Plate Tectonics CPR Paper
by
The Professor
Introduction
The theory of plate tectonics models the earth's surface as a relatively small number of rigid "plates" that move over a semi-plastic base. The rigid portion is called the "lithosphere" and the underlying material is called the "asthenosphere." The boundaries of the plates consist of "divergent" boundaries, where plates move apart, "convergent" boundaries where plates collide, and "transform" boundaries, where plates move sideways relative to each other. The theory of plate tectonics is able to describe many of the most striking features on the earth's surface. Long mountain ranges, deep ocean trenches and high ocean ridges are caused by plate tectonic motions. In addition, earthquake patterns are explained by motion across plate boundaries. The majority of volcanoes also occur as a result of plate motion.In this paper, I will describe models of two contrasting plate boundary types and show how the elevation, quake and other data support my models [:(Cmnt by William Prothero: This is a concise introduction that satisfies the assignment ):] .
Methods
The data that I will use for this study comes from the "Our Dynamic Planet" module of the EarthEd Online software provided for GS-4W, Introduction for Oceanography, by Prof. William Prothero. The kinds of data I will use are:1) Elevation data from the ETOPO5 elevation database. This dataset was compiled from data from the US naval Oceanographic Office, US Defense Mapping Agency, Bureau of Mineral Resources (Australia), Department of Industrical and Scientific Research (New Zealand), and US Navy Fleet Numerical Oceanographic Center. It averages the depth data into 5 minute rectangles (about 5 nautical miles), but some areas, like Asia , South America, and Canada are averages of 1 degree rectangles, about 60 nautical miles on a side [:(Cmnt by William Prothero: The elevation database does not have clearly defined errors, because they vary depending on location. However, the resolution is worth mentioning. ):] .
2) Earthquake data from the Worldwide Network of Seismic Stations, collected at the National Earthquake Information Center, Boulder, Colorado. The accuracy of these quakes varies between a few km to as much as 50 km in oceanic areas far from seismic stations. Depth errors can also cause artifacts where locations seem to lie at a constant depth. These artifacts are introduced by the earthquake location programs, which put all quakes with a large depth error at a specific depth.
3) Volcano data is from the Smithsonian Instituion Global Volcanism Program. The information as been extracted from the geologic literature on volcanoes. Many volcanoes that we know reside on the seafloor are not listed, because they have not been observed, or published in the literatur [:(Cmnt by William Prothero: All of the data sources are mentioned in the lab workbook ):] e.
Observations
{link: ptCPR_LocMapF.jpg}Figure 1 show a world map with the two regions of study marked as A and B.Region A is named the Tonga-Kermadec Trench on world maps. {link: ptcpr_LocMapF2.jpg}Figure 2 shows a detailed map of region A. The locations of earthquake hypocenters and 3 elevation profiles across a deep trench are shown. The elevation profiles clearly show the trench in f{link: ptCPRTonga3Pr.jpg}igure 3. You can see that the earthquake epicenters follow along the Tonga-Kermadec trench and the 3 profiles across it show the trench with a depth of 7,000 to 8000 meters.
{link: ptcpr_Tonvol.jpg}Figure 4 shows a plot of volcanoes in the region. The volcanoes lie on a line parallel to the trench. So, the trench feature, the earthquake epicenters, and the volcanoes all lie along an axis parallel to the trench.
{link: ptcprqks_prof3.jpg}Figure 5 show an earthquake profile along at the location of profile 3 shown in figure 3. Notice that the earthquakes lie along a descending line to a maximum depth of 700km.
Region B is in the Pacific Ocean in a region centered on 50S latitude,115W longitude.. {link: ptcpr_elqk2.jpg}Figure 6 is a detailed map of this region. Two profiles were made across what appears to be a linear rise. The earthquakes are on features that are perpendicular to this linear rise. {link: pt_epr2Profiles.jpg}Figure 7 shows the 2 elevation and age profiles. Notice that the age varies from zero to 30-34ma at the ends of the profile(s), which are about 2,600km and 2,000km long, respectively. For profile 4, estimating the distance along the profile from the 0 age to the 34ma age at about 1200km, the spreading rate would be 1200km/34ma = 35millimeters/year. This is the half-spreading rate, and the rate at which the two plates separate is 70mm/year.
