Precambrian

I

Introduction

Precambrian, span of time starting at the beginning of geologic time, when rocks first formed, and extending to the beginning of the Cambrian Period about 542 million years ago, when multicellular life first became abundant. Earth’s oldest known rocks have been dated at just over 4 billion years old. Precambrian rocks have been found in Canada, Scandinavia, Africa, Brazil, Western Australia, and East Antarctica.

Although Earth has been determined to be about 4.6 billion years old, evidence of the first 500 million years of the planet’s history has not yet been found in the rock record. Even the geologic record of the Precambrian, written in the surviving rocks, is sparse. Earth scientists must use the rock record that has been preserved to hypothesize and form theories about Earth’s origin.

During the Precambrian, continents began to form and grow, driven by a mechanism similar to what is now called plate tectonics. Also during this time, the oceans and the atmosphere started forming from the gases escaping the extremely hot, semiliquid interior of the planet. Life in the form of primitive bacteria may have originated as early as four billion years ago, perhaps at hot springs on the sea floor.

The Precambrian is divided into pre-Archean time (from the formation of the earth to 3.8 billion years ago), the Archean Eon (3.8 billion to 2.5 billion years ago), and the Proterozoic Eon (2.5 billion to 570 million years ago).

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II

The Archean Eon

Archean rocks, the oldest known rocks on Earth, are preserved as blocks of old continental crust, embedded in the crust of younger continents formed in the late Archean Eon. The oldest known rocks are the Acasta gneisses (deformed granites) in the Yellowknife region of northwestern Canada, dated at 4.03 billion years old by an isotopic method (see Dating Methods) using uranium-lead isotopes. This technique measures the relative amounts of radioactive isotope and its decay product to determine the time of rock formation. Gneisses are metamorphic rocks—that is, rocks that have been deformed and recrystallized by heat and pressure. Other Precambrian rocks, younger than the Acasta gneisses, are found on all the continents as isolated pieces of older continental crust.

A

Rocks and Mineral Deposits

Important Archean rocks include greenstones, granites, komatiites, and various sedimentary rocks (formed from deposits of weathered rocks and remains of organisms). Greenstones, generally a grayish green, are basalts (igneous rocks, or rocks that solidified from molten lavas) that were formed beneath the ocean and later metamorphosed. Komatiites, lavas with a high magnesium content, require very high temperatures for their formation. The presence of komatiites is evidence that, at the time they were formed, Earth was much hotter than it is today. Granites are igneous rocks that intruded and fractured the greenstones. Sedimentary rocks indicate that erosion was occurring.

Economic mineral deposits from the Archean Eon include copper, zinc, and nickel, all found in Archean greenstones, and gold, which is found in granites and sedimentary rocks. Over half of the gold that has been recovered on Earth comes from the 2.9-billion year old Witwatersrand sediments in South Africa.

B

The Origin of Continents

Earth scientists use the theory of plate tectonics and the evidence of Archean rocks to form a hypothesis of how the continents were probably formed. Komatiites from the Archean Eon indicate that Earth’s mantle (the layer between the planet’s crust and core) was much hotter in the Archean than it is today. Because of the greater heat, convection in the mantle (movement created when hotter parts rise and cooler parts sink), the driving force of plate movement, was greater. Archean continents probably grew much as continents do today, but at a faster rate. Continents grow at their edges when pieces of ocean crust are scraped off as the ocean plate descends under the continent and when two continents collide and stick together.

Since only small pieces of early Archean continental crust are now preserved, scientists hypothesize that the first continents were smaller than they are now and probably sank back into the mantle as a result of subduction (when one plate descends, or dives, beneath another when the two collide). Today’s continents are light and buoyant and tend not to sink. One of the important consequences of the early and comparatively rapid plate tectonics is that heat from Earth’s interior was lost at a greater rate than at the present time.

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