introduction to geology plate tectonics, structural geology
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introduction to geology plate tectonics, structural geology...
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Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
Lecture 2: Journey to the Centre of the Earth
Prepared by:
Ronald Parker, Parker, Senior Geologist Fronterra Geosciences, Denver, Colorado Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Earth’s Surface
Our experience with Earth is limited to its surface. Dramatic elevation changes— changes—mountains, canyons— canyons—are tiny “scratches” on this surface. Our Earth is much more vast and complex than the surface suggests.
Chapter 2 Opener Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
A Journey through Space
A probe to Earth from an extraextra-solar planet, would:
Traverse the profound vacuum of interstellar space. Encounter the diffuse edge of the Solar System, which is: Defined by the faint pull of the Sun’s gravity. Populated by fragments of ice and rock, the Oort Cloud. 50,000 AUs from the Sun. (One AU, an astronomical unit, is the distance between Earth and the Sun, 93 million miles).
Chapter 2 Another View Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
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Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
The Solar System
The boundary of the heliosphere is 200 AU from the Sun.
The Kuiper Belt is an diffuse band of icy objects.
This is the outer reach of the solar wind.
Some are up to 1,200 km across. Comets originate here. Neptune is the inner edge.
Neptune’s orbit defines interplanetary space.
Fig. 2.1 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
The Planets
All planetary orbits lie on the same plane (the ecliptic). Eight planets constitute the Solar System. Outer iceice-giant planets (Neptune and Uranus) Gas--giant planets (Saturn and Jupiter) Gas Terrestrial planets (Mars, Earth, Venus, and Mercury)
The asteroid belt lies between Jupiter and Mars.
Fig. 2.5 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Asteroids
Asteroids: small bodies of rock or metal orbiting the Sun. Planetesimals that were never part of a larger planet. Fragmented larger planetesimals. Most occur in a belt between Jupiter and Mars. Range in size.
Up to 30 km across Millions >1km in diameter Too small to be reshaped by gravity
Asteroids can strike Earth.
Fig. Bx2.1a Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
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Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
Comets
Comets: icy planetesimals that orbit the Sun.
They traverse highly elliptical orbits around the Sun. When approaching the Sun, they develop a long coma (tail). The tail is comprised of evaporating gas and dust. The tail always points away from the Sun.
Short-period comets originate in the Kuiper Belt. ShortLonger period comets may derive from the Oort Cloud. “Dirty snowballs” made of:
H2O, CO2, CH4, NH3 Other volatiles Organic compounds Dust
Comets can strike Earth. Fig. Bx2.1b
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Magnetic Field
Space visitors would notice Earth’s magnetic field. Earth’s magnetic field is like a giant dipole bar magnet. The field has north and south ends. The field grows weaker with distance. The magnetic flux is directional.
Flows from S pole to N pole along the bar magnet Flows from N to S along field lines outside the bar
Fig. 2.3a Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Magnetic Field
Earth’s magnetic field is like a giant dipole bar magnet. The N pole of the bar is near Earth’s geographic S pole.
A compass needle aligns with the field lines. The N compass arrow points to the bar magnet S pole. Opposites attract.
Magnetic field lines: Extend into space. Weaken with distance. Form a shield around Earth (magnetosphere).
Fig. 2.3b Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
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Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
Magnetic Field
The solar wind distorts the magnetosphere.
Shaped like a teardrop Deflects most of the solar wind, protecting Earth
The strong magnetic field of the Van Allen belts arrests deadly cosmic radiation.
Fig. 2.3c Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Magnetic Field
The magnetic field is revealed by spectacular aurorae. Some charged particles make it past the Van Allen belts. These are channeled along magnetic field lines. They cause atmospheric gases in polar regions to glow.
Northern lights: aurora borealis. Southern lights: aurora australis.
