Physical Environment: Earth Origin, Age & Structure

EVPP 110 Lecture
Fall 2003, Instructor: Dr. Largen

Brief History of Universe & Earth

• Origin of the universe
• unknown for certain
• actively researched
• many theories
• Big Bang theory
• inflation theory
• cold dark matter theory
• theories difficult to test
• Age of the universe
• unknown for certain
• actively researched
• several methods for calculating age of universe
• age estimates vary
• range from 8 billion to 14 billion years old
• origin and age of the universe
• most "popular" origin theory
• Big Bang Theory
• most popular age estimate
• ~ 12 billion years old
• size of universe
• continually increasing since its creation
• universe is thought to have had a dynamic adolescence
• from ~12 billion years ago (BYA) to ~7 BYA
• galaxies, stars and planets of universe were formed, destroyed, re-formed
• steps leading to birth of Earth
• ~7 BYA
• red giant star in vicinity of Earth exploded
• ~4.6 BYA
• remnants of explosion formed our solar system

Earth in context of our solar system

• After collapse of red giant
• rotating, dense cloud (solar nebula) remained
• cloud cooled, condensed and contracted
• rotating faster
• forming flattened disk, thinnest at edges
• contraction continued, rings of material separated from cloud
• condensed to form planets
• resulted in 9 planets of our solar system
• grouped as
• terrestrial planets
• Jovian (non-terrestrial) planets
• Pluto
• terrestrial planets
• closest to sun
• are "earth-like"
• rocky with metallic centers
• heavier materials that stayed nearer sun
• Mercury, Venus, Earth, Mars
• Jovian (non-terrestrial) planets
• farther from sun
• are similar to Jupiter
• composed mostly of liquids and gases
• lighter materials that boiled away from areas nearest to the sun
• Jupiter, Saturn, Uranus, Neptune
• Pluto
• anomalous
• terrestrial but at outer limits of solar system
• Earth is unique in our solar system
• why is the Earth so "special" relative to other planets?
• temperature
• presence & composition of atmosphere
• water
• continued tectonic activity

Age of the Earth

• ~4.6 billion years old - current estimate
• age of Earth not always known or agreed upon
• Greek philosophers
• Earth ageless - no beginning or end to time
• Biblical scholar
• Bishop Ussher (1664)
• put age at 5,668 years
• concluded Earth was formed on October 26, 4004 B.C, based on a literal translation of Bible

Early ideas about physical features of the Earth

• Throughout much of human history
• believed major physical features of Earth were fixed and unchanging
• continents, oceans, mountains, valleys were in their "original" location, would always remain in those locations, unchanged
• as time passed, knowledge grew
• generally held belief of an unchanging earth gave way to concept of catastrophism

• Catastrophism
• subscribed to by most natural scientists up through early 19th century
• proposed that supernatural forces caused catastrophic events that re-shaped the physical landscape
• earthquakes
• volcanic eruptions
• floods
• rise of scientific thought and explorations
• evidence against catastrophism grew
• fundamental principles of modern geology were developed
• principle of superposition
• principle of original horizontality
• principle of uniformitarianism
• Nicolaus Steno (Danish, 1636-1686)
• formulated in 1669
• Principle of Superposition
• Principle of Original Horizontality
• Principle of Original Lateral Continuity
• principle of superposition
• in unaltered series of rock layers
• layers on bottom were deposited first, are oldest
• oldest at bottom, youngest at top

• principle of original horizontality
• strata initially more nearly horizontal than vertical
• any strongly sloped stratum had to have been tilted by external forces after it was formed
• principle of original lateral continuity
• holds that
• strata originally are unbroken, flat expanses
• original continuity of a stratum can be broken by erosion
• as when a river cuts downward to form a valley
• James Hutton (Scottish, 1726-1797)
• formulated in1785 the
• principle of uniformitarianism

• Principle of uniformitarianism
• holds that geologic processes happening today operated in a similar fashion in past
• provide guidance in studying earth’s history
• fundamental to modern science of geology
• laws of nature have not changed over time, were same in past as now
• its application is sometimes called "actualism"
• example; when ripples seen on ancient rock composed of hardened sand (sandstone)
• can assume they developed in same way that ripples develop today
• under influence of certain kinds of water movement or wind

• James Hutton
• believed that rocks of past formed as a result of same processes that were currently operating
• such as
• volcanic activity
• accumulation of grains of sand and clay under the influence of gravity

Nature and Origin of Rocks

• rock
• consist of interlocking or bonded grains of matter
• typically composed of single minerals
• most formed of two or more minerals
• mineral
• naturally occurring inorganic solid element or compound with
• particular chemical composition (or range of compositions)
• characteristic internal structure

