1. VIZUALIZING EARTH HISTORY By Loren E. Babcock Chapter 8 Archean World

2. Overview of Archean Time Major geologic and biologic events of the Precambrian. The “Precambrian,” or the time of the Archean Eon plus the Proterozoic Eon, was a time in which many crucial events in Earth’s physical, chemical, and biological evolution took place. Precambrian - Initial formation of the Earth, solidification of the crust, development of the cratons (cores of continents), initiation of plate tectonic activity, formation of the oceans and atmosphere, the first prokaryotic and eukaryotic life, and early icehouse-greenhouse cycles including glacial episodes. The Precambrian rock record is fossil poor.

3. Overview of Archean Time The Phanerozoic is better understood than the Archean. Archean Eon - The unit of geologic time beginning with Earth’s formation, perhaps 4.56 billion years ago, and ending at the beginning of the Proterozoic Eon, 2.5 billion years ago. Proterozoic Eon - The unit of geologic time beginning 2500 million years ago (2.5 billion years ago) and ending at the beginning of the Phanerozoic Eon, 542 million years ago. The Archean Eon begins with this planet’s formation, 4.56 billion years ago. The end of the Archean is defined by the beginning of the next eon, the Proterozoic, a point arbitrarily chosen as 2.5 billion years ago.

4. Overview of Archean Time The Phanerozoic is better understood than the Archean. Craton - The core of a continent— the part of the Earth’s continental crust that has attained relative stability and received little deformation for at least 1 billion years.

5. Origin of Earth and the Solar System Using meteorites and moon rocks to decipher early Earth history We have access to at least two extraterrestrial sources of information about the age of Earth: meteorites and Moon rocks. Meteorites are rocks that have fallen to Earth (or some other planet or moon) from space. Meteorite - Relatively small rock that falls to a planetary surface from interplanetary space.

6. Origin of Earth and the Solar System Using meteorites and moon rocks to decipher early Earth history Rocks brought back from the lunar surface by US astronauts during the Apollo missions of the 1960s and 1970s are silicate rocks of basaltic (mafic) and ultramafic composition. They are comparable in composition to stony meteorites (mostly ordinary chondrites) and to Earth’s mantle.

7. Origin of Earth and the Solar System Using meteorites and moon rocks to decipher early Earth history Radiometric analyses of stony meteorites and Moon rocks using uranium-lead, uranium-thorium, potassium- argon, and rubidium-strontium dating methods provide ages that cluster between 4.5 and 4.6 billion years.

8. Origin of Earth and the Solar System Birth of the Universe and the Solar System Astronomers refer to the formation of the universe as the “big bang.” Initially, all matter is assumed to have been concentrated at a single point. Upon explosion, matter shot out in all directions. Eventually gravitational attraction caused its assembly into galaxies, which are disk-shaped clusters of stars. The universe is thought to be 15 to 18 billion years old based on calculations of the wavelengths of light radiating from distant stars.

9. Origin of Earth and the Solar System Early evolution of the Earth and Moon The hypotheses for the origin of the Moon most accepted today is that the moon formed after the Earth was impacted by a body about the size of Mars. About 4.5 billion years ago, the magma ocean of the Moon had solidified to form a crust, and the Earth’s second magma ocean had solidified to form its second basaltic crust.

10. Origin of Earth and the Solar System Describe how the Earth’s core, mantle, and crust were formed. The Earth shows layers of differing composition and zones of differing rock strength. The compositional layers consist of an iron-nickel-rich core, a silicate-rich mantle, and a silicate crust. The crust is thicker beneath the continents than it is under the oceans. The boundary between the crust and the mantle is called the Moho (or Mohorovičić discontinuity). Zones of differing rock strength are the cool, rigid lithosphere (on the outside of the planet), the hot, more plastic asthenosphere, and the hot but strong mesosphere.

11. Origin of Earth and the Solar System Describe how the Earth’s core, mantle, and crust were formed.

12. Origin of the Atmosphere and Ocean Explain how the Earth’s atmosphere was initially developed. The early atmosphere must have formed after the Earth had coalesced and was large enough to retain gases in its gravitational field. While in the molten state, volatiles easily escaped to the surface in a process called outgassing (or degassing). Outgassing - The process of releasing gases, including water vapor, from magma.

13. Origin of the Atmosphere and Ocean Explain the sources of water for Earth’s early ocean, and explain the source of salts dissolved in it. Earth’s outgassing vapors condensed to form liquid water of the ocean. Comets and volcanoes also release salt-free water. The source of the ocean’s salt comes from the chemical weathering of rocks, particularly on land and at the shoreline.

