Earth

Four stages of Planetary Development:

1. Differentiation (heat from in-falling and radio decay), 2. Cratering, 3.Flooding (lava and water), 4. Slow surface evolution

Only planet with liquid water, 70% covered, could cover surface to 3 km depth

Approximately 1/2 shrouded by water clouds at any time

Six large continents, high mountains, and deep sea-floor trenches

Geological activity; magnetic field implies differentiated interior, liquid iron core

Avg. uncompressed density = 4.3 g/cc, consistent with presence of a metallic core

Mature planet - cyclic processes: water, land, CO₂ ,

Biological evolution causes non-cyclic changes

Temperature

Interior structure from analyzing seismic waves using seismograph

Seismic Waves:

P - compression wave - goes through solid or liquid

S - transverse wave, solid only, not liquid

Rayleigh & Love (R and L) - along surface only

R: rock motion elliptical in vertical plane of propagation direction - like water waves

L: no vertical motion, lateral motion only

Speeds different in different material :

P: 5.5 km/sec in granite 1.5 km/sec in water

S: 3.0 km/sec in granite 0 km/sec in water

Distance (km) to focus (origin) ~ 10 (S - P), S and P in seconds

Waves transform at boundaries within planet: reflect and refract as different types

Wave Types

(a) longitudinal or compressional

(b) transverse

Focus: origin of quake

Epicenter: point on surface above focus

Sources of Earthquakes

Elastic Rebound Theory - Strain builds up at boundaries between slowly moving plates at Earth's surface where slip cannot occur uniformly.

Energy stored as in spring

Sudden slip releases stored energy as seismic waves

Richter Magnitude M_L

Log₁₀ A, where A = amplitude in increments of 10^-6 meters measured with Wood-Anderson seismograph at distance of 100 km from epicenter. Surface waves only.

A is max displacement of needle, not actual total energy, although related.

1 Richter magnitude increase ~ 30x energy increase

Log₁₀ = exponent of 10

e.g., log 0.1 = log 10^-1 = -1

log 1= log 10⁰ = 0

log 10 = log 10¹ = 1

log 100 = log 10² = 2

Example: A = 1 cm (or 10^-2 m)

Divide A by 10^-6 : 10^-2 / 10^-6 = 10⁴

log 10⁴ = 4 Þ M_L = 4

A = 2 cm Þ M_L = log (2 x 10⁴) = log 2 + 4 = 0.3 + 4 = 4.3

Great Alaska earthquake Mar 27, 1964 in Prince William Sound: M_L = 8.6

Brick dropped from table to ground: M_L = -2.0

Richter not used widely in Research, designed mainly for local earthquakes, instead:

M_s Surface waves at any distance

m_b P wave - deep or shallow, any distance

M_w Seismic moment magnitude - measures all wave types

Most physically meaningful measurements

Strong Motion Accelerometer

Near source of strong earthquake shaking can overdrive, send offscale, or even damage seismograph

Strong Motion Accelerometer incorporates inertial spring devises - low sensitivity, coupled with digital storage. Data read into laptop at location or transmitted via phone lines or Internet.

Pacoima Dam Accelerogram: Feb 9, 1971- Several strong motion accelerometers measured 0.25 g

Peak > 1 g!

Largest US Earthquake in 40 Years: 4:58 AM June 28, 1992

Landers in Mojave Desert

M_s (surface wave magnitude) = 7.5

Epicenter between Landers and Yucca Valley

Relatively light damage due to location in desert area - 1 dead, 25 serious injuries

Richter Scale

Richter Magnitude: How many kilograms of TNT would have this much energy?

0  0.6 

1.0  20  

2.0   600  Smallest quake people can normally feel

3.0  20 000   Most people near epicenter feel the quake

 Nearly 100, 000 occur every year of size 2.5 - 3.0

4.0  60 000  A small fission atomic bomb

 Quakes above 4.5 can cause local damage

5.0  20 000 000   A standard fission bomb, similar to the first bomb tested

6.0  60 000 000   A hydrogen bomb; can cause great damage locally

 About 100 shallow quakes of size 6.0 every year

7.0  20 billion  Major earthquake; about 14 every year

 Enough energy to heat New York City for 1 year

  Large enough to be detected all over globe

8.0  60 billion  Largest known: 8.9 in Japan and in Chile/Ecuador

  San Francisco destroyed by 8.25 in 1906

9.0  20 trillion  Roughly the world's energy usage in a year
 

Mercali Scale

The Mercali Scale measures observable results or effects the damage caused, the sensations described by people, etc. (Mercali numbers do not correspond directly to Richter numbers).

