User:Oceanfr/sandbox

Week 6

Juan Fernandez Plate (Edited)

Introduction

The Juan Fernandez Microplate is a List of tectonic plates in the Pacific Ocean. With a surface area of approximately 10⁵ km², the microplate is located between 32° and 35°S and 109° and 112°W. The plate is located at a triple junction between the Pacific Plate, Antarctic Plate, and Nazca Plate. Approximately 2000 km to the west of South America, it is, on average, 3000 meters deep with its shallowest point coming to approximately 1600 meters, and it’s deepest point reaching 4400 meters.

Discovery

The Juan Fernandez Microplate was first discovered in 1972 via seismicity charts, which showed semi-circular patterns at the Pacific-Nazca-Antarctica triple junction. This implied that Shear zone were present that were inconsistent with existing plate theories in the area. The microplate, as it is known today, was first Seafloor mapping and named in 1983, during a Sea Beam survey that specifically mapped the East Pacific Rise (EPR) between the Easter and Juan Fernandez microplates.

The first sonar mapping of the JF Microplate was performed in 1983. Shortly after, the RV Endeavor performed another geological survey with the goal of defining the boundaries of the plate and clearly identifying the key features of the triple junction of which the Juan Fernandez Microplate is the center. Since then, the growth of the plate has been theorized to be influenced by Mid-ocean ridge between the Pacific and JF Plates, accretion (geology) onto the plate by the Nazca and JF shear zones, and compression (geology) of both the northern most and south eastern points of the JF Microplate.

Morphology and Geological History

This microplate is estimated to have formed approximately 3-4 Ma ago. Once one single spreading center, the East Pacific Rise at this location bifurcated and resulted in two separate spreading ridges between the Nazca Plate and the Pacific Plate. The two spreading ridges, along with the Chile Rise, bounded the JF microplate. These two spreading centers then accreted material onto the JF microplate until it grew into the recognizable shape it is today. As it grew, the northern boundary of the microplate most likely experienced compression due to extreme clockwise rotational force provided by the eastern shear of the Nazca Plate and the western shear of the Pacific Plate. As the Nazca plate has a close coupling to the northern boundary of the plate, rotation of the JF Microplate was at its peak during its beginning stages of formation nearing rotational speeds of 32° per Ma. However, approximately 2.5 Ma ago, the south-eastern tip of the microplate collided against a fracture zone within the Antarctic Plate thus causing compression of the microplate against the Antarctic Plate and therefore stagnating its rotational motion down to approximately 9° per Ma at approximately 1 Ma ago. The original couplings causing shear then changed from the Pacific-JF and Nazca-JF boundaries to the Atlantic-JF boundary and Nazca-JF boundaries. The past 1 Ma has not seen any significant change in plate rotation and growth rate since.

Rotation

Since the earliest surveys of the area, theories about the movement of the JF Microplate have evolved to include compression of the plate through rotational movement and also shear zones between two major plates and the JF Microplate driving this clockwise rotational movement. The Pacific Plate is estimated to have a spreading rate anywhere between 13 and 16 cm/yr in relation to the Juan Fernandez MicroPlate. This spreading ridge supplies magma to the west of the plate, acting as a lubricant. In the development stages of the microplate, the Pacific Plate also shared a coupled shear section to the south of the microplate and, together with the Nazca Plate, drove fast rotation of the microplate. Approximately 2.5 Ma ago the JF Microplate began decoupling from the Pacific Plate and coupling with the Antartic Plate. The latter severely braked the rotational movement of the microplate. Now, the Nazca and Antarctica Plates, are the plates that share the current shear zones with transform faulting between them and the JF Plate. The Nazca Plate shears to the east while the Antarctica Plate shears relatively to the west, and this continues to drive the rotation of the microplate clockwise as well as consequential compression however at a much slower rate. Compression has been observed on the Juan Fernandez Microplate at the Pacific-Nazca-JF triple junction and the Nazca-Antarctic-JF triple junction, due to the slightly irregular shape of the plate which does not rotate perfectly about its center.

Possible Future Fates

Some researchers claim that as the triple Junction that the JF Microplate shares with the Pacific Plate and the Antarctic Plate shifts southwest, the plate will continue to have unimpeded clockwise rotational movement for the foreseeable future. Other researchers propose that due to the extent of compression between the JF Microplate and the Antarctic Plate, the microplate will accrete onto the Antarctic plate within the next million years and simply extend the triple junction between the Pacific, Antarctic, and Nazca plate to the current location of the Pacific-Nazca-Juan Fernandez triple junction.

Week 4

Juan Fernandez Plate

This article could use detail on the boundaries that the Juan Fernandez Plate shares with the Nazca Plate, Antarctica Plate, and Pacific Plate, including spreading rates and movement rates of the ridges. Boundary descriptions could also be bolstered with explanations ridge axes bathymetry. It could also use a clear description of its origin including how and when it was formed. This plate also needs to be defined as a microplate. The rotational motion of the plate was touched upon but could be elaborated upon greatly.

