Download Core and Mantle Studies

Core and Mantle Studies:
1. Background
Dr. Michael J. Passow
Earth2Class Workshops for Teachers at
Lamont-Doherty Earth Observatory
Originally Presented 16 Dec 2000
Technology has often led to new
discoveries
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Telescope
Microscope
X-ray machines
Radar and Sonar
Satellite multispectral cameras
Hubble Space Telescope
Understanding the nature of Earth’s
interior can be considered one of the
great discoveries of the past century.
• Entirely hidden by surface rocks and oceans
• Key to understanding how many surface
features formed
• Creates Earth’s magnetic field, which
changes over time
• Better understanding will lead to new
questions not yet conceived
• Concepts that were the focus of intense research
and scientific debate when today’s senior teachers
were in school and college are routinely taught
today at the high school and even intermediate
level.
• Some examples in most Earth Science courses
--Structure and characteristics of Earth’s Layers
--Locating epicenters
--Plate Tectonics: boundaries and motions
In most college, high school, and middle
schools, we routinely teach about models
of what lies beneath us such as the one
presented in the next slide.
http://www.seismo.unr.edu/ftp/pub/l
ouie/class/100/interior.html
“(J. Louie) The earth is divided into four main layers: the inner core,
outer core, mantle, and crust. The core is composed mostly of iron
(Fe) and is so hot that the outer core is molten, with about 10% sulphur
(S). The inner core is under such extreme pressure that it remains solid.
Most of the Earth's mass is in the mantle, which is composed of iron
(Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O)
silicate compounds. At over 1000 degrees C, the mantle is solid but can
deform slowly in a plastic manner. The crust is much thinner than any
of the other layers, and is composed of the least dense calcium (Ca) and
sodium (Na) aluminum-silicate minerals. Being relatively cold, the
crust is rocky and brittle, so it can fracture in earthquakes.”
How did we gain such understandings?
Today, we have a widely
held model of an Earth
consisting of concentric
layers arranged more or
less as shown in this
diagram.
Source for this and the next few images:
http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part1.html
What’s near the surface?
The outer parts of the solid
Earth are the continental
and oceanic crusts.
Together with the rigid
mantle, they form the
lithosphere. Below lies a
less-rigid zone, the
asthenosphere. Within
this, convection occurs.
How do these layers interact?
In the early 1960s,
Hess proposed that
deep-sea trenches were
zones where denser
oceanic lithosphere
plates moved
downward through the
less-dense
asthenosphere.
Throughout the 1960s, Vine, Matthews, and many
other researchers explored this theory and
developed the modern concepts of sea-floor
spreading and plate tectonics.
Representations of the surface boundaries of the
tectonic plates are very familiar by now.
Seismology has been the key to
understanding Earth’s interior
• Careful studies of seismic waves lead to
recognition of the mantle and two-part core
• But just as early microscopes could show
only some of the complexity in a cell or a
drop of pond water, early seismology
studies revealed only the largest-scale
features of the hidden interior.
Great advances are being made now by
IRIS – Incorporated Research
Institutions for Seismology
• Consortium of 96 institutions sharing
resources and research philosophy
• Global Seismographic Network (GSN)—
130 seismological observatories worldwide
• PASSCAL—Program for the Array Seismic
Studies of the Continental Lithosphere
• Data Management System
• Education & Outreach Program
What are some of the science reseatch
areas supported by IRIS facilities?
• Lithospheric structure and dynamics—Plate
tectonics explains ocean plates, but not the
evolution of continents. GSN & PASSCAL can
reveal depths of continental lithosphere, along
with variabilities in structures that indicate
such complex processes as modification by
volcanism, stacking through collisions, erosion
during rifting, and small-scale convection.
Other areas:
• Volcanic processes and magmatism
Volcanoes are surface indications of
deep-Earth processes. PASSCAL and other
research permits better understanding of
what’s happening inside our planet.
• Heterogeneity in the lowermost mantle and
core—IRIS can provide some data to solve
questions about why Earth has layers,
variations in composition and motions
within these deep layers, and many other
important clues to what’s happening inside
the planet. These clues combine with what
geochemists and others learn through their
work to provide higher resolution
understandings of Earth’s secrets.
Global Tomography
Key to much of these new understandings is
a technique that utilizes complex computerbased technology to represent convection
within the mantle. Global tomography may
be compared somewhat to a CAT scan of
the brain—it represents a 2-D image of a 3D environment.
Example of Adam M. Dziewonski’s research
using global tomography
http://www.seismology.harvard.edu/projects/3D/S20A/S20A.html
In the image, the red “dots” indicate seismic
events. The color patterns indicate variations
in seismic velocities at different depths.
“The motivation for studying 3-D structure of the
Earth's interior is that it may offer the best
information on the dynamic processes in the deep
interior of the Earth. As the seismic wave speeds
change with temperature, it is plausible to obtain
3-D snapshots of the convection pattern in the
Earth.”—Adam Dziewonski
http://www.seismology.harvard.edu/projects/3D/
What do we now know about the
mantle?
• Much more complex and dynamic than
previously able to prove
• Great variation at different depths and
locations
• Mapping at various depths may provide
essential knowledge needed to predict where
and possibly when earthquakes and
volcanoes may occur, as well as other useful
applications
Where can you find out more?
www.iris.edu
• Dr. Paul G. Richards is the Mellon Professor
of Geological Sciences at Columbia. Because
the American Geophysical Meeting in San
Francisco overlaps this workshop, he is
unable to be with us today.