Mercali Magnitude Observable Results and Effects

I  Most people do not notice, animals may be uneasy, detected by seismograph

II Hanging objects sway back and forth

III Many people feel the movement, parked cars may rock

IV Doors, windows, and shelves may rattle, people indoors can feel movement

V Light furniture moves, pictures fall off walls, objects fall from shelves

VI Nearly everyone feels movement, light furniture falls over, windows may crack

VII Some people fall over, walls may crack

VIII Heavy furniture falls over, some walls crumble

IX Many people panic, some buildings collapse, dams crack

X Railroad lines are bent, most buildings are damaged, roads crack

XI Bridges collapse, buried pipes break, most buildings collapse

XII All manmade structures are destroyed

Continental Drift

Alfred Wegener 1912 in Germany: Supercontinent broke up 300 million years ago into numerous plates, which then began to move around

Evidence: shapes of continents like pieces of puzzle

Wegener believed centrifugal force from Earth's rotation drove movement of plates through rigid rocky upper mantle

Problem: Sir Harold Jeffreys said no natural force known which could overcome greater opposite (resistance) force of rigid rocky mantle to drive the plates

By late 1960's seismic studies had shown: a rigid upper layer, the lithosphere, the top of which is the crust of varying thickness, but it continues as a rigid layer to depths of >150 km. Boundary below crust is Mohorovicic discontinuity.

Lithosphere merges with asthenosphere, upper mantle - softer, almost molten

Plate Tectonics (forces that stress a planet and the response of the crust to such forces)

Seven large plates and many smaller ones - stable and relatively rigid slabs of rock, lithosphere, cover globe and in motion relative to each other

Move over softer rock below

Greatest geological activity at plate boundaries - including high seismicity

Convection in asthenosphere pulls on bottom of lithosphere causing plate motion

Subduction Zone

(Rift)

Assumptions:

· New oceanic lithosphere generated by seafloor spreading from mid-ocean ridges

· New oceanic lithosphere covers moving plate; may or may not include continental material

· Earth's surface area conserved (constant): new material = material consumed at subduction zones

· Plates relatively rigid over large horizontal distances ” relative motion almost entirely at plate boundaries

Subduction zones

Oceanic crust destroyed (returned to mantle) at subduction zones

Responsible for most deep oceanic trenches

High mountain ranges can form, e.g., Andes (Nazca - So. American plates)

Compression (head-on collisions of plates)

Thrusting builds high mountain ranges, e.g., Himalayas

Himalayas: Mountain building results largely from plate collisions. (a) The subcontinent of India, imaged in infrared from orbit, lies at the northernmost tip of the Indian plate. As this plate drifts northward, the Indian landmass collides with Asia, on the Eurasian plate. The impact causes Earth’s crust to buckle and fold, thrusting up the Himalayan mountain range (snow covered at upper right). (b) Mount Everest (the dark peak in background).

Lateral Faults

Many plates shear past one another. Most famous active region in North America—the San Andreas Fault - boundary between Pacific and North American plates. The motion is neither steady nor smooth. The sudden jerks that occur when they do move against each cause major earthquakes.

Avg. period in So. Calif. area of 140 years. Last slip in 1857 Þ 1997!

Hawaiian Islands: Volcanoes and Hot Spots

The Hawaiian Islands, near the center of the Pacific plate are associated with a "hot spot" in Earth's upper mantle that melts the crust above it. Over millions of years the motion of the Pacific plate across the underlying hot spot (volcanic plume in mantle) has resulted in a chain of volcanic islands.

Atmosphere

Half within 5 km of the surface, and all but 1 percent is found below 30 km. Atmosphere below about 12 km is called the troposphere. Extending up to 40 to 50 km, lies the stratosphere. Between 50 and 80 km from the surface lies the mesosphere. Above about 80 km, in the ionosphere, the atmosphere is kept partly ionized by solar ultraviolet radiation.

