Chapter 8 Earth's Formative Stages and the Archean Eon

Chapter 8 Earth's Formative Stages and the Archean Eon

Archean Eon Archean (Archean Eon) is the oldest unit on the geologic time scale. It began 4.56 billion years ago and ended 2.5 billion years ago. Archean lasted for 2.1 billion years (2,100,000,000 years).

Earth's Oldest Rocks Earth's oldest rocks are found in Canada. They are about 4.04 billion years old. But there are even older mineral grains. Sand-sized zircon grains in metamorphosed sedimentary rocks from Australia are 4.4 billion years old.

Earth's Oldest Rocks There are no rocks on Earth that date back to 4.6 billion years ago because the Earth is geologically active, and the oldest rocks have been recycled by plate tectonics or by weathering and erosion as part of the rock cycle. Much of our knowledge of the Earth's earliest history comes from indirect evidence - meteorites.

Archean and Proterozoic Eons comprise in interval of time informally called Precambrian, which spans 87% of the geologic time scale. Precambrian

The Big Bang Calculations indicate that the Big Bang occurred 18-15 billion years ago. The Big Bang marked the instantaneous creation of all matter in the Universe.

The Solar System The Sun and the planets, moons, asteroids, comets and other objects that orbit it, comprise the Solar System.

Lines of evidence that must be considered for any hypothesis on the origin of the Solar System 1. Planets revolve around sun in same direction - counterclockwise (CCW) 2. Planets lie roughly within sun's equatorial plane (plane of sun's rotation) 3. Solar System is disk-like in shape

4. Planets rotate CCW on their axes, except for: a. Venus - slowly clockwise b. Uranus - on its side c. Pluto - on its side 5. Moons go CCW around planets (with a few exceptions)

6. Distribution of planet densities and compositions is related to their distance from sun a. Inner, terrestrial planets have high density b. Outer, Jovian planets have low density 7. Age - Meteorites are as old as 4.56 billion years

Solar Nebula Hypothesis or Nebular Hypothesis 1. Cold cloud of gas and dust contracts, rotates, and flattens into a disk-like shape. 2. Roughly 90% of mass becomes concentrated in the center, due to gravitational attraction. 3. Turbulence in cloud caused matter to collect in certain locations.

Solar Nebula Hypothesis or Nebular Hypothesis 4. Clumps of matter begin to form in the disk. 5. Accretion of matter (gas and dust) around clumps by gravitational attraction. Clumps develop into protoplanets. 6. Solar nebula cloud condenses, shrinks, and becomes heated by gravitational compression to form Sun.

Solar Nebula Hypothesis or Nebular Hypothesis 7. Ultimately hydrogen (H) atoms begin to fuse to form helium (He) atoms, releasing energy (heat and light). The Sun "ignites." 8. The Sun's solar wind drives lighter elements outward, causing observed distribution of masses and densities in the Solar System.

Solar Nebula Hypothesis or Nebular Hypothesis 9. Planets nearest Sun lose large amounts of lighter elements (H, He), leaving them with smaller sizes and masses, but greater densities than the outer planets. Inner planets are dominated by rock and metal. 10. Outer planets retain light elements such as H and He around inner cores of rock and metal. Outer planets have large sizes and masses, but low densities.

Solar Nebula Hypothesis or Nebular Hypothesis

How old is the Solar System? Based on radiometric dates of moon rocks and meteorites, the Solar System is about 4.56 billion years old.

Meteorites: Samples of the Solar System Meteors = "shooting stars." The glow comes from small particles of rock from space being heated as they enter Earth's atmosphere. Meteorites = chunks of rock from the Solar System that reach Earth's surface. They include fragments of: Asteroids Moon rock Planets, such as Mars (i.e., "Martian meteorites")

Types of Meteorites 1. Ordinary chondrites 2. Carbonaceous chondrites 3. Iron meteorites 4. Stony-iron meteorites

Meteorites: Ordinary Chondrites Most abundant type of meteorite About 4.6 billion years old, May contain chondrules - spherical bodies that solidified from molten droplets thrown into space during Solar System impacts

Meteorites: Carbonaceous Chondrites Contain about 5% organic compounds, including amino acids the building blocks of proteins, DNA, and RNA May have supplied basic building blocks of life to Earth Contain chondrules

Meteorites: Iron Meteorites Iron-nickel alloy Coarse-grained intergrown crystal structure About 5% of all meteorites

Meteorites: Stony-iron Meteorites Composed partly of Fe, Ni and partly of silicate minerals, including olivine (like Earth's mantle). About 1% of all meteorites. Least abundant type.

