Transcript lecture2sp07 - The University of Texas at Dallas

ISNS 3359 Earthquakes and Volcanoes
(aka shake and bake)
Lecture 2: Matter, earth and sources of
energy
Fall 2005
How We Understand the Earth
• Must think in terms of geologic time rather than human
time – thousands, millions and billions of years
• In 1788, Hutton introduced concept of geologic time:
– “No vestige of a beginning, no prospect of an end.”
– Everyday changes over millions of years add up to major
results
• Uniformitarianism: natural laws are uniform through
time and space; present is the key to the past
• Contrast to previously believed catastrophism
• Currently modified actualism: rates of Earth processes
can vary
Energy Sources
Four primary energy sources fuel Earth processes:
• Impact of extraterrestrial bodies
– Asteroids and comets; abundant in early Earth history, rare now
• Gravity
– Mass of Earth pulls objects (glaciers, hillsides) downhill
• Earth’s internal heat
– As Earth cools, heat flows from interior to surface
– Volcanic eruptions, earthquakes
– Plate tectonics; formation of continents, atmosphere and oceans
• The Sun
– Evaporation of water into atmosphere produces weather
– Hailstorms, lightning, tornadoes and hurricanes
Energy Sources
http://3dnworld.com/users/1/images/UltimateEarth.jpg
http://www.nikhef.nl/~pieth/astro/ph/clavius.htm
Origin of the Solar System
• Solar system began as rotating spherical cloud of gas, ice, dust
and debris
• Gravitational attraction brought particles together into bigger and
bigger particles
• Cloud contracted, sped up and flattened into disk
Origin of the Solar System
• Formation of Sun
– Greatest accumulation of matter (H and He) at center of disk
– Temperature at center increased to 1 million degrees centigrade
– Nuclear fusion of hydrogen (H) and helium (He) began,
producing solar radiation
Origin of the Sun and Planets
• Formation of planets
– Rings of concentrated matter formed within disk
– Particles within rings continued to collide to form planets
– Inner planets (Mercury, Venus, Earth and Mars) lost much gas
and liquid to solar radiation, becoming rocky (terrestrial)
– Outer planets retained gas and liquid, as gas planets
• Impact origin of the Moon
– Early impact of Mars-sized body with Earth
– Impact generated massive cloud of dust (from Earth’s crust and
mantle) and gas which condensed to form Moon
– Lightweight gases and liquids lost to space
– Lesser abundance of iron (from Earth’s core) in Moon
•
•
•
Earth began as aggregating mass of
particles and gases
– Aggregation took 30 to 100
million years
– Occurred about 4.57 billion
years ago
Process of aggregation created huge
amounts of heat
As temperature rose above 1,000
centigrade, iron melted
– Liquid iron is denser than
remaining rock,
• so sank toward center of
Earth to form inner and
outer core
– Release of gravitational energy
produced additional heat
– Remaining rock melted,
• allowing low-density
material to rise
– Low-density material formed
crust, oceans and atmosphere
Earth History
Earth History
•
•
•
•
3.9 billion years ago: large oceans, small continents
3.5 billion years ago: life (photosynthetic bacteria)
2.5 billion years ago: large continent
1.5 billion years ago: plate tectonics
• This last point - actually a very interesting debate
==> concept of uniformitarianism and how far back
does it apply
Earth History
Analogy with Mother Earth, 46-year-old woman:
(1 Mother Earth year = 100 million geologic years)
• First seven years unaccounted for
• 42 years old: life appeared on continents
• 45 years old: flowering plants
• 8 months ago: dinosaurs died out
• Last week: human ancestors evolved
• Yesterday: humans evolved
• Last hour: discovered agriculture, settled down
• 1 minute ago: Industrial Revolution
The Layered Earth
• Differentiated into layers of increasing density
• Center of Earth: Iron-rich core
– Inner core is solid
– Outer core is liquid and has viscous convection currents,
