Download Ch. 10 PDF Slides

Geology 1100
Lecture 18 Mon. Mar. 06, 2017
• Today:
– Finish last few slides of Ch. 9 -- Formation of Earth
– Chapter 10: Motion inside the Earth
– Preview Chapter 11: Deformation of rocks
• Use slides posted for Wednesday
• Reading: Finish Chapter 11: Deformation of rocks
1
For
Formation of Earth review,
use previously posted slides
2
Chapter 9: Motion in the Earth
• Convection:
– Causes of convection
• Thermal Convection
• Chemical Convection
– Evidence of Convection
• Heat flow patterns
• Seismic Tomography
– Patterns of convection in the mantle
• Earth's Magnetic field
– The GeoDynamo in the core
3
Heat transfer (from hot to cold)
• Radiation
– Energy transfer by
electromagnetic waves (light,
infrared, etc.)
• Conduction
– Transfer of vibration from one
molecule to the next
• Convection
– Moving warm material one way
and cold material the other
From our text, Smith and
Pun
• Note: heating at the top doesn't
produce much convection – so
most Earth activity must be due to
heat generated from within the
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Earth
Heat and Convection
From our text, Smith and
Pun
• Warm, less dense fluid rises Cold, more dense fluid sinks
• Cold dense plate sinks at subduction zone – pulling plate away from spreading ridge
• But it doesn't just take heat – it takes a temperature difference for this to work
• Something must keep providing heat or that temperature difference would go away
5
Ambrym Volcano,
Vanuatu
Marum and
Bembow Lava
Lakes
From Space Station,
by Mike Hopkins
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Convection in Marum Lava Lake
•
Partly thermal: hot (less dense) magma ascends, cool (denser) magma descends
•
Partly driven by gas release: Magma with gas less dense, ascends,
Magma sinks after loss of gas makes it more dense
7
Chemical convection in the core
From our text, Smith and Pun
• Outer core contains molten iron mixed with lighter elements:
• Near inner core boundary, as iron crystallizes from melt,
dense iron crystals sink, lighter iron-poor liquid rises
8
Convection in Solids
•
Some solids can "flow" over long periods of time:
– Ice
– Warm Tar
– Butter: (Example from text)
•
Flow (and convection) in solids aided by:
– Warmer temperatures (to lower viscoscity)
– Larger forces (to push more)
– Large size
• Large volumes produce large forces,
even if density differences are small
• Each volume element can distort slowly
but still produce large overall motions
Even in freezer, with enough
force and time, it will flow
From our text, Smith and Pun
9
Convection in the Earth
• Convection in outer (liquid) core.
– Viscosity is low because of high temperature (close to that of water).
– Very slow cooling causes growth of inner (solid) core
– Chemical density changes described earlier (plus some latent heat from
crystallization of iron) cause outer core to convect
– Velocities about 10 km/yr
• Convection in solid mantle:
– A few places (asthenosphere) are relatively low viscosity because
temperature is near melting point
– Over most of mantle viscosity is high (1020 × molasses)
but large size produces large forces, and only small velocities are
necessary (10 cm / yr)
– Some of lithosphere is denser than mantle and so wants
to sink (subduct) into it
•
No convection in solid inner core,
or within lithosphere (although it moves as a whole in plate tectonics)
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Surface Evidence of Convection
•
Plate Tectonics
•
Heat Flow Measurements:
– Without convection we would expect heat flow from continents to be higher than
from oceans, since radioactive elements are concentrated in continental crust.
– Upwelling mantle at mid-ocean ridges causes decompression melting,
creates new warm oceanic crust, so heat flow from ocean crust is actually higher.
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Seismic Tomography
and Convection
•
Seismic waves travel more slowly in
warm rock, revealing temperature
patterns
•
Lower diagram is cross-section along
the dotted "great circle" shown on top
•
Mantle transition zone may act as
boundary in convection pattern
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Helping or Inhibiting Convection
•
Why does transition zone act as a boundary?
•
What helps or inhibits convection?
– As rocks rise they:
• Expand as pressure drops, which lowers density and helps convection
• Cool, which tends to raise density so inhibit convection
– As rocks sink they:
• Contract as pressure increases, which raises density and helps convection
• Heat, which tends to lower density and inhibit convection
•
In most places the first term is more important, so convection takes place
– That may also help subducting plates sinking
•
At the transition zone minerals change form, absorbing or releasing latent heat
in a way that makes the second term important, inhibiting convection
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Patterns of Convection
• Hot material tends to rise in narrow columns
• Cold material tends to sing in long sheets
• Horizontal flow on the surface connects those two types of motion
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Convection in Marum Lava Lake
•
Partly thermal: hot (less dense) magma ascends, cool (denser) magma descends
•
Partly driven by gas release: Magma with gas less dense, ascends,
Magma sinks after loss of gas makes it more dense
15
Changes in Earth's Magnetic Field
• Magnetic Poles drift slowly
• Field reverses roughly every 200,000 yr
– Current interval longer than typical -has lasted 780,000 yr
• Normal Polarity Intervals:
field like current one
• Reversed Polarity Intervals:
magnetic N and S poles reversed
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GeoDynamo
• How is the field generated:
A Geodynamo in the liquid core
• Magnetism and Electricity are related:
– 1: Moving a conductor through a magnetic field generates
an electrical current (in a dynamo, i.e. a generator or alternator)
– 2: A conductor carrying current can generate a magnetic field.
– With "right" conditions you get a cycle:
1
– Convection in core "aligned" by earth's rotation,
creates those right conditions
2
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Computer Modeling of the Geodynamo
•
Computer models of geodynamo, by Glatzmaier and Roberts suggest
chaotic (turbulent) convective motion in the outer core, as influenced by the rotation
of the earth, can produce the right currents and magnetic fields to match observations.
– If there was only the liquid core, chaotic motions
would reverse the field ~ every 100 years.
– Temporary magnetization of the hot but solid inner core
stabilizes the overall field, and only allows the whole system to flip
if the outer core pattern stays reversed an unusually long time.
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