|Case Study - Theoretical limitations of inquiry and design
Science is often associated with words like ‘truth’, ‘reality’, ‘objectivity’. However, science is much more complex than these words allude to. It is not uncommon for a scientist to develop and validate a new hypothesis or new knowledge and have it met with fierce opposition from the rest of the scientific community. Even when considerable evidence substantiates the claims of scientists there are often still limitations and uncertainty that exist. This is especially the case when science is undergoing a revolution or a significant change in beliefs than those previously held. In light of this, two scientific myths continue to persist and these are “Observation provides direct and reliable access the secure knowledge” and “scientific inquiry is a simple, algorithmic procedure (Hodson, 1999).” If these statements were in fact true, opposition, resistance, and controversy would not be associated with science. Science is dynamic, continually changing and constantly being revised and updated. This is the nature of science – researchers do the best they can with the resources, techniques, and prior knowledge they have. That being said it is important to consider what a theory is and how it is developed.
What is a theory and how does it develop?
There are a variety of perspectives on what a theory is and how it develops however the following will attempt to generalize it for simplicity.
***Have students define what a hypothesis, theory, and law are before they continue
Theory: A theory is more like a scientific law than a hypothesis. A theory is an explanation of a set of related observations or events based upon proven hypotheses and verified multiple times by detached groups of researchers. Typically, one scientist doesn’t create a theory but creates a hypothesis. If this hypothesis is verified by others and explains observations at a given point in time then it will likely become a law or a theory. In general, both a scientific theory and a scientific law are accepted to be true by the scientific community as a whole. Both are used to make predictions of events. Both are used to advance technology. The biggest difference between a law and a theory is that a theory is much more complex and dynamic. A law governs a single action, whereas a theory explains a whole series of related phenomena
The following is a case study that demonstrates important ideas about science including what scientists mean by 'theory' and how it develops, particularly how the scientific community scrutinises claims of new knowledge. The story of how Wegener's hypothesis of continental drift developed into the theory of tectonic plates will be detailed. In addition, the limitations of theories will be highlighted. For example, our inability to predict major natural disasters such as earthquakes and volcanic eruptions shows the limits of the theory of plate tectonics. Finally, this case will demonstrate how scientists make ‘imaginative’ leaps and also draw on various sources of evidence.
Before Continental Drift
For centuries, Western Christian religious scholars, scientists, and the general public believed Genesis and that ‘God’ configured the world’s continents and separated them by oceans - this was considered fact and given little additional thought. These beliefs held fast for centuries even as evidence began emerging that something else might be at work. As early as 4 BC, Aristotle had puzzled over the existence of fossil marine creatures in rocks high above the sea, and in the 15th century, Leonardo da Vinci recorded the observation that fish once swam over the plains of Italy. How did the remains of these marine organisms end up on land? In the midst of this speculation about fossils, Sir Francis Bacon, in 1620, studied the first crude maps of the world drawn by sailors. He noticed strange similarities between the coasts of Africa and South America. He commented (in his famous Novum Organum) that these were 'no mere accidental occurrence'.
***Have students look at a map and have them describe what Sir Francis Bacon may have been talking about. If students have difficulty, the teacher should guide students to think of the continents like a jigsaw puzzle.
Furthermore, in 1634, Descartes reached the conclusion that the Earth was created by ongoing natural processes rather than the product of a divine ‘Creator’. However, he did not publish these conclusions, for fear of the all-powerful Church (Galileo was under house-arrest at the time for stating the Earth revolves around the Sun). This mounting evidence simply did not fit into the stories of the Bible. Therefore, in 1650, Archbishop James Ussher studied the bible and explained away much of the scientific evidence that was accumulating by attributing it to the Deluge or the floods that required Noah to build his ark. This satisfied most of the public. However, despite the church’s best efforts, the 18th century seemed to spark and foster uncertainty, skepticism and open discussion. Scientists were turning from God as the source of what was seen on earth to natural processes and phenomenon. In 1782, Benjamin Franklin observed fossils or “oyster shells mixed in stones” in England and wondered how they got there. He commented "The crust of the Earth must be a shell floating on a fluid interior.... Thus the surface of the globe would be capable of being broken and distorted by the violent movements of the fluids on which it rested". Little did Benjamin know he was not far off. Throughout the 1800s there were two features of the earth that continued to puzzle scientists – the discovery of similar fossils on continents that are now separated by oceans and the origin of mountain ranges - as evidence accumulated, so did the theories.
For instance, in the mid-nineteenth century, James Dwight Dana and Edward Suess proclaimed that the Earth was cooling and contracting and all geological features were the result of this contraction. As the earth cooled from its molten state, more dense materials contracted and sank towards the centre (forming ocean basins) and the least dense materials ‘floated’ and formed the crust. This theory gained wide-spread acceptance. Dana proposed that continents had previously been joined by land bridges and other continents, accounting for the similarity of fossils found in different areas, and that the contraction of the Earth had caused these bridges and continents to sink and be swallowed by the oceans (Ever heard of Atlantis?). This has been compared to the way an apple wrinkles and folds as it dries out and shrinks. Generally, all believed the earth’s crust moved up and down and not sideways.
***Teacher should create a demo out of modeling clay or some other pliable material to provide students with a visual demonstration of what Dana was referring to.
However, this sinking continent idea was challenged by the concept of geologic uplift or isostasy which was prominent in the late 19th century. The theory of isostasy, developed by Clarence Dutton, explained how different rock densities lead to an ‘equilibrium’ in the Earth’s crust. Lighter rock like granite rises, and heavier rock like basalt sinks. As mountains lose ‘weight’ through the erosion by wind and water, they slowly rise up from below the surface. The effect is now used to explain and account for the constant height of the Himalayas. The erosion rate that should lower the mountains is countered by "isostatic rebound" - they become lighter and ‘float’ higher. At this time, scientists began speculating about whether the Earth was indeed contracting. If the theory of isostasy is correct, then how could light, granite continents and land bridges sink into the oceans as hypothesized by the contracting Earth theory?
In 1908, Frank Taylor continued the controversy by proposing that continents had not sunk but had instead shifted horizontally. He believed that there was one continent that had broken into pieces and as these fragments collided they created mountain ranges. He also cited the Mid-Atlantic Ridge as a key to the geological movement – he believed that the ridge had been unmoved and instead the continents had ‘crept away in opposite directions’. However, his ideas were largely ignored and a short time later, Alfred Wegener proposed the theory of continental drift.
In 1911, Wegener, a meteorologist and astronomer, happened upon an article about the fossil evidence that supported the idea of sunken land bridges. He became very interested in the topic even though it was outside his discipline and through his research he noticed some fundamental flaws in the Contracting Earth Theory (still widely accepted at the time). Three principle flaws he described included:
The pattern of mountain ranges that occur on the Earth – ranges occurred in narrow curvilinear belts typically along the edge of continents. If the earth was contracting, these ranges should be randomly scattered
The age of the mountains – with the contracting earth theory, all mountains should be roughly the same age. It was already known that age varied enormously.
Gravity and the theory of isostasy – sinking land bridges of less dense rock ought to ‘pop up’
In 1912, Wegener, delivered an address entitled "The Formation of the Major Features of the Earth's Crust (Continents and Oceans)" which formed the basis of the theory of continental drift, to the Geological Association in Frankfurt. According to Wegener about 220 million years ago, a huge supercontinent he called Pangea existed and broke into pieces and moved apart (timing of the breakup was variable). He hypothesized that compression at the leading edge of the moving continent led to the formation of mountains. He also proposed that material beneath the earth’s crust acted like a slow-moving fluid. Wegener referred to the fit of the continental shelves (not the continental shorelines) to support his argument. Furthermore, he added climatic evidence and geographic indicators to his theory. He plotted the worldwide distribution of rock and fossils to determine and align the locations of tropical forests, deserts, coal fields, diamond fields, and icecaps.
