Home About Us Contact Us. Crustal Plates cover the surface of earth Earth's crustal plates Tectonic plates Crustal plates, also known as tectonic plates, form the outer layer of the Earth. Enjoy this page? Please pay it forward. Here's how Would you prefer to share this page with others by linking to it? Click on the HTML link code below. Copy and paste it, adding a note of your own, into your blog, a Web page, forums, a blog comment, your Facebook account, or anywhere that someone would find this page valuable.
See Our Range of Schoolbooks. See Our Fun Activity Books. The Paleogene rocks in Ireland, including at Cooley, Co. Louth and at the Giant's Causeway in Co. Antrim, formed at the opening of the North Atlantic approximately 60 million years ago. See volcanoes for more on the effects of divergent plate boundaries redirect to Topics section. Convergent or d e s t ruc t iv e pla te boundarie s This is when two tectonic plates move toward each other and collide.
The result depends on the type of plates involved. It is possible to have the collision of two oceanic plates, an oceanic plate and a continental plate or two continental plates. Subduction occurs when there is a difference in the density of the plates. Oceanic crust is typically denser than continental crust and is forced downwards into the hot mantle when it collides with continental crust.
The less dense conitinental crust is forced upward. At the start the Iapetus oceanic crust was subducted beneath continental crust, but eventually the oceanic crust was gone and continents collided. The Caledonian mountains formed at this time and it is thought they were as high as the modern Himalayas when they formed. The Himalayas are an example of the collision of two continental plates where the Indian plate is crashing into the Eurasian plate and is being forced upwards. They are continually growing at an average rate of 1cm per year, this will be 10km in 1 million years.
See earthquakes , tsunami , and volcanoes for more on the effects of convergent plate boundaries redirect to Topics section. For the 11 identified plates, a total of GPS velocity data was used. In contrast, both for this study and the next study of Tien Shan, Flesch et al.
Shen et al. The results reported here overcome these objections. They arrived at a model with eight blocks, of which only five had their boundaries complete or almost complete. The areas of these five blocks are given in Additional file 1 : Table S7.
The plates are in general larger than the plates in the Southern California Shear Zone. The blocks were originally determined using the locations of important faults running through the area. Details of the GPS observations are not extensive, but the number of sites used was These were occupied in campaign mode, probably about once a year Abdrakhmatov et al.
The concept of plates of low internal deformation is also agreed by Meade and Hager for this area. Figures show that each of the plates was observed using at least five GPS sites, and some plates were observed at far more sites than this. The velocity rates of the seven plates some incomplete in area relative to the Kazak Platform ranged between 2. Wallace et al. The plate names and velocities are given in Additional file 1 : Table S8.
The variation of sizes of these plates is strangely similar to those in Additional file 1 : Table S7. They found evidence of another plate, the New Guinea Highlands plate, but could not draw a complete boundary around the plate and so this plate is not included in the list of plates.
However, these authors did indicate that this and the other plates in the area N. Bismarck, S. Bismarck, and Woodlark plates are all acting as elastic tectonic plates with no permanent internal deformation. Other plates have been proposed. Since the boundaries of these proposed plates have not been drawn, I have left the Molucca Sea Plate, as described by Bird , in place. Kusky et al. Table 1 shows a list of the studies used to compile the catalogue of plates. Not many space geodetic observations were used to obtain plate velocities.
Most of the plates had been previously suggested but Bird occasionally modified their shapes or motions. The 23 plates in Additional file 1 : Table S2 were developed using similar techniques to those used by Bird, small changes in direction of motion, discrepancies in spreading rates, greater knowledge of recent geology volcanoes, earthquake locations , etc.
Fifteen had not appeared in global analyses, and 8 had been recognized earlier in one or more of the global analyses listed in Additional file 1 : Table S1.
Only 22 of these plate areas were used because the Tarim Basin appears in Additional file 1 : Table S6 with a slightly larger area, which has been used in the analysis. Many of the other studies used locations of faults determined by geological methods, as well as earthquake locations and mechanisms, volcanic activity, and also space-based methods such as GPS observations which have become important over the past decade, especially for the relative motions of small plates, and the locations of their plate boundaries.
Modeling will often accompany GPS data and will frequently allow for elastic strain at plate edges. The authors of all of the other analyses Additional file 1 : Tables S3—S8 state that a plate model, meaning no internal deformation and distances and angles are preserved, is the appropriate model for determining results that they obtained.
Elastic deformation is naturally occurring at plate edges, and some authors have shown that this gives a reasonable explanation of those GPS stations close to plate boundaries, the implication being that these elastic displacements will change because of earthquakes which happen at plate edges. There are situations where deformation is occurring across a broad region. For instance, motion across the Rio Grande Rift appears to be spread out over a width of hundreds of kilometers Berglund et al.
No plates are permanent but the time span for plates to remain roughly the same must vary, and it is probable that small plates last for shorter times than large plates. It also does not mean that all plates are caused by the same phenomena. Morra et al. They also found that the largest six to eight plates in the groups of large plates vary in size.
This is measured as the standard deviation of the areas in the plates under consideration, and it varies between 0. Large numbers indicate large variations in size heterogeneous tessellation whereas small values indicate more uniform size distribution homogeneous tessellation.
