One thing that I worry about when teaching is that I unknowingly perpetuate misconceptions. As I was reading this blog post, How Did Plate Tectonics Begin? by David Bercovici (Department of Geology & Geophysics, Yale University) I came across this passage,
“Although many text books sadly draw mid-ocean ridges as the site where two wheel-like convection cells upwell from the deep mantle, thereby prying apart the plates, that is probably very wrong.”
Ahhhhh! Many textbooks and online sources most certainly do and, according to my classroom text (and, unfortunately, me), explain it that way too! Facepalm. I emailed the author of the post, explained that my textbook does reference the type of model he was referring to in his post and asked if he had any suggestions of an alternate model/explanation to show my students. He very graciously responded and I received an excellent lesson from an expert in the field. This was his response:
“The textbook diagrams are not something criminal and at least they relay
the idea of a convecting mantle. But I think the better explanation is that
the subducting end of the plate, in the plate tectonic diagram, is really the
main form of convection itself; it’s where the colder rock near the surface
is cooled off and heavy enough to sink. When it sinks it pulls the trailing
plate behind it, and the mid-ocean ridges are basically where the plate
“tears”, and so are pulled apart from a distance (by the slab that is sinking).
In essence the convective “wheels” are driven by the plates, not the
other way around.”
I responded back:
“Thank you for your response! It is helpful. So, the mid-ocean ridges aren’t being pushed apart by upwelling, but being pulled apart by colder, denser sinking plate? How does that work for the mid-Atlantic ridge? I don’t see where subduction could be dragging the plates apart. I really do appreciate your time answering my questions. Thank you”
To which he responded:
“It’s very good question. I guess first thing to notice is the MAR (mid-atlantic ridge) is spreading slowly;
the N. American and S. American and Eursian plates are moving slowly (see Fig 2 in
the blog). So it’s really “slab pull” that is the strongest force. There are some other
weaker forces moving plates too, like the ill-named “ridge push” which is really just
gravitational sliding, i.e, the ridges are high and the sea-floor subsides away from them,
so there is a tendency to for the plates to slide down slope. But that doesn’t much
explain why the ridge got there in the first place, or why the Atlantic is opening,
or equivalently why the supercontinent Pangea broke up.
The simplest thing we can probably say is that the fast plates have slabs, and the
slow plates just get jostled and pushed out of the way. More specifically the slabs
dropping into the mantle stir the mantle, drive flow, and some of that mantle flow,
rising up in response to slabs dropping down, help push the slow plates around.
Also, it appears that slabs do not simply drive straight into the mantle, but often
roll and founder backwards, which is called trench retreat. This causes the plate
connected to the slab to shrink, but it will also tend to pull the over-riding
plate, above the slab, backward in the direction of rollback and that can cause
rifting and spreading. When Pangea broke up there were more subducting
slabs bounding N. America to help get the continent break-up going, but
a lot of those are gone, or there are tiny relics left. Like the Juan de Fuca
and Cocos plates in W. America used to be part of a much bigger Fallon plate
that subducted under the pacific coast. I hope that helps and doesn’t confuse things.”
I was so thrilled to get such a response and to learn more about how to teach this concept without perpetuating misconceptions. I then wondered what other misconceptions students were carrying with them to college based on what they learned in elementary/high school that I might be unwittingly supporting. I reached out on Twitter to a geologist and assistant professor at Kent State University, Chris Rowan, and asked what the top misconceptions in plate tectonics that he has come across. He also graciously responded and extended the question out to his followers. Here are the responses I received. I will update if I get more.
Chris Rowan @ :Geologist, assistant professor at Kent State University
- Lithosphere=plate but Crust≠lithosphere. The crust is *part of* the lithosphere, which includes the uppermost mantle.
- **The pull of subjecting slabs is generally the most important driver of plate motions, not convection beneath the plates. (Mark Tingay
@MarkTingay ^This is a great one ^. I wince every time I see a pic of a MOR (mid-ocean ridge) with a big upwelling convection cell underneath).
- The asthenosphere (mantle below the lithosphere) is not (usually) molten.
- Plate tectonics happened before Pangaea: many cycles of supercontinent break-up & amalgamation beforehand.
- Most devastating earthquakes not usually biggest: location (shallow, close to cities) & building strength more important.
@MarkTingayL: Geomechanics and pore pressure specialist and Associate Professor at the Uni of Adelaide.
- That the “Pacific Ring of Fire” is really a human construct. Related to several plates, not just pacific. There are a lot of volcanoes all round pacific simply because there are many different subduction zones there.
Kim Hannula@: Structural geology professor
- That the edges of continents are also the edges of plates. (Some are; some aren’t.)
- That continental collisions are like collisions between trucks. (They are more like subduction.)
- That plate boundaries are the only places where faults are found.
- That plates only move during earthquakes.
Loÿc Vanderkluysen@: Geoscientist at Drexel University
- That tectonic plates “float” on an ocean of magma. Nope. Crust and mantle are almost all solid
- One resource I found that helps me address and hopefully overcome misconceptions is from the American Association for the Advancement of Science (AAAS). On their website they have researched common misconceptions that students have about multiple science topics. They have an option where you can choose specific learning goals and create an assessment that tests students’ misconceptions. I call them “group quizzes”. My students take them together in small groups and are able to use any resource available to them (models, notes, tools, online animations, etc.) in class to help collect evidence for the correct response. Each answer must be supported with evidence or it is not counted as correct. As students are working (and often engaging in argument from evidence), I’m checking in with groups and informally assessing their progress with the content. I won’t give up answers but direct them to resources that might help them gather evidence. Many of the misconceptions mentioned by the geologists in their tweets are featured in this resource. The link for Plate Tectonics is found here.
“How the items were developed. Each item has gone through a rigorous developmental process. Items were revised multiple times using feedback obtained at each step of the process.
- A set of science ideas was initially selected to define the boundaries of each topic.
- From those ideas, more specific learning goals (key ideas and sub-ideas) were identified and clarified.
- A review of research on student learning was conducted to identify misconceptions and alternate ideas students have about the targeted ideas.
- Clusters of items were written to be closely aligned to the target learning goals, and student misconceptions were included in answer choices.
- Items were pilot tested, and written feedback about the items was obtained from students themselves.
- Items and target learning goals were reviewed by assessment specialists, scientists, science teachers, and other science educators.
- Items were field tested on a large national sample to obtain norming data.”
- **I am looking now to find some alternate visuals/videos/animations that I can show my students to support the sinking slab driving plate motion. I came across this resource: http://www.earthlearningidea.com/PDF/217_Slab_pull.pdf. Figure 2 in David Bercovici’s blog post supports this model as well. Also, this video https://www.youtube.com/watch?v=kwfNGatxUJI&feature=youtu.be describes the slab pull well. If anyone has any other suggestions or resources please comment below.