How to Measure Motion in Plate Tectonics

How to Measure Motion in Plate Tectonics The lithospheric plates are the sections of Earths crust and upper mantle that move- very slowly- over the lower mantle below. We know that these plates move from two different lines of evidence- geodetic and geologic- which allow us to trace their movements back in geologic time. Geodetic Plate Motion Geodesy, the science of measuring the Earths shape and positions on it, allows us to measure plate motion directly using GPS, the Global Positioning System. This network of satellites is more stable than the Earths surface, so when a whole continent moves somewhere at a few centimeters per year, GPS can tell. The longer we record this information, the more accurate it becomes, and in much of the world, the numbers are already quite precise. Another thing GPS can show us is tectonic movements within plates. One assumption behind plate tectonics is that the lithosphere is rigid, and indeed that is still a sound and useful assumption. But parts of the plates are soft in comparison, like the Tibetan Plateau and the western American mountain belts. GPS data helps us separate blocks that move independently, even if only by a few millimeters per year. In the United States, the Sierra Nevada and Baja California micro-plates have been distinguished this way. Geologic Plate Motion: Present Three different geologic methods help determine the trajectories of plates: paleomagnetic, geometric, and seismic. The paleomagnetic method is based on the Earths magnetic field. In every volcanic eruption, iron-bearing minerals (mostly magnetite) become magnetized by the prevailing field as they cool. The direction in which theyre magnetized points to the nearest magnetic pole. Because oceanic lithosphere forms continuously by volcanism at spreading ridges, the whole oceanic plate bears a consistent magnetic signature. When the Earths magnetic field reverses direction, as it does for reasons not fully understood, the new rock takes on the reversed signature. Thus most of the seafloor has a striped pattern of magnetizations as if it were a piece of paper emerging from a fax machine (only its symmetrical across the spreading center). The differences in magnetization are slight, but sensitive magnetometers on ships and aircraft can detect them. The most recent magnetic-field reversal was 781,000 years ago, so mapping that reversal gives us a good idea of plate movements in the most recent geologic past. The geometric method gives us the spreading direction to go with the spreading speed. Its based on the transform faults along the mid-ocean ridges. If you look at a spreading ridge on a map, it has a stair-step pattern of segments at right angles. If the spreading segments are the treads, the transforms are the risers that connect them. Carefully measured, these transforms reveal directions of spread. With plate speeds and directions, we have velocities that can be plugged into equations. These velocities match the GPS measurements nicely. Seismic methods use the focal mechanisms of earthquakes to detect the orientation of faults. Although less accurate than paleomagnetic mapping and geometry, these methods are useful for measuring plate movements in parts of the globe that arent well mapped and have fewer GPS stations. Geologic Plate Motion: Past We can extend measurements into the geologic past in several ways. The simplest one is to extend paleomagnetic maps of the oceanic plates out from the spreading centers. Magnetic maps of the seafloor translate precisely into age maps. These maps also reveal how the plates changed velocity as collisions jostled them into rearrangements. Unfortunately, the seafloor is relatively young, no more than about 200 million years old, because it eventually disappears beneath other plates by subduction. As we look deeper into the past, we must rely more and more on paleomagnetism in continental rocks. As plate movements have rotated the continents, the ancient rocks have turned with them, and where their minerals once indicated north, they now point somewhere else, toward apparent poles. When you plot these apparent poles on a map, they appear to wander away from true north as rock ages go back in time. In fact, north does not change (usually), and the wandering paleo-poles tell a story of wandering continents. Together, the methods listed above allow us to produce an integrated timeline of the movement of the lithospheric plates, a tectonic travelogue that leads smoothly up to the present.

What Athletes Need to Know About SAT Prep

What Athletes Need to Know About SAT Prep SAT / ACT Prep Online Guides and Tips Are you an athlete in high school? Then you know that you face challenges above and beyond everyone else: you need to attend practice and go to games; your schedule is already filled to the brink; and you still have to look out for athletic recruiters and scholarships! Here are some big SAT prep tips for athletes - some that apply to everyone but especially to athletes, and others that are athlete exclusive. 1. Are you a recruited athlete? Know your SAT score goal - it's different from everyone else's If you're aiming to be recruited by colleges, especially those that care about sports, then you don't have the same exact SAT target as everyone else. Why? Because colleges understand that being an athlete is difficult, and you don't have as much time to brush up. The rule of thumb is that achieving 80% of a school's median SAT score is sufficient pass their academic bar. For example, Harvard has a median SAT score that's around 1540. If you get a 1510, you're actually in a pretty good spot. This means that if you're already above this mark, you can study a bit less for the SAT - no need to go wild spending thousands of dollars on tutoring or hundreds of hours. What's a good way to study? Here at PrepScholar we measure your real score within the first two weeks. You know exactly where you stand, and then you can keep on prepping until you meet your goal. 2. Fit SAT Study around your athletic schedule Your main talent is in athletics, and you don't want your game to suffer because you're missing practices. A lot of SAT classes meet during prime practice time - right after school or for long periods during weekends. Given that, some great ways to study are ones that can be flexible to your schedule. If you study well from books, then this is a great way to study. Online test prep programs like PrepScholar work as well - you can sign in whenever and get the full training you need. Be careful - some online programs require you to schedule classes or have rigid sessions with live humans - that could put a damper in practice. 3. Get Started on SAT Prep As Soon as You Can You don't have the luxury of waiting until a week before the test to cram 40 hours into studying - you have to get started right now! If you're going too fast (less likely than you think), then just slow down before exam time. Your schedule is so busy that you need to spread your studying out to get all the time in. 4. Understand What College Coaches Care About College coaches want to make sure you have what it takes to pass muster in classes. They're afraid of their star quarterback being put on academic probation and not being able to make the big game. If your GPA is a little weak, you can use your SAT score to tell them that you've improved since freshman year, and you have what it takes to stay on the team. Thus, a good SAT score can make up for your GPA. Like the tips above? Get more in the Free SAT Guide: