2. Plate Patterns - Background

Teacher Background
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.

Earth's Tectonic PlatesEarth's Tectonic PlatesCombining 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:

  • Why are there earthquakes, volcanoes and ridges at the plate boundaries?
  • How come you don’t see earthquakes at the midocean ridges so much?
  • Why are there earthquakes but not volcanoes in the Himalayas above India?

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).
Types of plate boundaries: Image courtesy of the USGS.Types of plate boundaries: Image courtesy of the USGS.

  • With divergent boundaries, like the mid-ocean ridges, Iceland, and the African rift valley, you get few earthquakes because the plates are pulling apart, not storing up energy as they collide or rub past each other. Instead, a gap forms between the plates and magma is pushed up from the mantle below to fill in the hole. Thus, you get lots of volcanoes, thermal vents, and great broken rifts in the earth.
  • With transform boundaries, like the San Andreas fault (most of other transform boundaries lie on the ocean floor), you get lots of earthquakes and only occasional volcanic activity. The plates are moving past one another and storing energy between them until the friction holding them together gives way in the form of an earthquake.
  • With subducting convergent boundaries, as in the northwestern edges of the Pacific Plate, the west coast of South America, and the northeastern edges of the Indo-Australian Plate, you get a combination of volcanoes and earthquakes. The lighter plate floats on top while another plate dives below the edge in a process known as subduction. As this occurs, the submerging plate melts and bubbles up through cracks in the overlying crust as volcanoes.
  • With a continent-continent convergent boundary, like the Himalayas at the boundary between the Indo-Australian and Eurasian plates, you get lots of earthquakes but very little volcanic activity. That is because both continental crusts are light and resist subduction. Instead, they buckle and crumple against one another, gradually rising skywards inch by inch. In fact, the Himalayas continue to rise at the rate of approximately 5 mm a year.

For ways to model these different plate boundaries with students, see the Sea Floor Spreading activity.

Student Prerequisites
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.