Plate Update: Refreshing Ideas for Teaching Plate Tectonics

By Hawley, Duncan; Lyon, John | Teaching Geography, Spring 2017 | Go to article overview

Plate Update: Refreshing Ideas for Teaching Plate Tectonics


Hawley, Duncan, Lyon, John, Teaching Geography


Introduction

Somewhere in your curriculum you will teach about plate tectonics. Whatever your sources, it may be easy to think you are teaching ideas that reflect current understanding of the way the Earth works. But beware - our experience indicates that this is often not the case. We have found that some aspects of teaching about plate tectonics are based on sources of information and teachers' own knowledge that is often well outof-date and over-simplified (albeit telling a 'neat' story), and in some cases just plain inaccurate. Similar concerns have been raised by Trend (2008), Dove (2016) and research into science teaching by King (2012).

This article provides a 'refresher' that updates subject knowledge about some key recent developments in understanding how plate tectonics works and which consequently offer improved explanations for the distribution and characteristics of earthquakes, volcanoes and some surface landforms. It is intended to help teachers decide if and how they should adjust what they presently teach to reflect current understanding about the way plate tectonics operates, thereby developing students' critical sense of the plate tectonic 'story' encountered in textbooks, on diagrams, in the news, via the internet and in other media. Not every aspect of the topic can be covered in this short article, but some relevant further sources of information are recommended.

Crust and lithosphere

The outermost layer of the Earth is commonly described as being split into 'crustal plates', but this idea is inaccurate and conceptually misleading. Plates are a function of the mechanical (physical) properties of the Earth's cold outer shell, the strong, rigid (but still flexible) and mostly brittle layer, termed the lithosphere. This extends for some depth below the crust, hence the correct term is 'lithospheric plates' or 'tectonic plates'. The lingering use of the 'crustal' misnomer is a case of historical inertia; the concept of the crust was established long before the discovery of the lithosphere and plates (and probably endures in the popular imagination as a hard thin casing, as on a pie). However, the Earth's composition is, in scientific terms, less intuitive. Its outer layering exists in two ways: by chemical composition (crust, mantle) and by mechanical properties (lithosphere, asthenosphere). The lithosphere is a coupling of crust and uppermost mantle due to cooling; its thickness varies, but its lower boundary is considered to be where the mechanical properties change from elastic to plastic behaviour (around the 1300°C isotherm) (Figure 1). Rigidity allows it to bend (elastically) when subjected to a load, which helps explain the structure of ocean trenches and the deep waters around oceanic volcanoes. The lithosphere (i.e. plates) under oceans is 50-100km thick (thickest under older ocean basins) including a 5-8km veneer of basaltic crust, and is relatively warm and dense.

The lithosphere under continents is up to 300km thick, with a 30-40km crust of granitic rock (and sedimentary derivatives) that is relatively cool and buoyant. Plates can comprise portions of both oceanic lithosphere and continental lithosphere, although these are never vertically juxtaposed. Where one type passes laterally into the other is termed a passive margin; these are not seismically active but represent the site of former tectonic rifts, e.g. the Atlantic seaboards of South America and Africa.

The mechanics of material in the mantle

Some sources describe the mantle as 'semi-solid' or 'semi-liquid'. To many people 'semi-' implies around half, but in fact the mantle is almost entirely (99.9%) solid. We know this because the mantle will transmit seismic S (shear or shake) waves that can only pass through solid material. In the upper mantle is a zone called the asthenosphere, identified by low seismic S wave velocities. Here the rocks are ductile, so will not fracture; but they flow and deform plastically, a process known as viscoelasticity, when subjected to stress over long periods. …

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