technology has often led to new discoveries telescope microscope x-ray machines radar and sonar...
TRANSCRIPT
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
• 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/louie/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 reseatchareas 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 computer-based 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 3-D 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
• 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 seismo-grams 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 down-warping 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 Intermediate-Level 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/