for mass extinctions have been suggested. speculated that extinctions might have regular periodicity, and other scientists have . Around 30 million years is the average time between mass extinctions, which set scientists speculating whether galactic dynamics could be responsible. from supernovas have been proposed as one possible agent, as have , but the periodicity hypothesis has fallen out of favor. The periodic nature of mass extinctions could be because it takes millions of years for complex ecosystems to recover from the previous extinction events and build themselves into unstable states again, when new events cause the ecosystems to collapse.
There is also evidence that life itself can contribute to mass extinctions. When the eventually , organisms that could not survive or thrive around oxygen (called ) . When anoxic conditions appeared, particularly when existed, the anaerobes could abound once again, and when thrived, usually arising from ocean sediments, they . Since the ocean floor had already become anoxic, the seafloor was already a dead zone, so little harm was done there. The hydrogen sulfide became lethal when it rose in the and killed off surface life and then wafted into the air and near shore. But the greatest harm to life may have been inflicted when hydrogen sulfide eventually , which could have been the final blow to an already stressed ecosphere. That may seem a fanciful scenario, but there is evidence for it. There is fossil evidence of during the Permian extinction, as well as photosynthesizing anaerobic bacteria ( and ), which could have only thrived in sulfide-rich anoxic surface waters. Peter Ward made this key evidence for his , and he has implicated hydrogen sulfide events in most major mass extinctions. An important aspect of Ward’s Medea hypothesis work is that about 1,000 PPM of carbon dioxide in the atmosphere, which might be reached in this century if we keep burning fossil fuels, may artificially induce Canfield Oceans and result in . Those are not wild-eyed doomsday speculations, but logical outcomes of current trends and , proposed by leading scientists. Hundreds of already exist on Earth, which are primarily manmade. Even if those events are “only” 10% likely to happen in the next century, that we are flirting with them at all should make us shudder, for a few reasons, one of which is the awesome damage that it would inflict on the biosphere, including humanity, and another is that it is entirely preventable with the use of technologies .
Canfield’s original hypothesis, which seems largely valid today, is that the deep oceans were not oxygenated until the Ediacaran Period, which followed the Cryogenian; the process did not begin until about 580 mya and first completed about 560 mya. The wildest swing in Earth’s entire geological record begins about 575 mya and ends about 550 mya, and is called the Shuram excursion. Explaining the Shuram excursion is one of the most controversial areas of geology today, with numerous proposed hypotheses. When the controversies are finally resolved, if they resolved, the Shuram and excursions, even though they go in opposite directions, I suspect will likely be both related to the dynamics of ice ages and the rise of oxygen levels. Ediacaran fauna, the first large, complex organisms to ever appear on Earth, also first appeared about 575 mya, when the Shuram excursion began. I strongly doubt that Earth’s first appearance of large complex life at the exact geological timescale moment of the wildest carbon-isotope swing in Earth’s history will prove to be a coincidence. The numerous competing hypotheses regarding the Shuram excursion include:
In the early 19th century, a dispute was personified by , a British lawyer and geologist, and , a French paleontologist. Their respective positions came to be known as and . Just as , so did uniformitarianism prevail in scientific circles. Under the comforting uniformitarian worldview, there was no such thing as a global catastrophe. Changes had only been gradual, and only the present geophysical, geochemical, and biological process had ever existed. The British Charles Darwin explicitly made Lyell’s uniformitarianism part of his evolutionary theory and he proposed that extinction was only a gradual process. Cuvier was , which contradicted the still-dominant Biblical teachings, even in the . Although Cuvier did not subscribe to the , his catastrophic extinction hypothesis was informed by his fossil studies. But Lyell and Darwin prevailed. Suggesting that there might have been catastrophic mass extinctions in Earth’s past was an invitation to be branded a pseudoscientific crackpot. That state of affairs largely prevailed in orthodoxy until the 1980s, after the was posited for the dinosaurs’ demise. An effort led by a scientist publishing outside of his field of expertise (a ) removed from its primacy. Only since the 1980s have English-speaking scientists studied mass extinctions without facing ridicule from their peers, which has never been an auspicious career situation. Since then, many and mass extinction events have been studied, but the investigations are still in their early stages, partly due to a dogma that prevailed for more than a century and a half, and Lyell’s uniformitarianism is influential. The ranking of major mass extinctions is even in dispute, , and a was recently .