responsible for Earth’s magnetic field
• Surrounding core is Earth’s mantle, 2,900 km thick
– Stony in composition (like chondritic meteorites)
• Low-density elements extracted over time (mainly through volcanic
activity) from the mantle to form the crust, atmosphere and
oceans
The Layered Earth
Lithosphere =
crust + rigid part
of upper mantle
Mantle
convection
Driving force of
plate tectonics
http://wapi.isu.edu/Geo_Pgt/Mod04_Earth/EarthFig2.htm
Behavior of Materials
• Gas, solid and liquid are obvious terms, but
should be considered with respect to time
– Over longer time periods, solids may
behave as liquid
• Glacier is solid ice, yet flows downhill as
ultrahigh viscosity liquid over time span of
years
• Elastic deformation is recoverable – object
returns to original shape
• Ductile deformation is permanent – stress
applied over long time and/or at high
temperatures
• Brittle deformation is permanent – stress
applied very quickly to shatter or break object
The Layered Earth
• Asthenosphere is plastic
– “Possession of a structure weak enough to yield to an
influence, but strong enough not to yield all at once”
• About 250 km thick
• Comes to surface at mid-ocean ridges (and other
basaltic volcanoes)
• Lies more than 100 km below surface elsewhere
• Allows Earth to be oblate spheroid (flattened during
rotation; like Solar System during formation)
• Allows continents to ‘float’ atop the mantle, by
principles of isostasy
Isostasy
• Isostasy: Less dense materials float on top of more dense materials
(i.e. iceberg floating in ocean)
• Earth is a series of density-stratified layers
• Core – densities up to 16 g/cm3
• Mantle – densities from 5.7 to 3.3 g/cm3
• Continents (crust) – densities around 2.7 g/cm3
• Oceans – densities around 1.03 g/cm3
• Atmosphere – least dense
Isostasy
Examples of isostasy:
• Impoundment of water in Lake Mead behind Hoover
Dam caused area to sink 175 mm over 15 years
• Scandinavia is currently rising (about 200 m so far)
– Had been depressed under weight of ice sheets during last
glacial period (since 10,000 years ago)
– Ground ruptures and earthquakes are present
– Viking ship buried in the harbor mud of Stockholm was
lifted above sea level
– Another 200 m of uplift is likely
http://www.museum.state.il.us/exhibits/larson/glaciers.html
http://www.uwgb.edu/dutchs/EarthSC202Notes/folds.htm
Energy, Force, Work, Power and Heat
• Energy: Capacity to do work
– Potential energy: potential to do work (PE = mgh)
– Kinetic energy: energy of motion (KE = ½ mv2)-related to heat
•
•
•
•
Work: Force times distance (W = Fd = mad)
Force: Mass times acceleration (F = ma)
Power: Rate at which work occurs (P = W/T)
Heat: Capacity to raise the temperature of a mass
(energy)
m (mass), g (gravity), h (height), v (velocity), a (acceleration), d (distance)
Internal Sources of Energy
• Impact energy
– Tremendous numbers of smaller bodies hit the Earth early
after its formation, converting energy of motion to heat
• Gravitational energy
– As Earth pulled to smaller and denser mass, gravitational
energy was released as heat
Heat from both of these early sources is still flowing to
the surface today, as heat conducts very slowly
through rock
Also, loss of heat through advection - hydrothermal
circulation at the mid-ocean ridges
Gravity
• Gravity is attraction between objects:
– Two bodies attract each other with a force directly
proportional to the product of their masses and inversely
proportional to the square of the distance between them
along with a constant (Universal gravitational constant G)
• F= GMm/(dist)2
– I worked on a project to show it is not a constant
• Planetary mass can be considered concentrated at its center
• Gravity of a sphere same as a particle of infinite mass at the
center—can’t tell the difference
• Gravitational effects of Sun and Moon on Earth
– Moon’s mass is about 1/80 of Earth
– Sun’s mass is 332,000 times that of Earth
