Transcript ppt

An Izu-Bonin-Mariana Arc Thematic Mini-Lesson Package
Initiated by Jeff Ryan (University of South Florida), Rosemary Hickey-Vargas (Florida International University) & Leslie Peart (JOI/Smithsonian) at
the Margins Education Mini-Workshop: Bringing MARGINS Science to the Classroom, April 5-6, 2007, Arlington, VA
Rationale:
The idea is to develop a set of mini-lessons using the IBM subduction factory to
illustrate geologic concepts that would normally be taught in an upper level
undergraduate course, like Petrology, Sedimentology, Geophysics or
Geochemistry. The rationale for using the IBM arc system is twofold. First, this
focus area was chosen because many important subduction parameters vary
along its length and width, so it is ideally suited for illustrating how subduction
processes operate. A second rationale is that by making the mini-lessons, IBM
researchers can determine what aspects of IBM science, observations and
findings get into our college classrooms.
Question for IBM Education Group:
What aspects of IBM science,
observations and findings are most
important to get into our college
classrooms?
What is a mini-lesson?
(http://serc.carleton.edu/Margins/index.html)
MARGINS Mini-Lessons are modular learning materials that repurpose the
data resources, visualizations, and other information sources developed
through MARGINS and MARGINS-related research for use in examining
fundamental earth processes in undergraduate classrooms from a
multidisciplinary perspective. Several different types of learning materials are
being developed:
.
•Web-deliverable Laboratory/Classroom Exercises: exercises and activities that can be downloaded in their entirety from the website. These are available at
several scales and are suitable for use in either introductory or upper-level geoscience courses. Larger Mini-Lessons are scaled for use as a full laboratory session
or across several lecture meetings, while the smaller Lessons may be annotated sets of visualizations, short interactive activities, or other materials for use within a
lecture or laboratory session.
•Virtual Expeditions: These aim to bring the results of the MARGINS research across a number of courses (oceanography, marine geology, tectonics, geophysics,
structural geology, petrology). Each module consists of 20-25 linked web pages that trace the development of specific research projects through a combination of
short video segments, process-based animations, interactive graphics, and real-time data from land and ocean observatories. This approach to learning is driven
by a series of linked questions that progressively increase in sophistication, much like the evolution of a research project. These questions lead students through
the process of scientific discovery while learning about MARGINS research.
V) Backarc Basins
1) How is the development of a back arc spreading center related to stress in
the overlying plate? Compare bathymetric maps of the Izu-Bonin and
Mariana segments of the IBM arc, your findings in I to answer this question.
2) What is the difference between flux melting and decompression melting?
What aspects of magma geochemistry suggest that IBM arc magmatism is
dominated by flux melting and back arc magmatism is dominated by
decompression melting. .
3) How is the subducted crust metamorphosed? Look at published P-T
diagrams for the stability of the minerals in sediments from II-1 and for
seafloor basalt. Make a possible sequence of minerals that would appear in
the subducted plate with depth. Which minerals would release water and
which could carry water to the greatest depths.
4) Using published cross-sections of the Izu-Bonin and Mariana segments,
estimate the difference in depth to the top of the Pacific plate beneath the
back arc regions. How would this difference affect the sequence of minerals
determined in V-3?
These are some sample ideas
for IBM mini-lessons - “learning
activity snippets”. What is
missing? Feel free to add more.
Which are most important?
I Subduction parameters
1) How does the convergence direction and rate vary along the
IBM arc? Use Pacific and Philippine Sea plate velocities and
vectors to estimate the change in the rate and direction of
convergence along the arc.
2) How does the dip of the Pacific Plate vary along the IBM arc?
Use the location of earthquake foci and seismic tomographic
images to estimate the dip at different locations along the arc.
II) The downgoing Pacific Plate
1) What is the sedimentology/lithology of the downgoing plate?
Use the sediment column for the Reference Site to understand
oceanic sedimentology, the Geologic Time scale and the
interpretation of sediments in terms of depositional environments.
How are the sediments different along the arc? This is the oldest
seafloor on Earth, where did this seafloor originate?
2) What is the bulk composition of the downgoing plate? Use the
Reference column to calculate a bulk major elemental composition,
using average compositions for basalt, clay, etc. Compare this
weighted average with a typical arc basalt or andesite (see IV-2).
How do they compare?
3) What is the water content of the downgoing plate? Use ODP
data for pore water content/sample and density to estimate the
mass of water in a cubic kilometer section of downgoing plate.
4) How long would it take to recycle an “ocean’s” worth of water?
Use part II-3) and the rate of convergence (I-1) to estimate the
amount of water delivered per kilometer arc length per year.
III) Forearc
1) What are mud volcanoes and how do they
compare with “magma” volcanoes? Look at maps,
sonar images, cross-sections of serpentinite
seamounts and compare with the cross section of
well known stratocones of 2000 meter height. Why
are the shapes different?
2) What is serpentine and why does it form in the
forearc? Outline the serpentinization process: water
+ olivine, temperature constrained by polymorphs,
alkaline water. Where is there paleo-evidence for
this process?
3) What kinds of vent communities live on mud
volcanoes? Research examples of cold versus hot,
alkaline versus acid, low alkalinity versus high
alkalinity vents.
4) How much water can be held in the forearc? Use
III-2 and the breadth of the forearc to estimate how
much water could be contained in the forearc. How
does this compare with the total calculated in II-3?
IV) Arc volcanoes and arc crust
1) What influences the location of volcanoes? Look at a bathymetric map and measure the distance between
adjacent active volcanoes, and distance to the trench. Is there a regular spacing? Based on your answer to I-2,
are the volcanoes located the same distance above the subducting plate? Why or why not?
2) The Anatahan volcano has erupted several times since the IBM Focus area was established. What is the
average composition of lava erupted from this volcano? How does this compare with “average continental
crust”. Assume that the IBM crust is like average “continental crust” and think of explanations for differences
between the crustal composition and erupted lava.
3) Look at seismic profiles of arc crust in the Izu-Bonin and Mariana arc segments. How does the observed
layering of the crust support or refute your arguments from IV-2. Are the profiles like continental crust?
4) How much water is recycled by IBM arc volcanoes? Find a recent estimate for the water content for
undegassed IBM arc magma and published estimates of magma production rates for the arc. Estimate how
much water can be recycled by magmatic degassing (per km arc length per unit time) and compare this with
your estimate of subducted water in II-3 and II-4.
5) How are elements recycled from the slab to arc magmas? Do the calculation of II-2 for the elements Ba, Th
and K. Compare these with the composition of IBM arc volcanic rocks from the Izu, Bonin and Mariana
segments. Consider both the actual concentrations of the elements, and their ratios (i.e., Ba/Th, K/Th, K/Ba).
Think of some reasons that the elemental abundances may or may not match.