These results show that plates can change radically over the period of a Wilson cycle. However, what we have done in this paper is to look at a single age 0 Ma , while recognizing that information of some of the very small plates described here and elsewhere will probably not be available for older ages.
The variation in area of these plates is by a factor of over ,—from Mm 2 for the Pacific plate to km 2 for plate 5 of Hammond et al. This huge variation calls for a logarithmic axis when plotting area.
However, there is no reason why the plate number should not be plotted on a linear axis at this instant. This is what is shown in Fig. The black line is a line through all of the data. There is a distinct nick point at plate number 80 the Danakil plate, which by chance has the median area. In this diagram, the plates are ordered in size beginning at the smallest plate. Table 1 gives the number of plates in each study that are smaller in area than the nick point.
Plot of plate area logarithmic scale versus plate number from smallest to largest. Equations are given for the two straight lines fitting 35 plates on either side of the nick point.
The smallest plate 5 from Hammond et al. Analysis of the data below and above the nick point reveal that the slopes of the points below and above are significantly different. In Fig. This will be discussed in greater detail during a discussion of the data shown in Fig. Reduced area plotted against plate number smallest to largest for plates numbered from 46 to The reduction emphasizes the statistically significant difference in slope between the smaller data set plates from 46 to 80 in contrast to the larger plates plates from 81 to This suggests that the nick point seen in Fig.
The data for this figure were obtained from some of the data shown in Fig. Z values are the transformed correlation coefficients Fisher , and the Sigma Z values are the standard errors which depend only on the number of data pairs of the Z values. The multiplication factor for X is a slope which is halfway between the slopes of the lines above and below the nick point in Fig. This results in lines that have positive and negative slopes that are about equally different from zero.
The slopes are completely different. I have analyzed the distribution of plate sizes of the plates to compare it with a law proposed by Koehn et al.
They proposed that the number of plates N above a certain size as measured by a linear scale, L km, follows a power law, as follows. Figure 3 shows the plot of the number of plates larger than a certain value of L km.
Being on a log-log scale, this shows the division of plates into those large plates seven of them, in blue falling on a steeply inclined power law curve. This type of plot is the same as the one used by Bird Bird suggested that the sizes of these largest plates were governed in some way by the finite size of the Earth.
The equation governing the distribution of these seven largest plates is given by the following equation and is shown by the straight red line at the bottom right of Fig. Log-log plot of plate number largest to smallest against characteristic plate length square root of area, in km. I have made the assumption that the smaller plates starting at plate 81 are caused by a different process than the larger plates, and the lower graph assumes that these plates are renumbered starting from 1.
Straight line fits are given in colored lines with equations in the appropriate colors. Red is for the seven largest plates. Blue is for the next group of 73 plates.
Green is for the smallest group of 79 plates. These 79 plates are renumbered from 1 to The largest 32 of these plates is fit by a straight light blue line. The light orange equation and line is from the results of Koehn et al. The data are shown by the thin black lines , and for the renumbered set, the individual data are shown by black dots joined by a thin black line.
This has a much larger negative slope than do the model studies discussed by Koehn et al. The characteristic length of these seven largest plates varies from to km, i.
The large middle section omitting the 75 smallest plates and the seven largest plates has a very good straight line approximation which translates into an equation as follows. This has a much smaller negative slope the same thing as the fractal dimension than the curve proposed by Koehn et al. It is shown as a straight blue line following the black line joining the points.
If the seven largest plates had fallen on this line rather than the steeper line shown in Fig. This middle section goes from plate 8 to plate These plates have characteristic lengths between km plate 8 and km plate If the largest plates had followed the same power law as to middle section, then plate 7 would have an area equal to 4. The sizes of these largest plates must be controlled by the sizes of the convection currents in the asthenosphere.
The third straight line for the smallest plates has a fractal dimension of 0. This line is shown as the green line following the black line connecting the points. Turcotte has studied fragmentation in three-dimensional situations and comes up with a fractal dimension of about 2.
In order to investigate further the arrangement of plate sizes, I supposed that the hypothesis of Koehn et al. I therefore renumbered the 79 smallest plates, those smaller than the nick point in Fig. This is the black line with individual results as black dots in the lower left side of the figure.
Since there is so much curvature in this line, I separated it into two segments, the one with the larger plates has 32 plates, and one having the remainder of the plates has 47 plates. The sizes of the 32 larger plates are well described by a line with an equation given below. This is shown by the straight light blue line that follows the first 32 points. The plates analyzed for this section were chosen so that the correlation coefficient between the logarithm of plate size abscissa and the logarithm of plate number ordinate , starting at new plate 1, was maximized.
In other words, if the total number of plates analyzed was either greater or less than 32, the correlation coefficient was smaller than the number given in Fig. This line is close in fractal dimension to the model results of Koehn et al. The Koehn et al. The characteristic length of these 32 plates varies from The intercept will change as the number of plates within this size range increases, so it is somewhat of a coincidence that the intercept here is similar to that of Koehn, but the agreement in slope could indicate that the plates in this group are formed by similar forces that formed the experimental results of Koehn et al.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Image San Andreas Fault Tectonic plate boundaries, like the San Andreas Fault pictured here, can be the sites of mountain-building events, volcanoes, or valley or rift creation.
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