The pattern of age, elevation, and quakes shows two distinct patterns of interest. {link: pt_epr_elqk3.jpg}Figure 8 labels the map regions as A and B. The A region has no quakes, but zero, or close to zero age. The B region has earthquakes. {link: ptEpr_elprof5.jpg}Figure 9 shows an elevation-age profile across the B region. Notice the age jumps about 10ma as it crosses the quake zone [:(Cmnt by William Prothero: This section demonstrates how to concisely use figures to represent the data plots. It is important to also describe what the figure shows. ):] .
Interpretations
Region A{link: ptCPRTonga3Pr.jpg}Figure 3 shows that the 3 elevation profiles that identify the trench feature. {link: ptcprqks_prof3.jpg}Figure 5 shows the descending pattern of quake sthat identifies the feature as a subduction zone and indicates fault motion along a descending plane. {link: ptcpr_Tonvol.jpg}Figure 4 shows a plot of the volcanoes that lie behind the trench on the side beneath the downgoing slab. These features identify Region A as a convergent plate margin containing a subduction zone.
Region B
The data show that this region is a divergent plate boundary, or "spreading center." The strongest data is shown in {link: pt_epr2Profiles.jpg}figure 7, where the age profiles show zero (or very young) ages at the peak of the ridgecrest and nearly linearly increasing ages as distance from the ridgecrest increases. The lack of quakes at the spreading boundary is common, but not shown here. Where the ridge has offsets, perpendicular boundaries called "transform faults" connect the ridge segments. The transform faults show earthquake activity at depths less than 50 km ({link: pt_epr_elqk3.jpg}figure 8). In figure 8, the spreading zones and transform zones have been labelled A and B.
Therefore, region A is a convergent boundary, or subduction zone, and region B is a divergent boundary, or spreading center. [:(Cmnt by William Prothero: Each region has a short interpretation backed up with the data and a sketch model. Notice that the correspondence between the data and the model is discussed explicitly. ):]
References
Prothero, W, 2004. Introduction to Oceanography Lab Manual, pub by UCSB Bookstore, p 3-21. [:(Cmnt by William Prothero: The only necessary reference here is to the lab book. In later assignments, the references section will need to be more extensive. ):]
Figure 1: Location map of the two regions for this study. Region A is the Tonga-Kermadecc trench region and Region B is the East Pacific Rise region. {fig: ptCPR_LocMapF.jpg}
Reagion A detailed map. The arrows point to the approximate endpoints of the Tonga-Kermadec trench. {fig: ptcpr_LocMapF2.jpg}
Three elevation profiles across the Tonga Trench shown in the profilelocation map of figure 2. The Tonga Trench is marked by the slanting line through the 3 profile plots. {fig: ptCPRTonga3Pr.jpg}
Volcano plot of the Tonga-Kermadec trench region. Notice that the volcanoes lie on a line approximately parallel to the trench. {fig: ptcpr_Tonvol.jpg}
Cross section of earthquake locations along profile 3. {fig: ptcprqks_prof3.jpg}
Map of the East Pacific Rise, centered at 50S, 115W. Three elevation profiles and the earthquake epicenters are shown. {fig: ptcpr_elqk2.jpg}
Two elevation and age profiles across the crest of the East Pacific Rise. {fig: pt_epr2Profiles.jpg}
Two zones are marked A and B. The A zone has very young age and the B zone connects the A zones, and has earthquakes that are coincident with it. {fig: pt_epr_elqk3.jpg}
Elevation-age profile 5, as indicated in figure 6. {fig: ptEpr_elprof5.jpg}
Model of Region A, the Tonga-Kermadec trench region. This shows the subducting slab with a descending pattern of quakes, a topographic trench, and the volcanoes that are a result of melting of ocean crust on the slab. {fig: cartoon1.jpg}