***With students, chart out locations of major mountain ranges, diamond and coal fields, and key types of rock
The majority of scientists rejected Wegener’s ‘bizarre’ hypothesis and his thoughts won him few friends. Wegener’s hypothesis totally altered the way scientists viewed the Earth and evolution and this theory coming from a non-geologist (remember, Wegner was an astronomer) made matters worse. Although Wegener continued to revise his theory with new evidence, he continued to face unrelenting resistance and rejection from the scientific community. Part of this rejection was because he couldn’t explain the process of movement or why continents would move in the first place (how do continents plow through the ocean floor?) and perhaps primarily because he was considered an ‘outsider’ who had no right crossing disciplines. Wegener’s theory laid rejected for another thirty years after his death in 1930. The world still accepted the Contracting Earth Theory and the permanence of the continents even though many discrepancies existed.
Dusting off Continental Drift
The earth sciences underwent a revolution in the 1960s and culminated in the theory of plate tectonics and finally ended a 55 year old controversy over continental drift. The following will highlight some of the events that lead to this turn of events?
Sea Floor Spreading
In 1947, The National Geographic Society commissioned Maurice Ewing, a physicist, to explore the mid-Atlantic Ridge and the sea floor around it, of which very little was known at the time. Ewing found some unexpected surprises. Upon analyzing his first core samples from almost a mile down, he found that they contained a layer of recent sediment lying directly on top of another layer what was more than 20 million years old. However, there was no trace of the material from the period in between. Ewing and others had believed that 3 billion years of sediment buildup would make the shelf about 20 km thick. Ewing found that the shelf was comparatively thin – only a few 1000 feet at the thickest sections. Furthermore, Ewing began finding ‘glassy rocks’ (volcanic rock) that had withstood great heat and pressure. In addition, he found that the sea floor consisted of dense basalt, while the continents typically consisted of lighter granite. If continents and land bridges were sinking wouldn’t the sea floor consist of the same material as the continents themselves? Piecing these finds together, it became clear that the very thin sediment layer and glassy rocks indicated that the ocean floor was very young, and of volcanic origin – this was very surprising to most scientists at the time! One final surprise would surface before Ewing was through. He decided to use echo-sounding to create a topographical map of the mid-Atlantic Ridge. With Ewing’s data, cartographer Marie Tharp found herself drawing a deep canyon down the middle of the mid-Atlantic Ridge. The existence of this canyon was doubted until known earthquakes were mapped against it and each fell in that valley along the ridge. Ewing concluded that that valley was in fact a crack in the Earth's crust, out of which hot material from the mantle was rising to the surface. In 1956, Ewing disclosed some of his findings but they were rejected because they implied a widening of the ocean (by way of volcanic material being added through a crack in the crust) and that didn’t make sense when the Earth was known to be contracting – or was it? With Ewing’s discoveries there continued to be more puzzles than answers. Why did the ocean floor only contain rocks no older than about 150 million years old? Where was the ‘missing’ sediment from the last 20 million years? Why was the ocean crust so thin? Why was there such high heat flow along the ridge? More evidence was needed and this came from the discipline of marine geology.
Based on the findings of Ewing and others, and a strong belief in continental drift, Harry H. Hess (in 1959), produced a ‘radical’ hypothesis, that the ocean floors were moving 'like conveyor belts, carrying the continents along with them'. Wary of the backlash from the scientific community, Hess was very conservative and even referred to his ideas as borderline fantasy and was very noncommittal about his hypothesis that, “The sea floor is not permanent, but is constantly being renewed. The Mid-Ocean Ridge is indeed a crack in the crust. Through it, hot material from the underlying mantle continually wells up and spreads outwards...the continents are carried passively on the mantle with convection.” The effect that Hess describes is now termed 'sea-floor spreading'. Hess estimated that new crust is generated at the rate of about half an inch (just over 1cm) a year, on each side of the ridge. At this pace, all the ocean floors of the world would have been formed during the last 200 million years - less that 5% of the Earth's geological history. Hess went on to point out that as the Earth is not expanding, old crust must simultaneously be destroyed - and he correctly suggested that this happens in the deep ocean trenches, which lie near to the edges of continents. This process, whereby the old oceanic floor is pulled into the deep trenches, was later termed 'subduction'. It turns out that Hess was wrong about the continents moving by convection but it didn’t really matter as his ideas went the way of Wegeners and were rejected by the scientific community. Hard proof was required but the stage was set.
