Text of Earth Science 13.1 Precambrian Time Earth’s History: Precambrian Time
Earth Science 13.1 Precambrian Time Earths History: Precambrian Time
Precambrian Earth Geologists travel to Isua, Greenland to study some of Earths oldest rocks. Radiometric dating shows that the rocks of Isua greenstone belt are about 3.8 billion years old. Geologists think that the Isua greenstone belt began as volcanic rock that formed underwater. Over billions of years, heat and pressure changed the rock to greenstone, a metamorphic rock.
Precambrian Earth Geologists know little about early Precambrian time. Most rocks from this time have been eroded away, subducted, or greatly metamorphosed. Relative dating of Precambrian rocks is difficult because the rocks rarely contain fossils. Yet key geological events occurred during Precambrian time. Earth formed about 4.56 billion years ago. During Precambrian time (4.56 billion to 542 million), the atmosphere and oceans formed and plate tectonics began to build up continental landmasses.
Earth forms Scientists hypothesize that Earth formed as gravity pulled together dust, rock, and ice in space. As Earth grew, its gravity increased, pulling in more of these materials. The high-velocity impact of rocks from space caused the planet to melt Radioactive decay was also generating much more heat than at the present time.
Earth forms Melting permitted the most dense materials, such as iron and nickel, to sink towards Earths center. Less dense materials, such as silicates, floated, forming layers that became the mantle and crust. Over several hundred million years, the crust and mantle cooled and hardened, forming rock.
The Atmosphere Evolves Today, the air you breath is a stable mixture of nitrogen, oxygen, a small amount of argon, and trace gases like carbon dioxide and water vapor. Our planets atmosphere several billion years ago was far different. Earths original atmosphere was made up of gases similar to those released in a volcanic eruption: water vapor, carbon dioxide, nitrogen and several trace gases, but no oxygen. Scientists think that as earths crust cooled, gases that had been dissolved in molten rock were gradually released.
The Atmosphere Evolves Some of Earths earliest life forms radically changed the makeup of Earths atmosphere by using carbon dioxide and releasing oxygen. Slowly the atmospheres oxygen content increased. The Precambrian rock record suggest that much of this oxygen combined with iron. Iron combines with oxygen to form iron oxides, rust. Large deposits of iron-rich Precambrian sedimentary rocks, called banded iron formations, provide evidence of this process.
The Oceans Form Earths oceans formed as the planet continued to cool. The water vapor condensed to form clouds, and the great rains began. At first, the rainwater evaporated in the hot air before reaching the ground or quickly boiled or evaporated when it did reach the ground. This evaporation sped up the cooling of Earths surface.
The Oceans Form Torrential rains continued and slowly filled low areas, forming the oceans. This reduced not only the water vapor in the air but also the amount of carbon dioxide, which became dissolved in the water. A nitrogen-rich atmosphere remained.
Continents develop Geologists think that small continents began to form about 500 million years after Earths formation. As on Earth today, mantle convection drove the processes of plate tectonics. Tectonic forces caused early continents to grow by accretion and to split apart by rifting.
Continents develop At the same time, the rock cycle began. The early continents were made up of granitic igneous rock that was less dense than the volcanic rock of the oceanic crust. Weathering and erosion led to the formation of sedimentary rocks. Tectonic forces and volcanic activity in the crust helped to form metamorphic rock.
Continents develop Today, if you view Earth from space, you will find very little Precambrian rock left exposed. As we learned from the Law of Superposition, most Precambrian rocks are buried beneath more recent rocks. But we can still find Precambrian rocks exposed where younger rock layers are extensively eroded. Examples include the Grand Canyon and some mountain ranges.
Continents develop There are however large core areas of Precambrian rocks that make up the surface of some continents. These areas are called shields because they roughly resemble the a warriors shield in shape. The Canadian Shield forms the core of the continent of North America. Most shields are made up of deformed metamorphic rock. Canadian Shield
Precambrian Life Most Precambrian fossils are so small they are called microfossils. To study microfossils, geologists make very thin slices of rock that can be viewed under a microscope. Using microfossils and other evidence, geologists have begun to reconstruct lifes early history. The earliest life probably evolved in the oceans.
Precambrian Life Later one-celled organisms evolved that used light energy to produce food through photosynthesis. These organisms consisted of simple cells called prokaryotes. Slowly, more complex cells, called eukaryotes, evolved. Later in the Precambrian, multicelled organisms with soft bodies developed.
The Earliest Life: Scientists do not know when life first appeared on Earth or what it may have looked like. Because water is needed for most life processes of living things, the earliest life probably evolved in the oceans. Some scientists think the first organisms might have resembled bacteria found near hydrothermal vents in the deep ocean.
The Earliest Life: Photosynthetic Organisms Precambrian rocks from about 3.5 billion years ago contain fossils of one-celled organisms. These organisms resemble modern cyanobacteria. Cyanobacteria are one- celled organisms that make their own food through photosynthesis. In photosynthesis, organisms use carbon dioxide and the energy of sunlight to make food in the form of hydrocarbons. Photosynthesis releases oxygen into the atmosphere.
The Earliest Life: Photosynthetic Organisms Geologists have found traces of cyanobacteria from Precambrian time. The fossils are stromatolites. Stromatolites are layered mounds of calcium carbonate deposited by cyanobacteria. Stromatolites closely resemble similar deposits made by modern cyanobacteria. Today, stromatolites are relatively rare. They occur in warm, shallow water along sheltered coastlines. Today, geologists infer that Precambrian stromatolites formed in similar conditions. stromatolites
The Earliest Life: Prokaryotes and Eukaryotes: Precambrian time, two different groups of organisms evolved; prokaryotes and eukaryotes. Cyanobacteria belong to a group of simple one-celled organisms called prokaryotes. A prokaryote is a cell that lacks a nucleus. Earths earliest organisms were prokaryotes.
The Earliest Life: Prokaryotes and Eukaryotes: About 1.8 billion years ago, cells evolved that were larger and more complex than prokaryotes. These cells, called eukaryotes, had nuclei. Geologists have found fossil eukaryotes; red and green algae that were are about one billion years old. Many early eukaryotes were one- celled organisms. Unlike prokaryotes, eukaryotes can also be made up of many cells. Eukaryotic fossil
The Earliest Life: Multicelled Organisms: The development of eukaryotes led to dramatic evolutionary change. Fossil evidence shows that the earliest animals had evolved in the late Precambrian Period. In the 1940s, geologists found strange looking fossils in the Ediacara Hills is southern Australia. The fossils were about 600 million years old. The Ediacara fossils were molds and casts of soft bodied animals. They resemble worms, jellyfish, and corals living in the oceans today. To e preserved as fossils, these animals must have been rapidly buried in fine sediment.