Kirschvink noted that reappeared in the geological record during the possible Snowball Earth times, after vanishing about a billion years earlier. Kirschvink noted that iron cannot increase to levels where they would create BIFs if the global ocean was oxygenated. Kirschvink proposed that the sea ice not only killed the photosynthesizers, but it also separated the ocean from the atmosphere so that the global ocean became anoxic. Iron from volcanoes on the ocean floor would build up in solution during the , and during the greenhouse phase the oceans would become oxygenated and the iron would fall out in BIFs. Other geological evidence for the vacillating icehouse and greenhouse conditions was the formation of cap carbonates over the glacial till. It was a global phenomenon; wherever the Snowball Earth till was, cap carbonates were atop them. In geological circles, deposited during the past 100 million years are considered to be of tropical origin, so scientists think that the cap carbonates reflected a tropical environment. The fact of cap carbonates atop glacial till is one of the strongest pieces of evidence for the Snowball Earth hypothesis. Kirschvink finished his paper by noting that the eon of complex life came on the heels of the Snowball Earth, and scouring the oceans of life would have presented virgin oceans for the rapid spread of life in the greenhouse periods, and this could have initiated the evolutionary novelty that led to complex life.
Another key set of tensions are those between theorists, empiricists, and inventors. Theorists attempt to account for scientific data and ideally predict data yet to be adduced, which tests the validity of their hypotheses and theories. often produce that scientific data. Inventors create new technologies and techniques. Albert Einstein is the quintessential example of a theorist, who never performed experiments relating to his theories but accounted for experimental results and predicted them. , who performed the experiment that produced results that various scientists wrestled with for a generation before Einstein proposed his , never suspected that their experiment would lead to the theories that it did. The most important experiments in science’s history were often those producing unexpected results and were usually called failures. Einstein’s had no experimental evidence when he proposed it (it , but that was the only evidence for it when the theory was proposed), but it has been confirmed numerous times since then. Einstein expected that his theories would eventually be falsified by experimental evidence, but that the best parts of his theories would survive in the new theories.
When you fire an arrow from a bow you will see that different angles of elevation give different ranges. In a vacuum it can be seen that the maximum range occurs for an elevation of 45º. However, in air, the range and elevation are not related so simply (see the two graphs below). This suggests several good EEIs. You could merely find out which elevation gives the best range when "draw" is kept constant, but you could also propose an hypothesis along the lines of "45° gives the greatest range" and that angles above or below this give a shorter range. Further, complementary angles are said to give the same range - but this could be tested (see diagram below). Lastly - is "range" the only dependent variable you want to look at. Perhaps an archer is more interested in "time of flight" as this may give better accuracy (less time for air resistance to apply). I have attached two pages from my text that may be helpful in designing this experiment. to download them. For safety information about archery, see the comments in the EEI suggestion above.
Here's a neat EEI from Sandgate State High School courtesy of physics teacher Ewan Toombes. It goes thus: Stage1 - Design and build a Robot Submarine using plastic bottle ranging from a 1.25L softdrink bottle up to a 4L juice container which can be trimmed to neutral buoyancy so that it "floats" just above the bottom of the pool at a depth of 1 metre. Stage 2 , The Escape - Release or inject a known volume of gas into the ballast tank(s) by remote control (something that operates above but works under water that allows you to inject a known volume of gas into your submarine) that will allow it to escape to the surface carrying a "treasure" of known mass that was resting on the bottom and attached to the submarine by a slack piece of string. Stage 3 - Measure the acceleration of the submarine as it rises. Stage 4 - Calculate the acceleration it should have had due to the excess gas and use your research to explain any difference between the two. That's the start. Now think of some variables to manipulate, propose an hypothesis, justify it, design an experiment and go and investigate. Photo taken at Sandgate SHS.
The of an ice age is only a few hundred years old, and was , who got his first ideas from and others. There had also been . By the 1860s, most geologists accepted the idea that there had been a cold period in Earth’s recent past, attended by advancing and retreating ice sheets, but nobody really knew why. Hypotheses began to proliferate, and in the 1870s, proposed the idea that variations in Earth’s orientation to the Sun caused the continental ice sheets. Because of problems in matching his hypothesis with dates adduced for ice age events, it fell out of favor and was considered dead by 1900. Croll’s work regained its relevance with the publication of a paper by (usually spelled Milankovitch in the West) in 1913, and by 1924, Milankovitch was widely known for explaining the timing of advancing and retreating ice sheets during the current ice age.
Scientific practice is ideally a process of theory and experimentation that can lead to new theories. There are three general aspects of today's scientific process, and it , which used to formulate his theory of evolution. First, facts are adduced. Facts are phenomena that everybody can agree on, ideally produced under controlled experimental conditions that can be reproduced by other experimenters. Hypotheses are then proposed to account for the facts by using inductive (also called ) logic. The hypotheses are usually concerned with how the universe works, whether it is star formation or evolution. If a hypothesis survives the fact-gathering process – often by predicting facts that later experiments verify – then the hypothesis may graduate to the status of a theory. Scientific theories ideally can be , which means that they can be proven erroneous. The principle of hypothesis and falsification is primarily what distinguishes science from other modes of inquiry.