fossil-bearing rocks represent the most recent part of Earth's history, called the Phanerozoic Eon Ma to present; Fig. Archean rocks make up the oldest of the Precambrian rocks, and are subdivided into two units or terranes the word terrane indicates an area of a particular kind of rock on the basis of their age and metamorphic history.
The Superior Province is subdivided into subprovinces, which are more or less east—west, linear belts of rocks of similar geologic history and age. The oldest group of Archean rocks, which are Paleoarchean to Neoarchean in age Figs.
These rocks are exposed in the valley of the Minnesota River between New Ulm and Ortonville, and similar rocks underlie the southwestern quarter of the state beneath younger rocks and glacial sediment. Most of them are different kinds of gneiss pronounced "nice" , a family of coarse-grained, streaky, or banded rocks Fig. These gneisses have undergone multiple events of igneous intrusion and were squeezed and deformed deep in the Earth's crust like taffy, producing the banded appearance we see today.
The oldest components of the gneiss are between 3, and 3, Ma in age, but later, at around 2, Ma, after most deformation and metamorphism had ceased, the gneiss was intruded by several large bodies of granite, which have not been metamorphosed. These late granites can be seen near Fort Ridgely, south of Sacred Heart, and near Ortonville, among other places. The gneissic rocks in this continental fragment that makes up the Minnesota River Valley subprovince are about million years older than the Archean volcanic rocks of northern Minnesota.
This group includes the Wawa granite-greenstone subprovince, the Quetico subprovince metamorphosed sedimentary rocks and gneiss derived from them , and the Wabigoon granite-greenstone terrane Fig. The Wawa and Wabigoon subprovinces originally were parts of volcanic chains that were later deformed and intruded by granitic rocks; the Quetico subprovince was likely a large sedimentary basin on or between those volcanic arcs.
Greenstone a dark greenish-gray, fine-grained, weakly metamorphosed basalt; Fig. As these volcanic chains collided they were intruded by large amounts of granite and related coarse-grained rocks that crystallized at depth from the molten state, but are now revealed at the surface by deep erosion over time.
The granites welded the greenstone belts together to form an Archean continent. The Neoarchean rocks, collectively, are called the greenstone-granite terrane. The Ely greenstone is a well-known example of Neoarchean rock. Photo courtesy of James St. Rocks of the gneiss terrane are quarried in the Minnesota River valley for use as building stone and are also crushed for road construction and railroad ballast, among other things. This early crust was frequently turned and subsumed by the molten interior.
There are few terrestrial rocks from Hadean time, just a few mineral fragments found in sandstone substrates in Australia. Between 4 billion and 2. Approximately 70 percent of continental landmass was formed during this time. Water from comets and hydrated minerals condensed in the atmosphere and fell as torrential rain, cooling the planet and filling the first oceans with liquid water.
Exactly when or how it happened is unknown, but microfossils of this time indicate that life began in the oceans about 3. It is probable that these microscopic prokaryotes began as chemoautotrophs, anaerobic bacteria able to obtain carbon from carbon dioxide CO 2.
By the end of the Archean, the ocean floor was covered in a living mat of bacterial life. The Proterozoic Eon is also called the Cryptozoic "age of hidden life".
About 2. It is generally accepted that different types of prokaryotic organisms formed symbiotic relationships.
0コメント