Fig. 2.3d Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
The Atmosphere
Our atmosphere is mostly nitrogen (N2) and oxygen (O2). The remaining gases, (totaling less than 1%) include:
Argon (0.93%), carbon dioxide (0.039%), neon (0.0018%) Other, less common, gases (helium, methane, krypton).
The atmosphere thins away from Earth. Atmospheric layers have distinct characteristics.
Pressure, temperature, density, moisture composition
Fig. 2.4a,b Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
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Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
The Atmosphere
The atmosphere is more dense closer to Earth. Sea--level atmospheric pressure: Sea
14.7 pounds per square inch (psi) 1.01 bar
With increasing elevation: Pressure decreases. Density decreases. Oxygen content decreases.
Fig. 2.4c Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
Chapter 2: Journey to the Center of the Earth
© 2011, W. W. Norton
The Atmosphere
Atmospheric layers are separated by pauses.
Troposphere (0– (0–11 km): The lower mixing layer. Weather is confined to this layer. Temperature decreases upward.
Stratosphere (12– (12–47 km): No mixing occurs. Temperature increases upward.
Mesosphere (47– (47–82 km):
Thermosphere (82 km+):
Temperature decreases upward. Outermost layer. Temperature increases upward. Fig. 2.4d Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
Chapter 2: Journey to the Center of the Earth
© 2011, W. W. Norton
The Earth System
Orbiting around Earth, space visitors would notice: Atmosphere—the gaseous envelope. Atmosphere— Hydrosphere— Hydrosphere —the blue liquid water. Cryosphere Cryosphere— —the frozen world. Biosphere Biosphere— —the wealth of life. Lithosphere Lithosphere— —the solid Earth.
Interaction of these components comprises the Earth System. Earth is in the habitable zone. Only Earth has liquid water, and hence, life.
interactions,
Fig. 2.2 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
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Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
Earth’s Surface
Land and water are the most prominent surface features. Icecaps and living organisms are also in evidence.
Fig. 2.6 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Hypsometric Curve
Most land lies within 1 km of sea level. Most ocean floors are close to 5 km in depth. Extreme highs or lows of depth or height are rare. The two dominant “levels” reflect continental vs. oceanic crust.
Fig. 2.7 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Elemental Composition
91.2% of Earth is comprised of just four elements. Iron (Fe)— (Fe)—32.1% Oxygen (O)— (O)—30.1% Silicon (Si)— (Si)—15.1% Magnesium (Mg)— (Mg)—13.9%
The remaining eightyeighty-eight are naturally occurring elements: 8.8% These ratios differ for Earth’s crust.
Fig. 2.8 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
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Earth: Portrait of a Planet, 4th edition by Stephen Marshak © 2011 W.W. Norton & Company
Chapter 2 Journey to the Center of the Earths
Earth Materials
Elements combine in a variety of Earth materials.
Organic compounds— compounds—carboncarbon-containing compounds. Most are residue from onceonce-living creatures. These include wood, peat, lignite, coal, and oil. Geologically rare (organic in contact with oxygen).
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Earth Materials
Elements combine in a variety of Earth materials.
Minerals— Minerals —naturally naturally--occurring crystalline solids. Crystal Crystal— —a single coherent mineral with geometric faces. Grain Grain— —an irregularly shaped fragment of a larger crystal. Minerals comprise rocks and, therefore, most of the earth.
Glasses— Glasses —noncrystalline solids. Glasses form by rapid cooling— cooling—too fast for crystal growth.
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Earth Materials
Elements combine in a variety of Earth materials.
Rocks— Rocks —aggregates of minerals, grains, and/or glass. Igneous Igneous— —cooled from a liquid (melt). Sedimentary Sedimentary— —debris cemented from preexisting rock. Metamorphic Metamorphic— —rock altered by pressure and temperature.
Rocks may be made of a single mineral.
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
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Earth: Portrait of a Planet, 4th edition by Stephen Marshak © 2011 W.W. Norton & Company
Chapter 2 Journey to the Center of the Earths
Earth Materials
Elements combine in a variety of Earth materials.