It has been known for centuries that the Earth is magnetic, with poles at either end however what causes this magnetic field is not entirely understood. What is known is that like a needle on a compass, magnetic components of molten rock are magnitised and align in parallel with the Earth's magnetic field. As the rocks harden they become a snapshot or record of the direction of the magnetic field at a point in time.
***Teacher should demonstrate how rocks with magnetic components will spin toward and in the direction of a magnet
From this, scientists know that at certain intervals in the past there had been reversals in the polarity of the global magnetic field. So, by studying the positions of these rocks it was suggested by Patrick Blackett, that North American and England had once been 30° closer together - or, there had once been no Atlantic Ocean separating them. Critics quickly pointed out that paleomagnetism wasn’t exact, and thus questioned the reliability and completeness of the data. Again more evidence was sought.
Meanwhile, Fredrick Vine and Drummond Matthews were firmly convinced of continental drift and in 1962 conducted a mid-Ocean ridge survey and began studying some spatial variations in the earth’s magnetic field on the ocean floor (by studying the molten rocks). They reasoned that if hot mantle material was welling up in the Mid-Ocean Ridge, it would be magnetised in the direction of the Earth's magnetic field as it cooled. If the seafloor was spreading, then this band of magnetised rock would be carried slowly away from the ridge. They published their results in an article for Nature entitled "Magnetic anomalies over Oceanic Ridges" but they had little response. They had integrated two wholly unproven and unrelated hypothesis – the creation of the sea floor at mid-ocean ridges and recurrent reversals of the Earth’s magnetic field. Although, the resistance to these ideas was widespread, mounting evidence would soon help launch a revolution in earth sciences. From the interests of Hess, Vine, Matthews, Morley, and a Canadian scientist Tuzo Wilson (identified transform faults), they were able to prove Wegener’s theory of continental drift but more appropriately described the mechanism that would carry the continents around the world.
Theory of Plate Tectonics
Plate tectonics links the concepts of continental drift, seafloor spreading, and subduction of oceanic crust. We now know that the Earth is covered in a number of rigid 'plates' that move across its surface, over and on a partially-molten internal layer. Using geological terms, the plates form the lithosphere, which is the Earth's solid rock. The lithosphere comprises all of the crust, and the brittle part of the uppermost mantle. The rigid lithospheric plates can be considered to 'float' on the underlying, ductile asthenosphere, which 'flows'. There are 9 major such plates, and many smaller ones. These are known as "continental plates", with the smaller ones being termed "micro continents". It is now known that at mid-ocean ridges (seafloor spreading centers) convection in the Earth’s mantle brings molten rock to the surface at these ridges and new oceanic crust if formed from Earth's interior and pushes the plates apart and the seafloor spreads. At so-called convergent boundaries, the plates collide and crash into each other, sometimes forming mountains and volcanoes and often producing earthquakes. Where plates collide, one plate is subducted below the other, returning rock to the mantle. Plates may slide past each other, generating earthquakes as along the San Andreas fault; or continents may collide, resulting in mountains such as the Rockies or Himalayas. The Earth’s mantle is seen to be quite active and the mountains, ocean ridges, trenches, faults, and volcanoes of the crust are consequences of the restless plates.
***Referring to the questions raised during the section on seafloor spreading, students should try to answer them based on what they now know about plate tectonics.
Why did the ocean floor only contain rocks no older than about 150 million years old?
Where was the ‘missing’ sediment from the last 20 million years?
Why was the ocean crust so thin?
Why was there such high heat flow along the ridge?
Did you know?
Scientists believe the Himalaya mountain range in Asia was formed as a result of the collision of the Indian-Australian and Eurasian plates.
Coal that is mined in Pennsylvania was actually formed from tropical plant life near the Equator. How did it travel northward to Pennsylvania? Scientists believe that 200 million years ago, when the dinosaurs dined upon tropical ferns and tall tropical vegetation, what is now Pennsylvania was at a different location, namely the equatorial region.