The Sun The Sun is a star Composition: 70% hydrogen 27% helium 3% heavier elements Size: About 1.5 million km in diameter Contains about 98.8% of the matter in the Solar System.

The Sun Temperature: may exceed 20 million o C in the interior. Sun's energy comes from fusion, a thermonuclear reaction in which hydrogen atoms are fused together to form helium, releasing energy. The Sun's gravity holds the planets in their orbits.

Sun's energy is the force behind many geologic processes on Earth Evaporation of water to produce clouds, which cause precipitation, which causes erosion. Uneven heating of the Earth's atmosphere causes winds and ocean currents. Variations in heat from Sun may trigger continental glaciations or change forests to deserts. Sun and moon influence tides which affect the shoreline.

The Planets 1. Mercury 2. Venus 3. Earth 4. Mars 5. Jupiter 6. Saturn 7. Uranus 8. Neptune

The Planets Terrestrial planets: Small Dense (4-5.5 g/cm 3 ) Rocky + Metals Mercury, Venus, Earth, Mars Jovian planets: Large Low density (0.7-1.5 g/ cm 3 ) Gaseous Jupiter, Saturn, Uranus, Neptune

Mercury Smallest of the terrestrial planets Revolves rapidly around the sun; its year is 88 Earth days Densely cratered Thin atmosphere of sodium and lesser amounts of helium, oxygen, potassium and hydrogen Weak magnetic field and high density suggest an iron core No moons

Venus Similar to Earth in size, mass, volume, density and gravity No oceans or liquid water Very high atmospheric pressure Atmosphere is 98% carbon dioxide Dense clouds of sulfuric acid droplets in atmosphere Greenhouse effect causes temperature on planet's surface to reach 470 C, hot enough to melt lead

Venus Rotates once on its axis (one day on Venus) in 243 Earth days Rotates on axis in opposite direction to other planets, possibly due to collision with other object Has volcanoes Has craters Surface rocks resemble basalt No moons

Earth Diameter = nearly 13,000 km (8000 mi) Oceans cover 71% of surface Atmosphere = 78% nitrogen and 21% oxygen Surface temperature approx. -50 and +50 o C Average density = 5.5 g/cm 3 Surface rock density = 2.5-3.0 g/cm 3 Core about 7000 km in diameter; Mantle surrounds core. Extends from base of crust to depth of 2900 km. Geologically active. Plate tectonics. Only body in the Universe known to support life.

Earth's Internal Layered Structure The Earth is internally layered, with a basic structure consisting of: Crust Mantle Inner and outer core The Earth's internal structure may be primary (formed initially as the Earth formed), or secondary due to later heating.

Factors that make Earth hospitable for life Distance from Sun maintains temperatures in the range where water is liquid. Temperature relatively constant for billions of years. Rotation allows all sides of Earth to have light and heat. Atmosphere absorbs some heat from the Sun and reflects some solar radiation back to space. Magnetic field protects life from dangerous high energy particles and radiation in the solar wind.

Earth's Moon Diameter = about 1/4 that of Earth. Density = about 3.3 g/cm3 (similar to Earth's mantle). Rotates on its axis at same rate as it revolves around Earth (29.5 days). Results in same side of Moon always facing Earth. Far side of moon is more densely cratered No atmosphere. Ice is present at the poles.

Geology of the Moon Dominant rock type is anorthosite (related to gabbro; rich in Ca plagioclase feldspar). Basalt is also present. Two types of terrane Lunar highlands Maria (singular = mare)

Lunar highlands Light-colored Rough topography Highly cratered Rocks more than 4.2 billion years old

Lunar maria Large, dark areas Immense basins covered with basaltic lava flows Age of basalt is 3.8 to 3.2 billion years Maria have few craters. This indicates a decrease in meteorite bombardment after about 3.8 billion years ago.

Origin of the Moon Moon may have formed as a result of an impact of a large body with Earth about 4.4 billion years ago. Debris from the impact was thrown into orbit around Earth and collected to form the Moon. Heat from impacts led to melting and differentiation (or segregation of materials of different density; low density materials rose and high density materials sank).