Sediment— Sediment —an accumulation of loose mineral grains. Weathered and eroded from preexisting rocks. Precipitated from evaporating water.
Metals— Metals —solids composed of metal atoms (Al, Fe, Cu, Sn). Metals have high density, are shiny and conduct electricity.
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Earth Materials
Elements combine in a variety of Earth materials.
Melts— Melts —rocks that have been heated to a liquid. Magma Magma— —molten rock beneath the surface. Lava Lava— —molten rock at the surface.
Volatiles— Volatiles —materials that turn into gas at the surface. H2O, CO2, CH4 and SO2 Volatiles are released from volcanic eruption.
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Earth Materials
Most rocks on Earth are silicates (based on Si and O). There are four fundamental silicate igneous rock types. Vary by silica (SiO4) to iron (Fe) and magnesium (Mg). Felsic Felsic— —most SiSi-rich, Fe and Mg Mg--poor. Ex: Granite Intermediate Mafic Ultramafic Ultramafic— —most Si Si--poor, Fe and MgMg-rich. Ex: Peridotite
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
8
Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
A Layered Earth
How do we know that the Earth has a layered interior? Early speculations sought to explain: The source of lava. Gem and mineral enrichment. Spring waters. Earthquakes.
Early guesses were wrong.
Open caverns to the interior Flowing lava, air, water
Fig. 2.9 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
A Layered Earth
The first key to understanding Earth’s interior: density. A plumb bob is deflected by a nearby mountain mass. Degree of deflection can be used to calculate Earth’s mass. The density from this method (4.5 g/cm3) is much higher than the density of the thin outer crust (2.5 g/cm3). This suggests that density must increase with depth.
Fig. 2.10 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
A Layered Earth
The first key to understanding Earth’s interior: density.
In 1896, Emil Wiechert made important contributions. He determined that metal must be present in Earth’s interior. He deduced that the metal must occur at the Earth’s center.
We now know he was correct. His ideas led to a model: Earth is like an egg.
Thin, light crust (eggshell) Thicker, more dense mantle (eggwhite) Innermost, very dense core (yolk) Other density observations: The land doesn’t have large tides, hence Earth must be solid.
Fig. 2.11 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
9
Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
A Layered Earth
Earthquakes: seismic energy from fault motion.
Seismic waves provide insight into Earth’s interior. Seismic wave velocities change with density. We can determine the depth of seismic velocity changes. Hence, we can tell where densities change in Earth’s interior.
Fig. 2.12 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
A Layered Earth Changes with depth.
Pressure (P) The weight of overlying rock increases with depth.
Temperature (T) Heat is generated in Earth’s interior. T increases with depth.
Geothermal gradient
The rate of T changes with depth. The geothermal gradient varies.
° °
High of 50 C per km Low of 15 C per km Determined by tectonic setting Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Fig. 2.13 Chapter 2: Journey to the Center of the Earth
Layer Compositions
Geologists strived to understand the nature of the layers.
Studied meteorites as analogues for core and mantle. Conducted laboratory experiments. Density measurements of rocks from the interior Characteristics of mantlemantle-derived rocks and minerals Determined high P and T stability field of rocks and minerals
Fig. Bx.2.2c Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
10
Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
Layer Compositions End result of a century of investigation?
We know much about the nature of Earth’s interior. This knowledge continues to evolve.
Earth’s layers consist of the crust, upper, transitional, and lower mantles, and liquid outer and solid inner cores. Much complexity characterizes even these layers.
Geology at a Glance Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
The Crust
The outermost “skin” of our planet; highly variable.
Thickest under mountain ranges (70 km or 40 miles). Thinnest under midmid-ocean ridges (7 km or 4 miles).
Relatively as thick as the membrane of a toy balloon. The Mohoroviči Mohorovičić discontinuity (Moho) is the base.