Sources: Kleinrock, Martin C. and Robert T. Bird. "Southeastern Boundary of the Juan Fernandez Microplate; Braking Microplate Rotation and Deforming the Antarctic Plate." Journal of Geophysical Research, vol. 99, no. B5, 10 May 1994, pp. 9237-9261. EBSCOhost, doi:10.1029/93JB02510.

Hey, RN, Martinez, F, Pardee, Debra R, Hey, Richard N, and Martinez, Fernando. "Cross-sectional Areas of Mid-ocean Ridge Axes Bounding the Easter and Juan Fernandez Microplates." Marine Geophysical Researches. 20.6 (1998): 517-31. Web.

Larson R.L.et al. Larson H., 1992. Roller-bearing tectonic evolution of the Juan Fernandez microplate, Nature , 356, 571–576.

Searle R.C. Bird R.T. Rusby R.I. Naar D.F., 1993. The development of two oceanic microplates: easter and Juan Fernandez microplates, East Pacific Rise, J. Geol. Soc. Lond. , 150, 965–976.

Looks like you're off to a good start! Erik 04:02, 24 April 2017 (UTC)

Agreed that this is a good start. Have you searched with GeoRef in UW libraries? Looks like all your references come early in a google search. You will likely get a more complete listing of research articles in Georef. Also a google search seemed to suggest that there might be books that cover this topic so it is worth following up and seeing if any are in the UW library William Wilcock (talk) 06:07, 24 April 2017 (UTC)

I actually did look at GeoRef. These were the most cited and used articles in GeoRef. I may not have been searching as efficiently as possible. Using words like microplate instead of just plate results in much different results in GeoRef, so I may need to search different keywords.Oceanfr (talk) 17:29, 24 April 2017 (UTC)

Week 3

For your week 3 assignment, your addition is a bit confused.

The accreted material is often referred to as an accretionary wedge, or prism. These accretionary wedges can be identified by ophiolites (uplifted ocean crust consisting of sediments, pillow basalts, sheeted dykes, gabbro, and peridotite).

I think you need to cite something more specific than an encyclopedia - i.e., an article in an encyclopedia. I looked up "Accretionary Complex" in the encyclopedia and it clearly explains that accretionary wedges/prisms/complexes (all the same thing) are made of sediments. Opthiolites are another form of accreted material. You need to be clear that accretionary prisms are just one type of accreted material and not another name for accreted material. William Wilcock (talk) 00:50, 17 April 2017 (UTC)

What did I learn: Plate tectonics.

Right off the bat, I was not aware of the origin of the word "tectonics".

The idea of previous theories upon a shrinking and or growing globe was completely new to me. I had no idea that was ever a theory.

I always thought of slab pull as a force of gravity rather than a "drag".

Tectonic plates are actually made up of the lithosphere rather than just the crust.

I was not aware that convection was still considered to be a possible driving force for slab subduction. Subduction has been currently taught to us as a primarily slab pull, ridge push system, with several other more complex forces related with friction and plate boundaries.

I was unaware of any theories involving the earths rotation as a mechanic behind continental drift.

I didn't know the many minutia of the plate tectonics theory development.

I learned about possible plate tectonics on other planets. It's interesting that this has been researched before.

Thoughts on the page: Plate tectonics.

This article has very good citation and has copious amounts of contributors.

From what I could tell, all the references were from notable journals.

The talk page shows real thought put into credible sources and several instances where fringe theories are discussed but reasonably argued against.

There were a few instances where I felt there was notably different voices used across the article. This can be seen with seemingly different terminology used depending on the writer.

There is also some confusing differing numbers such as the ones about plate movement given in the introduction [0-100mm/yr], and the "Key Principles" [10-160mm/yr].

What did I learn: Divergent boundary.

I had not heard of the term "triple junctions" before, though I have heard of the theory of hotspots breaking apart the lithosphere.

Thoughts on the page: Divergent boundary.

This article has poor citation. There is only one citation for the whole paper. Even simple things like theories and hypotheses have little to no backing.

At least the one citation used is a from a credible source.

There is clear use of adjectives that provide no educational benefit such as the "famous" in front of East African Great Rift Valley.

The phrase "many bearing names" in reference to fracture zones is completely useless in this context. It does not provide any actual example of a fracture zone just acts as filler.

The talk section is filled with uninterested readers and I believe even the author stated that they weren't interested in the content itself. It seems like this article could do with a little bit of editing.

You have done a really good job with this assignment. I like your suggestion on the talk page for a section on mid-ocean ridges. The Divergent boundary page is in pretty bad shape and yes it does look like it was written by somebody with no interest in the topic William Wilcock (talk) 06:10, 10 April 2017 (UTC)

Oceanography is the study of the ocean.