Mars Has white polar caps made of frozen carbon dioxide ice Has seasonal changes. Polar ice caps expand and contract Rusty orange color due to iron oxides on surface Heavily cratered due to early bombardment by meteorites and asteroids

Mars Diameter is about half that of Earth Mass is only about 10% of Earth's mass, so gravity is much less Thin atmosphere (less than 1% as dense as Earth's). Dominant gas is carbon dioxide; small amounts of nitrogen, oxygen and carbon monoxide. No greenhouse effect.

Mars Previously had a denser atmosphere Evidence of abundant liquid water in the past. An ocean once existed, at least 0.5 km deep and larger than all 5 U.S. Great Lakes. Temperatures range from -85 o C to 21 o C (21 o C is about room temperature)

Mars Lower density than other terrestrial planets Little to no magnetic field, suggesting only a small iron-rich core. Lack of magnetic field exposed planet to solar winds which swept away atmosphere and liquid water. Two small moons, Phobos and Deimos

Asteroid Belt Thousands of asteroids, primarily between Mars and Jupiter. Asteroids are composed of rocks and metal (Fe & Ni). Size of asteroids ranges from a few km in diameter to about 1/10 the size of Earth.

Jupiter Largest planet in the Solar System. (Diameter 11 times greater than Earth) Low density. (Density is about 1/4 that of Earth) Most of planet's interior is probably liquid metallic hydrogen. Rotates on axis rapidly. One day on Jupiter is 10 hours on Earth. Rotation causes bands in atmosphere

Jupiter Note Great Red Spot, a cyclonic storm Atmosphere composed of H, He, with lesser amounts of methane and ammonia Has a faint ring of debris which encircles the planet

Jupiter Has more than 60 moons Four largest moons are: Io- covered by sulfur volcanoes Europa - has sea of liquid water beneath an icy surface Ganymede - planet-sized body larger than Mercury. Cratered with sinuous ridges; has sea of liquid water beneath an icy surface Callisto - highly cratered; has sea of liquid water beneath an icy surface

Saturn Second largest planet Has prominent rings of debris encircling planet in equatorial plane

Saturn Density is less than that of water; it could float. (Density = 0.7g/cm 3 ) Mostly H and He; also contains methane, ammonia, and water; may have iron core Has magnetic field, radiation belts, and internal heat source Has more than 30 moons.

Uranus About 4 x larger than Earth Low density (density = 1.3 g/cm 3 ) Axis of rotation is tipped on its side, possibly due to collision with another Solar System object Has more than two dozen moons Atmosphere of hydrogen, helium, and methane Has planetary ring system

Neptune Similar in size and color to Uranus Low density (density = 1.6 g/cm3) Atmosphere = H, He, and methane Has more than a dozen moons Has Great Dark Spot, a cyclonic storm Has planetary ring system

Solar Nebula Hypothesis or Cold Accretion Model (Secondary Differentiation) Earth formed by accretion of dust and larger particles of metals and silicates. Earth was originally homogeneous throughout - a random mixture of space debris. Origin of layering requires a process of differentiation. Differentiation is the result of heating and at least partial melting.

Possible sources of heat for melting: 1. Accretionary heat from bombardment (meteorite impacts) 2. Heat from gravitational compression as material accumulated 3. Radioactive decay

Differentiation after Accretion Iron and nickel sink to form core. Less dense material (silicon and oxygen combined with remaining iron and other metals) forms mantle and lighter crust (dominated by silicon and oxygen). Presence of volatile gases on Earth indicates that complete melting did not occur. Earth was repeatedly partly melted by great impacts, such as the Moon-forming impact.

An alternative model: Hot Accretion (Primary Differentiation) Internal zonation of planets is a result of hot heterogeneous accretion. Hot solar nebula (over 1000 o C). Initial crystallization of iron-rich materials forms planet's core. With continued cooling, lower density silicate materials crystallized.

Which Model? Solar Nebula Hypothesis also known as the Cold Accretion Model (secondary differentiation) OR Hot Accretion Model (primary differentiation)??? Parts of both models may have been in operation.

Archean Crust Once differentiation occurred, Earth's crust was dominated by Fe and Mg silicate minerals. If Earth experienced heating and partial melting, it may have been covered by an extensive magma ocean during Archean. Magma cooled to form rocks called komatiites.

Komatiites Komatiites are ultramafic rocks composed mainly of olivine and pyroxene. Komatiites form at temperatures greater than those at which basalt forms (greater than 1100 o C). This rock formed Earth's Archean crust.