Seismic velocity change between crust and upper mantle. The crust is the upper part of a tectonic plate.
Fig. 2.14a Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
The Crust
There are two kinds of crust: continental and oceanic oceanic..
Continental crust underlies the continents. Average density of ~2.7 g/cm3 Average thickness 35– 35–40 km Felsic (granitic) to intermediate in composition.
Oceanic crust underlies the ocean basins. Average density of ~3.0 g/cm3 Average thickness 7– 7–10 km Mafic (basaltic and gabbroic) in composition.
Fig. 2.14a Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
11
Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
The Crust
There are two kinds of crust: continental and oceanic oceanic.. Crustal density controls surface position.
Continental crust
Oceanic crust
Less dense: “floats higher” More dense: “floats lower”
Fig. 2.14c Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Crustal Composition
98.5% of the crust is comprised of just eight elements. Oxygen is the most abundant element in the crust.
This reflects the importance of silicate (SiO4) minerals. Oxygen is large. It occupies ~93% of crustal volume.
Fig. 2.15 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
The Mantle
Solid rock, 2,885 km thick, 82% of Earth’s volume. The mantle is entirely the ultramafic rock peridotite. Convection below 100 km mixes the mantle.
Like oatmeal on a stove: hot rises, cold sinks. Convection aids tectonic plate motion.
Upper, transitional, and lower.
Fig. Bx2.3c
Geology at a Glance Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
12
Earth: Portrait of a Planet, 4th edition
Chapter 2 Journey to the Center of the Earths
by Stephen Marshak © 2011 W.W. Norton & Company
The Core
An ironiron-rich sphere with a radius of 3,471 km. Seismic waves segregate two radically different parts.
The outer core is liquid; inner core solid. Outer core Liquid ironiron-nickel nickel--sulfur 2,255 km thick Density is 10– 10–12 g/cm3
Inner core Solid ironiron-nickel alloy Radius of 1,220 km Density is 13 g/cm3
Outer core flow generates Earth’s magnetic field. Geology at a Glance
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Summarizing Earth’s Layers
Earth has a layered interior.
Crust Continental Oceanic
Mantle Upper Transitional Lower
Core Outer Outer— —liquid Inner Inner— —solid
These layers are subdivided on the basis of seismic waves. Fig. 2.14b
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
Lithosphere--Asthenosphere Lithosphere
We can also regard layering based on rock strength.
Lithosphere— Lithosphere —the outermost 100– 100–150 km of Earth. Behaves rigidly, as a nonflowing material. Comprised of two components: crust and upper mantle. This is the material that makes up tectonic plates.
Asthenosphere— Asthenosphere —upper mantle below the lithosphere. Shallow under oceanic lithosphere; deeper under continental. Flows as a soft solid.
Fig. 2.16 Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
13
Earth: Portrait of a Planet, 4th edition by Stephen Marshak © 2011 W.W. Norton & Company
Chapter 2 Journey to the Center of the Earths
Resources
Read Marshak Chapter 2 NASA Solar System Exploration http://solarsystem.nasa.gov/index.cfm AGU Geomagnetism and Paleomagnetism http://www.agu.org/sections/geomag/background.html NOAA Weather and Atmosphere http://www.education.noaa.gov/Weather_and_Atmosphere/ USGS Earthquake Hazards Program http://earthquake.usgs.gov/earthquakes/ NESTA Structure of the Interior of the Earth http://www.windows2universe.org/earth/Interior_Structure/interior.html British Geological Survey earthquake database http://www.earthquakes.bgs.ac.uk/
Earth: Portrait of a Planet, 4th edition, by Stephen Marshak
© 2011, W. W. Norton
Chapter 2: Journey to the Center of the Earth
PowerPoint slides prepared by Ronald L. Parker, Fronterra Geosciences, 700 17th Street, Suite 900, Denver, CO, 80202
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