AS and a Level Geology Learner Resource 5 Global Tectonics

Learner resource 5–What makes the Earth’s plates move?

Introduction

What makes the tectonic plates move? The simple answer is mantle convection but how does that actually work? The first mechanism was suggested in the 1940s to explain continental drift; that the plates were carried along like rafts on currents in the upper mantle. Once it was realised that it was tectonic plates rather than blocks of crust that moved it was suggested that perhaps it wasrising magma forcing the plates apart at mid-ocean ridges, or the subducting slab dragging the plate behind it. By investigating the correlation between the mean absolute motion of each plate and different characteristics of the plates it we can try to identify what the likely mechanism(s) is(are).

Activity

Learners should have access to a map identifying the different tectonic plates. Absolute motion is taken to be the motion of the plates relative to the hot spot reference frame which was assumed to be static; this has been confirmed by more recent precise GPS observations which have demonstrated that any motion in the hot spot reference frame is below the resolution of current measuring techniques. It is important to realise that the motion of tectonic plates are not linear (hence the use of mean absolute velocity) as moving two plates apart on the surface of a sphere is actually a rotation about a common point. Understanding this helps to explain why all plates show some motion as every plate must be rotating to some degree (like the cogs in a clock).

Each learner needs to have access to the data table and a sheet of graph paper. Each student should investigate a different potential mechanism for the motion of the plates by plotting the mean absolute velocity versus the appropriate variable:

The plates are carried along mantle drag. velocity v total area
The plates are pushed along by mantle drag on the continental keels: velocity v continental area
The plates are moved by “ridge push” : velocity v effective length of divergent plate boundary
The plates are moved by “slab pull” : velocity v effective length of convergent plate boundary
The plates are moved by shearing on transform faults : velocity v effective length of transform plate boundary
The plates are moved by total “ridge push” : velocity v total length of divergent plate boundary

Version 11© OCR 2017

Global Tectonics

Plenary questions:

•Is there likely to be a single mechanism causing all plate motion?

Extension activity

•What do you think is the actual “ridge push” mechanism? – intrusion of dykes, mantle upwelling, gravity acting on raised seabed at mid-ocean ridge

•What is actually happening during slab pull at a subduction zone?

•Plate motion has been described as buoyancy driven, analyses how density and gravity could be moving the plates

Version 11© OCR 2017

Global Tectonics

Tectonic Plate / Total Area
/ 106 km2 / Continental Area / 106 km2 / Mean absolute velocity / cm a-1 / Circumference
/ 102 km / Effective length of plate boundary / % circumference
Divergent / Transform / Convergent
North America / 60 / 36 / 1.1 / 388 / 22 / 31 / 3
South America / 41 / 20 / 1.3 / 305 / 23 / 35 / 1
Pacific / 108 / — / 8.0 / 499 / 24 / 36 / 23
Antarctica / 59 / 15 / 1.7 / 356 / 5 / 37 / —
Indo-Australian / 60 / 15 / 6.1 / 420 / 26 / 30 / 20
African / 79 / 31 / 2.1 / 418 / 14 / 28 / 2
Eurasian / 69 / 51 / 0.7 / 421 / 8 / 13 / —
Nazca / 15 / — / 7.6 / 187 / 29 / 26 / 28
Cocos / 2.9 / — / 8.6 / 88 / 33 / 18 / 28
Caribbean / 3.8 / — / 2.4 / 88 / — / 50 / —
Philippines / 5.4 / — / 6.4 / 103 / — / 31 / —
Arabian / 4.9 / 4.4 / 4.2 / 98 / 28 / 37 / 31
Scotia / 1.6 / 0.3 / 5.5 / 78 / 1 / 74 / 12

Tectonic plate data sheet, based on Forsyth and Uyeda (1975), effective length is the length of plate boundary which is not balance on the opposite side of the plate

Global Tectonics

Teacher answers – Expected results

Global Tectonics