Origin of Mafic Crust The first mafic, oceanic crust formed about 4.5 billion years ago by partial melting of rocks in the upper mantle.

Earth's Crust Today Earth has two types of crust today: 1. Dense, mafic (Mg- and Fe-rich) oceanic crust dominated by basalt. 2. Less dense, silicic (Si- and Al-rich) continental crust dominated by granite.

Origin of Continental Crust Continental crust developed after the initial mafic to ultramafic crust. Continental crust is silicic or felsic (such as granite). Dominated by light-colored minerals such as quartz and feldspar. Felsic crust began forming around 4.4 billion years ago.

Origin of Continental Crust Felsic crust formed in subduction zones where descending slabs of crust partially melted. The early-melting, less dense components of the melt rose to the surface where they cooled to form continental crust.

Earth's Oldest Rocks One of the oldest dated felsic Earth rocks is the 4.04 billion year old Acasta Gneiss from northwestern Canada. Dates are from zircon grains in tonalite gneisses. (Tonalite gneiss is metamorphosed tonalite, a rock similar to diorite, with at least 10% quartz).

Earth's Oldest Rocks The Amitsoq Gneiss from Greenland is another old tonalite gneiss (3.8 b.y. old). Patches of old felsic crust have also been found in Antarctica (3.9 b.y. old).

Earth's Oldest Land Surface A 3.46 b.y. old fossil soil zone (or paleosol) associated with an unconformity in the Pilbara region of Australia indicates that Archean continents stood above sea level. This paleosol represents the oldest land surface known, and provides evidence that subaerial weathering, erosion, and soil formation processes were at work during Archean.

The Oldest Mineral Grains The oldest zircon grains are 4.4 b.y. old. Found in quartzite in western Australia. Sedimentary structures in the quartzite resemble those in modern stream deposits. Interpreted as fluvial (river) deposits. Derived from weathering of granitic rocks (some of the earliest continental crust), and deposited above sea level, indicating the presence of both liquid water and continental crust by 4.4 b.y. ago.

Oceanic Crust First appearance About 4.5 b.y. ago Continental Crust About 4.4 b.y. ago Where formed Mid-ocean ridges Subduction zones Composition Komatiite & basalt Tonalite & granodiorite, and later, granites Lateral extent Widespread Local How formed Partial melting of ultramafic rocks in upper mantle (few 100 km or mi) Partial melting of wet, sedimentcovered mafic rocks in subduction zones

Evolution of Earth's Atmosphere and Hydrosphere Earth's first, primitive atmosphere lacked free oxygen. The primitive atmosphere was derived from gases associated with the comets and meteorites which formed the Earth during accretion. The gases reached the Earth's surface through a process called outgassing.

Gases Associated with Comets Comets are made of frozen gases, ice and dust. Halley's comet is composed of: 80% water ice Frozen carbon dioxide (dry ice) Hydrogen cloud surrounds comet Dust near the nucleus contains iron, oxygen, silicon, magnesium, sodium, sulfur, and carbon

Gases Associated with Meteorites Carbonaceous chondrites are mainly composed of silicate minerals, but also contain: Nitrogen Hydrogen Water Carbon in the form of complex organic molecules (proteins and amino acids)

Water and gaseous elements would have been released from the newly accreted Earth by the heat associated with bombardment and accretion, or by melting and volcanism accompanying later differentiation.

Volcanic Outgassing Outgassing = release of water vapor and other gases from Earth through volcanism. Gases from Hawaiian eruptions consist of: 70% water vapor (H 2 O) 15% carbon dioxide (CO 2 ) 5% nitrogen (N 2 ) 5% sulfur (in H 2 S) chlorine (in HCl) hydrogen argon

Volcanic Outgassing Most of the water on the surface of the Earth and in the atmosphere was outgassed during the first billion years of Earth history. We know this because there are 3.8 b.y.-old marine sedimentary rocks, indicating the presence of an ocean by 3.8 billion years ago.

Formation of the Hydrosphere Once at the Earth's surface, gases and other volatile elements underwent a variety of changes. 1. Water vapor condensed and fell as rain. 2. Liquid water probably began to fall on the Earth's surface as early as 4.4 billion years ago. 3. Rain water accumulated in low places to form seas. The seas were originally freshwater (rain).