• kinds of rocks
• three basic types recognized, based on modes of origin
• igneous
• sedimentary
• metamorphic
• igneous rocks
• form by cooling of molten material to point at which it hardens
• molten material (magma) comes from within earth
• reaches surface through cracks, fissures in crust
• cools and hardens
• composed of bonded grains
• each consisting of a particular mineral
• sedimentary rock
• form by
• accumulation of grains of sediment in a variety of settings
• bonding together of grains to form solid sedimentary rock
• metamorphic rock
• forms by alteration of rocks within earth under conditions of great temperature and pressure (without melting them)

Geologic Time & Dating the Rock/Fossil Record

• geologic time
• expressed in two ways
• relative time (relative age)
• absolute time (absolute age)

• relative time (relative age)
• determined by relative position of sedimentary rocks to each other
• can be used to answer a question like, "Which is younger?"
• utilizes comparison of different geologic formations to determine which is oldest, next oldest, etc.
• governed by concepts such as
• principle of superposition
• looked at earlier
• principle of intrusive relationships
• principle of cross-cutting relationships
• principle of inclusions
• principle of faunal succession
• unconformities
• geologic correlation
• Principle of intrusive relationships
• intrusive igneous rock is always younger than rock it invades
• feature such as a dike that cuts formations is younger than formations it cuts
• dike
• molten magma that cuts upward through sedimentary or metamorphic rocks
• Principle of cross-cutting relationships
• break or fault in formation is always younger than formation itself
• fault that offsets beds is younger than beds it offsets
• Principle of inclusions
• when fragments of one body of rock are found in a second body of rock
• second body is always younger than first
• rock fragments in this conglomerate are older than conglomerate itself

Geologic Time & Dating the Rock/Fossil Record

• Principle of faunal succession
• proposed by William Smith (1769-1839)
• states that over time, organisms on earth have changed in a definite order that is reflected in fossil record
• rocks with recently evolved life forms are younger than those with older life forms

• unconformities
• gaps in rock record
• surface between group of sedimentary strata and rocks beneath those strata
• mark boundaries between rocks of different ages
• may result from
• non-deposition (a hiatus)
• deposition followed by erosion
• angular unconformity
• separates tilted beds from flat lying beds
• disconformity
• separates beds; upper beds rest on erosion surface that developed after lower beds were deposited
• nonconformity
• separates flat-lying beds from igneous or metamorphic rock

• Disconformity
• erosional disconformity separates earlier folding in the lower half from folding (above) after later ash flows were deposited(outcrop of volcanic ash, Japan)

• Unconformity
• boundary between unlayered igneous or metamorphic rocks, and overlying sequential sedimentary rocks
• lower rocks show evidence of erosion before deposition of sedimentary rocks
• geologic correlation
• seeks to establish age relationships between distant sequences of rock
• often through use of fossil assemblages, or index fossils
• a key bed, a distinctive stratum that appears at several localities, may also be used
• Index fossils
• organisms with specific characteristics:
• short lived (geologically)
• widespread occurrence
• readily recognized
• absolute time (absolute age)
• absolute ages are expressed in years, or millions or billions of years, before present
• usually determined using radiometric dating

Geologic Time & Dating the Rock/Fossil Record

• Radioactivity and absolute ages
• radioactive elements and products of their radioactive decay can be used to measure ages of rocks

Geologic Time & Dating the Rock/Fossil Record

• radioactive isotopes
• decay spontaneously, changing into atoms of another element
• each at own nearly constant rate
• age of rock determined by measuring amounts of parent and daughter isotope that remain in rock
• parent isotope
• isotope that undergoes decay
• daughter isotope
• product of parent isotope’s decay
• radioactive decay
• atoms change to those of another element by releasing subatomic particles and energy
• follows an exponential decay law
• exponential decay law
• no matter how much of parent element is present when decay begins
• after certain amount of time, half that amount will survive
• after another interval of same duration, half of surviving amount will survive
• and so on, and so on….
• characteristic interval is known as half life
• half life
• time necessary for half of original atoms of parent isotope to decay into daughter isotope

Radioactive Decay

• Radiometric dating
• requires a parent isotope that undergoes radioactive decays to yield a daughter isotope at a known rate
• Example:
• 14C ® ¹⁴N
• called radiocarbon dating
• half-life of ¹⁴C is 5730 years (relatively short)
• can only be used for dating materials less than 70,000 years old

• Geologic time
• Eons
• largest divisions of time, beginning with the Archean (4.6 to 3.8 billion years ago)
• Eras (subdivisions of eons)
• defined by dominant life forms
• Periods (divisions of eras)
• based on smaller scale changes
• Epochs (divisions of periods)
• based on detailed, smaller scale changes

