Instructions

Read and take notes, especially where the learning objectives below are addressed. Pay particular attention to Table 5.2, which provides an excellent summary.

Learning Objectives

Students can describe the geologic processes, the relative movement of plates, and the distinguishing landforms associated with each type of plate boundary
Students can differentiate between constructive and destructive geologic processes, can explain the energy driving these processes, and can identify examples of each
Students can relate tectonic stresses to plate boundaries, geologic structures, crustal deformation, and the building of mountains
Distinguish between the terms tectonic plate (plate), plate boundary, and plate tectonics
Recognize Earth’s major surface features
Identify the layers of Earth
Describe some of the evidence used to show the continents and tectonic plates move

Time Needed

30 to 90 minutes.

Types of Plate Boundaries

Tectonic plates can interact in three different ways: they can come together, they can pull apart, or they can slide by each other (Figure 5.4). The other factor that can be important in the interaction is the composition of the plates (oceanic or continental crust). These three types of motions along with the type of plates on each side of the boundary can produce vastly different structures and geologic events.

Convergent and Divergent Boundaries — **Figure 5.4**: The major plate boundaries and their associated movement (CC-BY 4.0; Chloe Branciforte, own work).

Divergent Boundaries

Two plates that are moving apart from each other are referred to as diverging. Divergent boundaries (Figure 5.5) are important because they split continents apart, breaking them into separate plates, and they serve as the site for the formation of new oceanic crust. In a continental rift, a divergent boundary forms within a continent and the region stretches apart. As the area is stretched, the resulting crust becomes thinner and a topographic low or valley is formed (such as the East African Rift Valley). This extension is not a smooth process, so the area is prone to earthquakes and volcanic activity. Eventually, the crust becomes so thin it will rupture and form a gap between the plates, which will fill with molten rock, forming new oceanic crust. This new, thin, and dense plate is topographically low and will eventually become covered in seawater, forming a narrow, elongate (linear) sea (such as the Red Sea). These continental rift zones eventually evolve into mid-ocean ridges (MORs).

Mid-ocean ridge and continental rift valley — **Figure 5.5**: Development of a mid-ocean ridge and continental rift valley. (CC-BY 4.0; Chloe Branciforte, own work).

Convergent Boundaries

Two plates that are moving together are referred to as converging. Convergent boundaries are important because they typically join (suture) continents together to form larger plates as well as where ocean crust is destroyed. The resulting structures we see at convergent boundaries depend on the types of tectonic plates. In a continent-continent convergent plate boundary (Figure 5.6), two thick, low density continental plates converge resulting in a large collision which produces mountains. This is a violent process resulting in earthquakes, deformation (folds and faults) of rock, intense heat and pressure, and the uplift of mountains (an orogeny).

Two converging continental crust plates — **Figure 5.6**: Cross-sectional view of two continental crust slabs colliding. (CC-BY 4.0; Chloe Branciforte, own work).

If a continental plate and an oceanic plate converge (continent-ocean convergent plate boundary) (Figure 5.7) the resulting process is subduction, the oceanic plate sinks downward toward the mantle underneath the continental plate. This produces several distinct features including a deep ocean trench, abundant earthquakes, and a line of volcanoes along the margin of the continent (continental volcanic arc). Associated with subduction zones is the Wadati-Benioff zone, a zone where earthquakes are produced; this zone ranges in depth from shallow (at the trench) to deep (~600km), indicating that the oceanic plate is sinking into the mantle.

Cross-sectional view of a subduction zone where a continental arc is produced. — **Figure 5.7**: Volcanic mountains are produced on a continent when oceanic crust subducts. (CC-BY 4.0; Chloe Branciforte, own work).

If two oceanic plates converge (ocean-ocean convergent plate boundary) (Figure 5.8) it will also result in subduction with deep ocean trenches, abundant earthquakes, and volcanoes. However, the volcanoes will appear on an oceanic plate and will form islands along the tectonic boundary (volcanic island arc).

Subduction zone where a volcanic island arc is produced — **Figure 5.8**: Volcanic island mountains are produced when oceanic crust subducts. (CC-BY 4.0; Chloe Branciforte, own work).

Transform Boundaries

When the two plates slide past each other it is called a transform boundary. This type of boundary differs from others in that no new crust is being formed and no old crust is being destroyed. In other words, constructive and destructive geologic processes are minimal along transform boundaries. Transform boundaries are often marked by abundant earthquakes that can be close to the surface as well as distinctive patterns of rivers that become offset as the land is moving underneath them. The most famous transform boundary is the San Andreas Fault (Figure 5.9). Some transform boundaries are located on the ocean floor and associated with mid-ocean ridges.

San Andreas Fault map bisecting the North American Pacific Plates — **Figure 5.9**: Map view of the San Andreas Fault, with the North American Plate moving to the southeast and the Pacific Plate moving to the northwest. (CC-BY 4.0; Chloe Branciforte via Google Earth, own work).

**Table 5.2:** Summary table of plate boundaries and their associated features, geologic processes, and geologic phenomena (modified from CC-BY 4.0, Chloe Branciforte, own work). Noteworthy features and processes are in **bold**.
Boundary Type	Plate Compositions	Identifying Features	Geologic Processes	Geologic Phenomena	Constructive or Destructive Geologic Processes
Divergent continental rift	Continent-Continent	Rift valley and volcanoes	Continental rifting	Volcanoes, earthquakes	Constructive
Divergent mid-ocean ridge	Ocean-Ocean	Rift Valley and the Mid-Ocean Ridge	Seafloor spreading	Submarine volcanic activity, earthquakes	Constructive
Convergent collision zone	Continent-Continent	Mountains	Deformation (of the crust)	Earthquakes	Constructive
Convergent subduction zone	Continent-Ocean	Deep-ocean trenches, continental volcanic arcs	Subduction	Volcanoes, earthquakes, tsunamis	Destructive (subduction) Constructive (volcanoes)
Convergent subduction zone	Ocean-Ocean	Deep-ocean trenches, volcanic island arcs	Subduction	Volcanoes, earthquakes, tsunamis	Destructive (subduction) Constructive (volcanoes)
Transform	Continent-Continent	Offset rivers	Faulting	Earthquakes	Neither
Transform	Ocean-Ocean	Fracture zones	Faulting	Earthquakes	Neither

Textbook Reading

Optional reading from Chapter 2, Plate Tectonics: Convergent, Divergent, and Transform Boundaries. The selected sections below present helpful and interesting information that will deepen your understanding of the learning objectives.

Most of section 2.3.1: Focus on the first paragraph, the animation, and the illustrations until you reach OCEANIC-CONTINENTAL SUBDUCTION and OCEANIC-OCEANIC SUBDUCTION, where you should focus your reading attention once again.
Section 2.3.2: read the first 3 paragraphs and carefully consider the animation
Section 2.4/2.4.1: read the first 3 paragraphs and carefully consider the animation
Section 2.4.2. the first 6 paragraphs and the Pangaea Breakup video
Section 2.5, the first 4 paragraphs, and the Plate Tectonics in a Nutshell animation