Formation of the Hydrosphere 4. Carbon dioxide and other gases dissolved in the rain made the water more acidic than today. Carbon dioxide and water combine to form carbonic acid. 5. Acid waters caused rapid chemical weathering of the exposed rocks, adding Na, Ca, K, and other ions to seawater. 6. A change to more alkaline water may have occurred rapidly as large amounts of Ca, Na, and Fe were introduced by submarine volcanism, neutralizing the acid.

Formation of the Hydrosphere 7. Ions accumulated in the seas, increasing the salinity. Sea salinity is relatively constant today because salts are precipitated at about the same rate they are supplied to the sea. Sodium remains in sea water due to its high solubility. 8. Later, when the seas became less acidic, Ca ions bonded with CO 2 to form shells of marine organisms and limestones (CaCO 3 ). 9. The presence of marine fossils suggests that sodium has not varied appreciably in sea water for at least the past 600 million years.

Hydrologic Cycle Today Earth's water is continuously recirculated through the hydrologic cycle (evaporation and precipitation, powered by the sun and by gravity).

Evolution of the Atmosphere Note - Gases released by volcanoes, condensation of water vapor, precipitation, and accumulation of liquid water, photochemical reactions in the atmosphere, and formation of carbonate rocks (limestones) later, after the seas became less acidic.

The Early Anoxic Atmosphere Earth's early atmosphere was strongly reducing and anoxic (lacked free oxygen or O 2 gas), and probably consisted primarily of: Water vapor (H 2 O) Carbon dioxide (CO 2 ) Nitrogen (N 2 ) Carbon monoxide (CO) Hydrogen sulfide (H 2 S) Hydrogen chloride (HCl)

The Early Anoxic Atmosphere The atmosphere composition would have been similar to that of modern volcanoes, but probably with more hydrogen, and possibly traces of methane (CH 4 ) and ammonia. If any free oxygen had been present, it would have immediately been involved in chemical reactions with easily oxidized metals such as iron.

Evidence for a Lack of Free Oxygen in Earth's Early Atmosphere 1. Lack of oxidized iron in the oldest sedimentary rocks. (Instead, iron combined with sulfur to form sulfide minerals like pyrite. This happens only in anoxic environments.) 2. Urananite and pyrite are readily oxidized today, but are found unoxidized in Precambrian sedimentary rocks. 3. Archean sedimentary rocks are commonly dark due to the presence of carbon, which would have been oxidized if oxygen had been present.

Evidence for a Lack of Free Oxygen in Earth's Early Atmosphere 4. Archean sedimentary sequences lack carbonate rocks but contain abundant chert, presumably due to the presence of an acidic, carbon dioxide-rich atmosphere. In an acidic environment, alkaline rocks such as limestone do not form.

Banded Iron Formation 5. Banded iron formations (BIF) appear during Precambrian (1.8 - about 3 b.y.). Cherts with alternating laminations of red oxidized iron and gray unoxidized iron. Formed as precipitates on shallow sea floor. Some iron probably came from weathering of iron-bearing rocks on continents. Most iron was probably from submarine volcanoes and hydrothermal vents (hot springs). Great economic importance; major source of iron mined in the world.

Banded Iron Formations

Additional Evidence for an Anoxic Atmosphere 6. The simplest living organisms have an anaerobic metabolism. They are killed by oxygen. Includes some bacteria (such as botulism), and some or all Archaea, which inhabit unusual conditions. 7. Chemical building blocks of life (such as amino acids, DNA) could not have formed in the presence of O 2.

Formation of an Oxygen-rich Atmosphere The change from an oxygen-poor to an oxygen-rich atmosphere occurred by Proterozoic, which began 2.5 billion years ago, at the end of Archean.

Formation of an Oxygen-rich Atmosphere The development of an oxygen-rich atmosphere is the result of: 1. Photochemical dissociation - Breaking up of water molecules into H and O in the upper atmosphere, caused by ultraviolet radiation from the Sun (a minor process today) 2. Photosynthesis - The process by which photosynthetic bacteria and plants produce oxygen (major process).

Evidence for Free Oxygen in the Proterozoic Atmosphere 1. Red beds - Sedimentary rocks with iron oxide cements (including shales, siltstones, and sandstones), appear in rocks younger than 1.8 billion years old. This occurred during Proterozoic, after the disappearance of the banded iron formations (BIFs).

Evidence for Free Oxygen in the Proterozoic Atmosphere 2. Carbonate rocks (limestones and dolostones) appear in the stratigraphic record at about the same time that red beds appear. This indicates that CO 2 was less abundant in the atmosphere and oceans so that the water was no longer acidic.