• He has given permission to use one of his
articles as an example of how scientists make
progress in understanding the core.
Core facts
• Composed principally of solid Fe
• Radius about 1220 km
• Mass 10**20 metric tons (~30% > mass of
Moon)
• Lies at center of larger fluid core (which has
~55% of Earth’s radius)
• Low viscosity, convects ~1 cm/s
Image on next page shows relative
sizes of fluid and solid cores
• As deepest interior loses heat through slow
convection through mantle, iron at base of
fluid core solidifies and solid core grow
(~1 cm/100 yr)
• Release of latent heat at base of fluid core
drives convection which maintains
magnetic field, applying a couple to the
inner core (Glatzmaier and Roberts)
http://www.ldeo.columbia.edu/~richards/Jefflec.html
What do we know about Earth’s Inner Core?
In March 1998, Paul Richards summarized
many key ideas in the Jeffreys Lecture:
“Earth’s Inner Core --Discoveries and Conjectures”
www.ldeo.columbia.edu/~richards/Jefflec.html
(Used with permission.)
Overview
• Inner core long recognized as part of process by
which fluid core convection maintained, influence
on magnetic field
• Recent studies reveal core is not homogenous, with
variations in structure and internal velocities
• Rotating in easterly direction relative to mantle at
rate fast enough to be perceived on human time
scales
• Glatzmaier & Roberts created numerical
solution for 3-D convective dynamo
motions that reproduced typically observed
magnetic field strength, dipolar pattern, and
propensity to reverse direction. Estimated
rotation to east compared with mantle with
changes on time scale of ~500 years
• Song and Richards looked for seismological
indications of such movements
• Studied differences in rate compressional
(P-waves) travel through interior.
• Especially observable in north-south, rather
than equatorial, so used South Sandwich Island
to Alaska quakes over several decades
Figure on next page shows three paths
by which seismic waves can travel from
South Sandwich Islands to Alaska. DF
passes through solid inner core. Blue
arrow shows direction of fastest axis of
anisotropy (differences). If inner core
rotates eastward, travel time for DF will
show changes (compared with travel
times for AB and BC paths not through
solid core.)
http://www.ldeo.columbia.edu/~richards/Jefflec.html
Figure on next page shows seismograms recorded at station COL (College,
Alaska) for South Sandwich Island
quakes over a 28-year period. Signals
have been aligned based on BC arrival.
They show a shift in the DF arrival that
increases over the decades. This
provided evidence of rotating core.
Figure courtesy of X. Song in http://www.ldeo.columbia.edu/~richards/Jefflec.html
Reactions to Song & Richards
conclusion
• Much popular attention to concept of
Moon-size object inside Earth making
revolution on time scale of centuries
• Attention from scientists because of
implications concerning dynamo theory,
past and present magnetic field, heat flow,
and interactions with gravity field
Can you teach these applicable Physical
Setting/Earth Science Core Concepts?
• 2.1a. Earth systems have internal and
external sources of energy, both of
which create heat.
• 2.1b. The transfer of heat energy within
Earth's interior results in the formation
of regions of different densities. These
density differences result in motion.
• 2.1j. Properties of Earth's internal
structure (crust, mantle, outer core,
inner core) can be inferred from the
analysis of the behavior of seismic
waves (including velocity and refraction.)
> Analysis of seismic waves allows the
determination of the location of
earthquake epicenters and the
measurement of earthquake intensity.
This analysis leads to the inference that
Earth's interior is composed of layers
that differ in composition and states of
matter.
• 2.1k. The outward transfer of Earth's
internal heat drives convective
circulation in the mantle that moves the
lithospheric plates comprising Earth's
surface.
• 2.1l. The lithosphere consists of
separate plates that ride on the more
fluid asthenosphere and move slowly in
relationship to one another, creating
convergent, divergent, and transform
plate boundaries. These motions
indicate Earth is a dynamic geologic
system.
>These plate boundaries are the sites of
most earthquakes, volcanoes, and
young mountain ranges.
> Compared to continental crust, ocean
crust is thinner and denser. New ocean
crust continues to form at mid-ocean
ridges.
> Earthquakes and volcanoes present
geologic hazards to humans. Loss of
property, personal injury, and loss of life
can be reduced by effective emergency
procedures.
• 2.1m. Many processes of the rock cycle are
consequences of plate dynamics. These
include: production of magma (and
subsequent igneous rock formation and
contact metamorphism) at both subduction
and rifting regions; regional metamorphism
within subduction zones; and the creation of
major depositional basins through downwarping of the crust.
• 2.1n. Many of Earth's surface features
are the consequence of forces
associated with plate motion and
interaction. These include: mid-ocean
ridges/rifts; subduction zones
trenches/island arcs; mountain ranges
(folded, faulted, and volcanic); hot
spots; and the magnetic and age
patterns in surface bedrock.
Can you explain these applicable IntermediateLevel Science Core Concepts?
• 2.2a. The interior of Earth is hot. Heat
flow and movement of material within
Earth cause sections of Earth's crust to
move. This may result in earthquakes,
volcanic eruptions, and the creation of
mountains and ocean basins.
• 2.2f. Plates may collide, move apart, or
slide past each other. Most volcanic
activity and mountain building occur at
the boundaries of these plates, often
resulting in earthquakes.
• 2.2c. Folded, tilted, faulted, and
displaced rock layers suggest past
crustal movement.
IRIS Education and Outreach has useful
Internet-based resources for teachers,
students, and others
http://www.iris.washington.edu/EandO/