As early as the 1920’s scientists recognized that earthquakes lined up along fault zones and were not randomly scattered across the globe. The technology improved dramatically in the 1960’s when standardized seismic monitoring stations were established around the globe to police the ban on above-ground nuclear testing. The location of active volcanoes also lines up along these same zones. For example, the Pacific Ocean is surrounded by volcanoes and earthquake zones – commonly known as the “Ring of Fire”. These zones mark the boundaries of the Pacific Plate. Other tectonic plate boundaries may also be identified in this way.
Yet to see all the borders you also need to look under the ocean. In the late 1950’s, the exploration of the oceans revealed enormous mid-ocean ridges that zig-zag across the ocean floor between continents, nearly encircling the globe in places. These mid-ocean ridges rise on average 4,500 kilometers above the ocean floor and reach peaks higher than most mountains on land. More recent explorations have revealed that the mid-ocean ridges are characterized by huge upwellings of magma similar to volcanoes on land. Incredibly, life, in the form of archaebacteria and other species, exists along the mid-ocean ridges, surviving on the chemicals and nutrients exiting from hydrothermal vents.
Combining information from all these sources (earthquakes, volcanoes and mid-ocean ridges, it is possible to draw the boundaries of all the Earth’s major plates. The seven largest plates are easily identified: African Plate, Antarctic Plate, Eurasian Plate, Indo-Australian Plate, North American Plate, Pacific Plate, South American Plate. The smaller Philippine and Caribbean plates can be outlined using the prominent volcano and earthquake data. The Cocos and Nazca plates can be distinguished using mid-ocean ridge data.
Only the Juan de Fuca, Scotia, and Arabian plates are easily overlooked. In fact, I generally don’t emphasize these 3 plates if my students don’t identify them themselves since it is not essential to me that they memorize all the world’s tectonic plates, only that they recognize how the crust is broken into moving plates and that they understand how the plate boundaries can be determined with earthquake, volcano and mid-ocean ridge information.
Several critical questions remain:
All these questions are related to the differences in what is happening at each of the plate boundaries. (I choose to hold off on discussing these issues with my students until after they learn about the interior of the Earth, convection cells, and sea floor spreading.)
Plate boundaries may be divided into 3 main categories: convergent boundaries where plates collide, divergent boundaries where plate pull apart, and transform boundaries where plates grind past each other. Convergent boundaries in turn have different characteristics depending on if it is 2 pieces of continental crust colliding (continent-continent convergent boundary) or if 1 piece of oceanic crust is diving down below a piece of oceanic or continental crust (subducting convergent boundary).
For ways to model these different plate boundaries with students, see the Sea Floor Spreading activity.
Students should have participated in labeling and discussing the large classroom map with earthquake epicenter data. Students should already know that earthquakes cluster in lines along faults and that these faults occur at the edges of pieces of land that are colliding or grinding past one another.