– Moon is 386,000 km from Earth
•Tidal energy is generated by
interactions of gravity between
Earth, Sun and Moon
•Tides are caused by differences
in gravitational pull from one part
of Earth to another
–Sun’s pull in mostly
uniform throughout Earth,
so small tides
–Moon’s pull is much
greater on facing side than
on opposite side, so Moon
tides are about twice
magnitude of Sun’s tides
Gravity
Gravity
• Tides affect land, water and air – most obvious in ocean
• Moon’s tidal bulges move relative to Sun’s tidal bulges
– Both tides coincide twice monthly (spring vs neap tides)
• Tidal motions within the Earth cause:
– Earth’s and Moon’s rotations to slow down
– Earth and Moon to move farther apart
– Days become longer
– Years have fewer days
• 370 million years ago, Earth day was 22 hours and year had 400 days
Internal Sources of Energy
Radioactive elements
• Unstable radioactive atoms decay and release heat
• Early Earth had much larger amount of short-lived radioactive
elements and therefore much greater heat production than now
• Radioactive decay process:
– Measured by half-life: length of time for half the present
number of atoms of a radioactive element (parent) to
disintegrate to decay (daughter) product
– Half-lives against time is negative exponential curve
Internal Structure of Atom (brief and simple)
• Atom = protons, neutrons, electrons (p + n = nucleons).
• Composition of an atom is expressed as the number of protons and
neutrons in the nucleus. The number of protons is the atomic
number (Z) and defines the location on the periodic table. N =
number of neutrons and A = Z+N = mass number.
• Nuclide = atom with a unique number of protons and neutrons.
Can thus define a nuclide by specifying Z and A e.g.:
Table of isotopes:
Internal Structure of Atom (brief and simple)
Internal Structure of Atom (brief and simple)
Internal Structure of Atom (brief and very simple)
Radioactive decay can be used to date rocks. Longer the half life the older
the rocks can be dated.
Internal Structure of Atom (brief and simple)
• Isotopes of an element - same # of protons (Z) but different # of
neutrons (N) i.e., same atomic number, diff. atomic weights
•Mass of proton = 1.6726231 x 10-27 kg
•Mass of neutron = 1.6749286 x 10-27 kg
•Mass of electron = 0.91093897 x 10-30 kg
• Stable isotopes = if not involved in radioactive decay scheme
• Radiogenic isotope = if formed by decay but not involved in a
decay scheme
• Radioactive isotope = if undergoes radioactive decay
Internal Sources of Energy
• Sum of internal energy from impacts, gravity and
radioactive elements (plus tidal friction energy) is
very large
• Internal temperatures have been declining since early
Earth maximum, but still significant enough to cause
plate tectonics, earthquakes and volcanic eruptions
Age of the Earth
• Oldest Solar System materials are 4.57 billion years
old
– Measured using radioactive elements in Moon rocks and
meteorites
• Oldest Earth rocks (found in northwest Canada) are
4.055 billion years old
• Oldest Earth materials (zircon grains from Australian
sandstone) are 4.4 billion years old
http://www.geology.wisc.edu/zircon/Earliest%20Piece/Earliest.html
Age of the Earth
http://www.geology.wisc.edu/zircon/Earliest%20Piece/Earliest.html
Age of the Earth
• Earth must be younger than 4.57 billion years old
materials that formed the planet
– Seems that Earth has existed as coherent mass since about
4.54 billion years ago
– Probably took 30 million years (0.03 billion years) for
Earth to form
– NOTE; new estimates for gas giants (Jupiter and Saturn) may be as little as a few hundred years
• Collision that formed the Moon seems to have
occurred between 4.537 and 4.533 billion years ago
• Earth must be older than 4.4 billion years old zircons
(which indicate low T surface conditions of
formation)
Radioactivity Disasters
• Chernobyl disaster of 1986, in Ukraine
– Explosion released 185 million curies of radioactive atoms