Geologic Time Scale

• Archean Eon (4.6bya-2.5bya)
• Proterozoic Eon (2.5bya-543mya)
• Phanerozoic Eon (543mya-present) "interval of well-displayed life"
• Paleozoic Era (543mya-251mya) "old life"
• 8 periods; Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvania, Permian
• Mesozoic Era (251mya-65mya) "middle life"
• 3 periods; Triassic, Jurassic, Cretaceous
• Cenozoic Era (65mya-present) "modern life"
• Paleogene Period (65mya-24mya)
• 3 epochs; Paleocene, Eocene, Oligocene
• Neogene Period (24mya-present)
• 4 epochs; Miocene, Pliocene, Pleistocene, Holocene

Geologic Time

Paleozoic Era (543mya-251mya)

• Trilobite fossil, early Paleozoic era

Mesozoic Era (251mya-65mya)

• "age of the dinosaurs"

Cenozoic Era (65mya-present)

• "age of mammals"

Components of the Earth System

• Ecosphere
• entire earth system
• includes all other spheres
• Lithosphere
• solid earth, including earth’s crust & part of upper mantle
• Hydrosphere
• liquid envelope of water which surrounds our planet
• Atmosphere
• layer of gas (air) which surrounds our planet
• Biosphere
• living organisms which inhabit all of above spheres.

Earth’s Structure

• "Layman’s" description
• hot, dense, solid inner iron core
• hot, dense, molten iron outer core
• thick, rocky mantle
• thin, rocky crust
• formally described two ways
• chemical-based description
• mechanical-based
• chemical-based description
• Crust
• Mantle
• Core
• chemical-based description
• crust
• outermost layer or shell
• represents <0.1% of Earth's total volume
• total depth is ~100km
• floats on upper mantle
• broken into 16 plates
• chemical-based description
• crust
• nine elements compose ~99% of mass
• oxygen = 45%
• silicon = 27%
• aluminum = 8%
• iron = 5.8%
• calcium = 5.1%
• magnesium = 2.8%
• sodium = 2.3%
• divided into
• continental
• 30-60km thick
• composed of Al, Ca, K-rich silicate ("granite")
• density ~2.8 g/cm³
• oceanic
• 6-10km thick
• Fe, Mg-rich silicate ("basalt")
• density ~3.0 g/cm³
• mantle
• zone below crust & above core
• ~3000km thick
• consists of soft rock, mostly Fe, Mg-rich silicates
• density ~3.2-5.0 g/cm³
• constitutes ~ 67% of Earth’s mass
• divided into
• upper mantle
• transition zone
• lower mantle
• core
• central zone
• ~3000km thick
• composed of metallic iron
• no silicate
• density ~10 g/cm³
• divided into
• inner core
• transition zone
• outer core
• Mechanical-based description
• Lithosphere
• Asthenosphere
• Mesosphere
• Outer Core
• Inner Core

• Mechanical-based description
• lithosphere
• solid portion of Earth
• compared with non-solid atmosphere & hydrosphere
• includes crust & part of upper mantle
• ~100 km thick
• rigid
• very strong, rigid
• cool
• asthenosphere
• layer or shell below lithosphere
• plastic - but solid
• very weak
• hot
• ~200km thick
• part of upper mantle
• mesosphere
• layer or shell below asthenosphere
• plastic
• weak, but stronger than asthenosphere
• hot
• ~2600km thick
• remainder of mantle
• outer core
• molten
• iron, nickel, dissolved sulfur and oxygen
• constitutes ~30% of Earth’s mass
• ~2200km thick
• convection currents in this region generate Earth’s magnetic field
• inner core
• solid
• mostly iron, some nickel
• ~1400km thick
• constitutes ~2% of Earth’s mass
• floats in middle of molten outer core
• pressure reaches ~3 million atmospheres
• temperatures range from 4000-5000°C

• Interior of earth
• hot and dense
• weight of upper layers presses on interior
• extreme compression leads to extreme heating
• since metals are heavy and rocks are light
• heavy metals sink to center (iron and nickel)
• lighter minerals float to surface (silicates)
• temperature
• increases nonlinearly with depth
• pressure
• increases linearly with depth
• density
• increases with depth
• combination of temperature and pressure determines when materials in Earth will be molten versus solid
• affects production of convection process in asthenosphere
• isostasy
• condition of equilibrium (comparable to floating) of units of lithosphere above asthenosphere
• Crustal loading (as by ice, water, sediments, or volcanic flows)
• leads to isostatic depression or downwarping
• Crustal unloading (as by erosion, or melting of ice)
• leads to isostatic uplift or upwarping