14. Earth’s Oldest Rocks Early continental crust The cores of continents are amalgamated from large podlike rock bodies welded along metamorphic zones called greenstone belts. The podlike bodies are mostly high-grade metaigneous rocks representing the felsic crust of Archean protocontinents, and the greenstones connecting them are metavolcanic and metasedimentary rocks rich in chlorite, a green mineral formed under low-grade metamorphic conditions.

15. Earth’s Oldest Rocks Early continental crust

16. Earth’s Oldest Rocks Greenstone belt - Elongate area within an Archean shield containing metamorphosed and deformed volcanic and sedimentary rocks, and characterized by abundant chlorite-rich greenstone. Mobile crust phase - Interval of Earth history during which amalgamation of the continental crust occurred. The oldest known block of continental crust is in the Acasta Formation, part of the Slave Craton, which is now part of northern Canada (3.8 to 4.0 billion years old).

17. Earth’s Oldest Rocks Banded iron formation - Sedimentary rock composed of thin chert (quartz) bands interlayered with iron oxide minerals. One banded iron formation, from the Itsaq gneiss complex- Isua greenstone belt of Southwest Greenland, was deposited 3.7 to 3.8 billion years ago.

18. Archean Life Forms The earliest organisms: The prokaryotes. The earliest fossils are of simple microbes preserved as molds with carbon in Archean chert. Chert, which is cryptocrystalline quartz, solidifies from a gel of silica. Cyanobacteria - Blue-green eubacteria, most of which are photosynthetic. Photosynthesis - The process by which plants, algae, and some bacteria create organic molecules from carbon dioxide and water using energy from sunlight.

19. Archean Life Forms The earliest organisms: The prokaryotes.

20. Archean Life Forms The earliest organisms: The prokaryotes. Autotrophy - “Self-feeding” by means of either harvesting light energy from the Sun or from oxidation of inorganic compounds to make organic molecules. Heterotrophy - A means of obtaining nutrients by ingesting or breaking down organic matter.

21. Archean Life Forms The earliest evidence of life Stromatolite - A thinly layered biogenic-sedimentary structure resulting from the trapping and binding of fine sediment in layers by photosynthetic cyanobacteria. These thinly layered features are persistent in Archean and Proterozoic sedimentary environments. The oldest known stromatolite is from the Fig Tree Group of South Africa, where they have been dated at about 3.2 billion years.

22. Archean Life Forms Abiotic synthesis of amino acids and the earliest life forms Amino acids, composed of carbon, hydrogen, oxygen, and nitrogen, are the chemical building blocks of life. In 1953, Stanley Miller and Harold Urey published results of a simple laboratory experiment demonstrating that amino acids could be created under abiotic conditions without the availability of free oxygen. Their experiment was designed to imitate the conditions under which life was thought, in the early 1950s, to have arisen on Earth more than 3 billion years ago.

23. Archean Life Forms Abiotic synthesis of amino acids and the earliest life forms

24. Archean Life Forms Abiotic synthesis of amino acids and the earliest life forms Photochemical dissociation - The splitting of molecules into their components by means of energy from sunlight or other light sources. In order for life to have arisen abiotically, it must have first developed under anoxic, aqueous conditions. It is possible that life arrived to this planet aboard a meteorite or a comet. The more likely option is that life arose on Earth. If so, it must have evolved in a place where contact with free oxygen of the atmosphere was limited. This “primordial soup” could have developed in small isolated ponds, lakes, and a range of marginal-marine settings.

25. Archean Life Forms Simple organic molecules can be created abiotically. One anoxic environment where amino acids could organize into more complex genetic and organic materials is along mid-ocean ridges. Earth’s early crust was probably rich in spreading centers and subduction zones. Hydrothermal vent - Opening in the Earth’s crust, usually associated with magmatic activity, where hot water, often enriched in ions, is released.

26. Archean Life Forms Simple organic molecules can be created abiotically. Archaebacteria - Organisms belonging to the domain Archaea, including the methanogenic, halophilic, and thermoacidophilic prokaryotes. Some archaebacteria not only survive under the extreme conditions that exist along mid-ocean ridges, but require them for their metabolism. They obtain energy through chemosynthesis, in which naturally occurring chemical reactions are allowed to occur within their cells.

27. Archean Life Forms Does life exist beyond Earth? Whether life exists elsewhere in the Solar System, and whether the early Earth may have been “seeded” with life forms arriving from beyond our planet are questions that remain speculative. In the search and test for fossilized life forms from elsewhere in our Solar System or elsewhere in the universe, we can use the same criteria for meteorites or rocks collected directly from extraterrestrial bodies as we use in the study of Earth’s earliest life forms.