Mainly nitrogen (78%) and oxygen (21%), all others exist as traces - highly variable in concentration

N2 and 02 can come from inorganic processes, e.g., vulcanism

On earth, due mainly to life

02 from photosynthesis: plants convert C02 and H20 to glucose and 02

N2 from nitrogen cycle - based on organic decay

Original atmosphere: Primary atmosphere would have consisted of most gasses in the early solar system. Hydrogen, helium, methane, ammonia, and water vapor. Almost all this light material, and especially any hydrogen or helium, escaped into space during the first half-billion or so years after Earth was formed

Subsequently, Earth developed a secondary atmosphere, which was outgassed from interior as a result of volcanic activity. Volcanic gases are rich in water vapor, methane, carbon dioxide, sulfur dioxide, and compounds containing nitrogen (such as nitrogen gas, ammonia, and nitric oxide). Solar ultraviolet radiation decomposed the lighter, hydrogen-rich gases, allowing the hydrogen to escape, as well as much of the nitrogen from its bonds with other elements. As surface temperature fell, the water vapor condensed and oceans formed. Much of the carbon dioxide and sulfur dioxide became dissolved in the oceans or combined with surface rocks. Oxygen is such a reactive gas that any free oxygen that appeared at early times was removed as quickly as it formed. An atmosphere consisting largely of nitrogen slowly appeared.

Global warming:

But, Robert Essenhigh, E.G. Bailey Professor of Energy Conservation in Ohio State's Department of Mechanical Engineering says that:

Some 90 billion tons of carbon as carbon dioxide annually circulate between Earth's ocean and the atmosphere, and another 60 billion tons exchange between the vegetation and the atmosphere.

Compared to man-made sources' emission of about 5 to 6 billion tons per year, the natural sources would then account for more than 95 percent of all atmospheric carbon dioxide.

As temperatures rise, the carbon dioxide equilibrium in the water changes, and this releases more carbon dioxide into the atmosphere. According to this scenario, atmospheric carbon dioxide is then an indicator of rising temperatures -- not the driving force behind it.

Essenhigh attributes the current reported rise in global temperatures to a natural cycle of warming and cooling.

Global temperatures have been oscillating steadily, with an average rising gradually, over the last one million years.

Average global temperatures have risen less than one degree in the last million years, though the amplitude of the periodic oscillation has now risen in that time from about 5 degrees to about 10 degrees, with a period of about 100,000 years.

"Today, we are simply near a peak in the current cycle that started about 25,000 years ago," Essenhigh explained.

As to why highs and lows follow a 100,000 year cycle, the Arctic Ocean acts as a giant temperature regulator - the "Arctic Ocean Model."

According to this model, when the Arctic Ocean is frozen over, as it is today, it prevents evaporation of water that would otherwise escape to the atmosphere and then return as snow. When there is less snow to replenish the Arctic ice cap, the cap may start to shrink. That could be the cause behind the retreat of the Arctic ice cap that scientists are documenting today.

As the ice cap melts, the earth warms, until the Arctic Ocean opens again. Once enough water is available by evaporation from the ocean into the atmosphere, snows can begin to replenish the ice cap. At that point, the Arctic ice begins to expand, the global temperature can then start to reverse, and the earth can start re-entry to a new ice age.

According to Essenhigh's estimations, Earth may reach a peak in the current temperature profile within the next 10 to 20 years, and then it could begin to cool into a new ice age.

Essenhigh's scientific opinion is a minority one.

Magnetosphere

The region around a planet that is influenced by that planet’s magnetic field

Van Allen belts: two doughnut-shaped zones of high-energy charged particles, located at about 3,000 km and 20,000 km above the surface. Named after the American physicist whose instruments on board one of the first satellites to initially detect them.

The particles that make up the Van Allen belts originate in the solar wind. Charged particles, mainly electrons and protons, from the solar wind can become trapped by Earth’s magnetic field. The outer belt contains mostly electrons; the much heavier protons accumulate in the inner belt.

Earth’s real magnetosphere is greatly distorted by the solar wind, with a long tail extending the sun's direction.

Impacts and Extinctions:Atmosphere shields us from bodies of less than 100,000 tons with energy release <10 megatons

Tunguska, Siberia, 1908:

60 m object exploded in atmosphere - 10 - 20 megatons

Flattened forest over 1,000 km2 - thousands of caribou died

Death of the Dinosaurs - Cretaceous/ Tertiary (K/T) boundary event 65 million years ago

Meteoroid, probably asteroid 10 - 15 km diameter blasted Chicxulub crater off Yukatan 200 km diameter ” Iridium layer - thin clay layer deposited over Earth at K/T boundary time

Iridium is a "platinum-group" metallic element that is very uncommon in the earth's crust. The amount of iridium in 1 gram of surface rock would be less than 1 to 2 billionths of a gram. The platinum-group metals include platinum, iridium, palladium, rhodium, rhenium, and osmium. The greatest concentration of iridium resides in Earth's core.

Much more common in asteroids and comets ” mixed with mud spread around planet from impact

Hazard of Impacts:

K/T - level extinction event every 100 million years