An estimated 500,000 detectable earthquakes occur in the world each year. Of those, 100,000 can be felt by humans, and 100 cause damage
The only object in the solar system that is known to support life (Earth), is the only object with tectonic activity?! (Mars has no tectonic plates, no earthquakes and no life) – Coincidence?!
OK, so now what?
Even with a great deal of evidence for plate tectonics, controversy still exists today. For example, while the theory of plate tectonics has been widely accepted, understanding the forces or mechanisms that drive the movement of the plates remains unknown. Some scientists have suggested that the way that mantle convection drives plate motion is through ‘slab-suction’, while others believe it is the result of ‘slab-pull’ forces, others believe it is a combination of the two, and still others believe it is none of these mechanisms. The forces behind plate movement are currently a very large area of research. Controversy is also rampant when continental interiors are considered. It has been suggested that the interior of continents behaves quite differently than the rigid edges of tectonic plates and the ocean floor. For example, in central Asia, India is colliding with Tibet but instead of Tibet remaining rigid it is acting more like putty and deforming under the pressure from India. “It’s as if India were colliding with a water bed (Bilham in Kerr, 2004).” It is suggested that buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of more diffuse tectonic activity. Why and how continental deformation happens is still unknown. An even larger area of controversy is related to the ability of scientists to accurately predict earthquakes or volcanic eruptions. Scientists have monitored everything from the incidence of small tremors to animal behaviour in order to develop a means for accurately predicting when an earthquake will occur. Currently, scientists rely on identification of active faults, historical data of past movement, and past earthquakes on those faults in order to produce a probability of an earthquake occurring. So, hopefully you see that science is as dynamic as the earth itself – there will always be more to know!
Have students research and design earthquake-proof buildings
Use everyday materials to demonstrate some of the movement in the Earth’s crust. Teacher will need sand paper, a couple of bricks (the mass affected by the ‘earthquake’, a winch (represents the steady motion of the plate interiors), and a bungee cord (elasticity of the earth's crust).
Have students investigate various methods of earthquake prediction and present their findings
Have students write a reflective paper about what they’ve learned about the nature of science from this case study
Additional Teacher Information:
This case is appropriate for the following grades and strands:
The Earth’s Crust (Parts of the case)
Earth and Space Science 12U
Physics 11 U
-Forces and Motion
-Electricity and Magnetism (partial)
Physics 12 U
-Forces and Motion: Dynamics
-Electric, gravitational, and magnetic fields
For the Teacher - Rationale for Using a History of Science Approach
It has been argued that using science writing from periods of great change or revolution in science can be used effectively in the classroom to foster student interest and advance scientific literacy (Goodney and Long, 2003). Such an approach helps students understand the true nature of science and that science is embedded in culture, society, and human nature. It is important for students to understand that the work of change in scientific thought is a culmination of research reported in papers and at conferences and it is this collective research from various authors that brings about revolution or a paradigm shift, not just the contents of a single paper. As with the case of continental drift and plate tectonics, change does not happen overnight but can sometimes be a gradual process of fits and starts. In the end, researchers must provide enough data from a number of specialist fields to develop a theory that is persuasive enough to cause a shift in world view.
Conrad, C. and Lithgow-Bertelloni, C. (2002). How Mantle Slabs Drive Plate Tectonics. Science, 298: 207- 209.
Frankel, H. (1988). From Continental Drift to Plate Tectonics. Nature, 335: 127- 130.
Goodney, D and Long, C. (2003). The Collective Classic: A Case for the Reading of Science. Science and Education, 12: 167-184.
Hodson, D. (1999) Going Beyond Cultural Pluralism: Science Education for Sociopolitical Action. Science Education, 83(6), 775-796.
Kerr, R. (2004). Hammered by India, Puttylike Tibet Shows Limits of Plate Tectonics. Science, 305:161.
Molnar, P. (1988). Continental Tectonics in the Aftermath of Plate Tectonics. Nature, 335: 131-137.
Smith, M. and Southard, J. (2001). Exploring the Evolution of Plate Tectonics. Science Scope, 25 (1): 46-49.