affecting everyone from Scandinavia to Greece
– 31 workers were killed and 165,000 later deaths blamed on
accident, with more to come
• Can such a thing occur in nature? Depends on relative amounts of
U-238 and U-235:
– U-235 makes up 0.7% of uranium ore
– Uranium ore used in reactors is enriched to 2-4% U-235
– Because U-235 decays more rapidly than U-238, at some point
in the past all uranium ore would have had about 2-4% U-235
– Sites in West Africa were natural nuclear reactors about 2
billion years ago, at about 400 degrees centigrade temperatures
External Sources of Energy
All sources of internal heat flow on Earth are dwarfed by 5,300
times greater heat flow to Earth from Sun
• Solar radiation:
–43% visible wavelengths
–49% near-infrared
wavelengths
–7% ultraviolet
wavelengths
–Short wavelength
• Sun’s energy:
–30% reflected back to
space at short wavelengths
(albedo)
–47% absorbed as heat
–23% evaporates water
and begins hydrologic
Hydrologic Cycle
•
•
•
•
Sun’s heat evaporates water and plants transpire water into atmosphere
Atmospheric moisture condenses and precipitates
Sun’s energy is stored in water and water vapor
Equatorial regions receive excess insolation (solar radiation), whereas in polar
regions radiation back to space exceeds insolation
• Imbalance between equatorial and polar regions cause ocean currents and winds,
transferring heat
Hydrologic Cycle
• 97.1% of Earth’s water is in oceans
• Of remaining 2.9%:
–
–
–
–
68% in glaciers
21% underground
10% in atmosphere
1% in rivers, lakes, inland seas and soil moisture
• Water in atmosphere holds, transports and releases solar
energy, distributing heat around the Earth
Water – The Most Peculiar Substance on Earth?
•
•
•
•
•
•
•
•
Present in liquid, solid and gas states
Highest heat capacity except liquid ammonia
Highest heat conduction
Highest latent heat of vaporization
Highest latent heat of fusion except ammonia
Bipolar molecule easily able to bond with ions
Highest dielectric constant of liquids
Expands 9% when frozen (most substances shrink)
Processes of Construction vs. Destruction
Rock Cycle:
• Internal heat melts rock to
magma, which rises and cools to
form igneous rocks
– Process of Construction
• External heat from Sun drives
hydrologic cycle that weathers
and erodes rocks into sediments,
which are transported and
deposited
– Process of Destruction
Processes of Construction vs. Destruction
Rock Cycle:
• Continents could be eroded to sea level in 45 million
years (1% Earth history), but processes of construction
keep raising continents and forming new landmasses
• The power of these Processes of Construction and
Destruction causes disasters!
Impacts with Asteroids and Comets
Earth travels 950 million km around the Sun each year with an orbital speed of
108,000 km/hr  kinetic energy = 2.7 x 1033 joules
• When this kinetic energy is brought into collision with another body, effects are
catastrophic and worldwide:
– Asteroid: 65,000 km/hr
– Comet: 150,000 km/hr
• Additional sources of energy are:
– Daily rotational motions of Earth on axis
– Monthly rotational motions of Earth-Moon system about center of gravity
Volcanoes and the Origin of the
Ocean, Atmospheres and Life
• Volcanic gases (H, O, S, C, Cl etc) combine to make:
– Water (H2O), carbon dioxide (CO2), sulfur dioxide (SO2),
hydrogen sulfide (H2S), carbon monoxide (CO), nitrogen
(N2), hydrogen (H2), hydrochloric acid (HCl), methane (CH4),
and others
• Dominant volcanic gas is water vapor – more than 90%
• Are volcanoes responsible for the oceans?
Volcanoes and the Origin of the
h
Ocean, Atmospheres and Life
• Volcanic rocks:
– Oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium
(Ca), magnesium (Mg), sodium (Na), potassium (K)
• 4.5 billion years of volcanism has brought light weight
elements to the surface to make up
–
–
–
–
Continents
Oceans
Atmosphere
CHON (carbon, hydrogen, oxygen, nitrogen) elements of life