Stories of the Present

How Do We Know?—Evidence, Reasoning, Conclusion:

Earth Science Essentials

by Russ Colson

 

One of the hardest things for non-science folks to do in science is to recognize what is evidence, what is conclusion, and what is the reasoning that connects the two.   Maybe this isn't so surprising, since many public reports of science focus on the theory or main conclusion with only slight reference to the evidence (reports often begin with "scientists tell us..."--as though the scientists telling us were the primary evidence).   However, spotting the evidence in science writing is a key science literacy skill.   We are going to go through a few exercises to practice thinking about evidence, reasoning, and conclusions.

 

Reading 1:   The discovery that Earth is layered

Below is a section from the website http://www.columbia.edu/~vjd1/earth_int.htm

Mohorovicic Seismic Discontinuity
Seismic stations within about 200 km of a continental earthquake (or other seismic disturbance such as a dynamite blast) report travel times that increase in a regular fashion with distance from the source. But beyond 200 km the seismic waves arrive sooner than expected, forming a break in the travel time vs. distance curve. Mohorovicic (1909) interpreted this to mean that the seismic waves recorded beyond 200 km from the earthquake source had passed through a lower layer with significantly higher seismic velocity.

This seismic discontinuity is now known as the Moho (much easier than "Mohorovicic seismic discontinuity" ) It is the boundary between the felsic/mafic crust with seismic velocity around 6 km/sec and the denser ultramafic mantle with seismic velocity around 8 km/sec. The depth to the Moho beneath the continents averages around 35 km but ranges from around 20 km to 70 km. The Moho beneath the oceans is usually about 7 km below the seafloor (i.e., ocean crust is about 7 km thick).

 

Which one of the following statements is Mohovoricic's primary conclusion?  

 

 

 

 

Reading 2:   Seafloor spreading and plate tectonics

 

Below is a section of text on Seafloor Spreading from Wikipedia:   https://en.wikipedia.org/wiki/Seafloor_spreading

 

Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to form new sea floor. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone.

 

The segment of text does not actually present any evidence for seafloor spreading or plate tectonics, but it does describe the theory and present predictions for evidence that might be found if one looked.   Which one of the following is a prediction of evidence that could be found that would support the theory of plate tectonics?

 

 

 

 

 

 Additional ideas for plate tectonic activities can be found at Issues in Earth Science, an online magazine with short stories containing an earth science element and ideas for classroom activities related to the stories. For example, plate tectonic activites are found at http://earthscienceissues.net/Fiction/Plate_Tectonics_Teacher_Resourses.htm and http://earthscienceissues.net/Fiction/Jigsaw_Teacher_Resourses.htm

Reading 3: Ancient Temples and Uniformitarianism

Below is a segment of text about the Temple of Sarapis in Alexandria, Egypt by Randy Moore, from the National Center for Science Education website: http://ncse.com/rncse/29/2/temple-serapis

  

When Lyell visited the temple's ruins in 1828, its three remaining marble pillars — each some 40-feet high — were still standing (the fourth column lies in pieces on the temple's floor). In Principles of Geology, after describing the columns as "smooth and uninjured to the height of about twelve feet above their pedestals," Lyell made his most important point: "Above this is a zone, about nine feet in height, where the marble has been pierced by a species of marine perforating bivalve, Lithodomus." (Lithodomus is a genus of clams that burrow into piers and boat moorings.) Since these clams cannot live above the low-tide line, Lyell concluded that the columns had at one time been underwater (many of the columns' holes still have shells of Lithodomus in them). The original temple had been built above sea level, but the presence of the mollusks on the columns meant that the columns had been partially submerged and were standing upright in the ocean. The columns had then been raised to their present level by the volcanic eruption that produced Monte Nuovo just northwest of Pozzuoli.

Image from Principles of Geology, by Charles Lyell.

 

 The key conclusion of Charles Lyell, according to this article is

 

 

 

 

 

 

Reading 4:   Earth's solid inner core

 This reading comes from EARTH: INSIDE AND OUT, edited by Edmond A. Mathez, a publication of the New Press. © 2000 American Museum of Natural History, as quoted on the website http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html

 

In 1929 a large earthquake occurred near New Zealand. Danish seismologist Inge Lehmann "the only Danish seismologist," as she once referred to herself—studied the shock waves and was puzzled by what she saw. A few P-waves, which should have been deflected by the core, were in fact recorded at seismic stations. Lehmann theorized that these waves had traveled some distance into the core and then bounced off some kind of boundary. Her interpretation of this data was the foundation of a 1936 paper in which she theorized that Earth's center consisted of two parts: a solid inner core surrounded by a liquid outer core, separated by what has come to be called the Lehmann Discontinuity. Lehmann's hypothesis was confirmed in 1970 when more sensitive seismographs detected waves deflecting off this solid core.

 

Based on this passage, the key conclusion of Inge Lehmann's work is that

 

 

 

 

The argument rests on reasoning that is not explained in this passage, but is simply summarized by "which should have been deflected by the core."   The idea here is that the liquid outer core produces not only an S-wave shadow, but also a P-wave shadow.   The P-wave shadow is related to refraction—the P-waves bend away from certain parts of the Earth and so never reach those areas.   Sketch a cutaway diagram of the Earth showing seismic waves with arrows and show where you think the P-wave shadow should occur (you can refer to the sketch of the Earth with the S-wave shadow in the text, drawn from the lecture screen shot). You can submit the picture as a pdf file in the dropbox that follows this lesson.

 

 

 

 

 

Additional activity ideas involving the Temple at Sarapis and Inge Lehmann's work can be found in the NSTA Press book Learning to Read the Earth and Sky.

 

 Last updated 11/21/2016.   All text and pictures are the property of Russ Colson except as noted.

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