5.1: Front Matter

The Grand Unifying Theory of Geology

Recall in the scientific method the exact meaning of a theory: a well-supported explanation for a natural phenomenon that still cannot be completely proven. A Grand Unifying Theory is a set of ideas that is central and essential to a field of study such as the theory of gravity in physics or the theory of evolution in biology. The Grand Unifying Theory of geology is Plate Tectonics, which defines the outer portion of the earth as a brittle outer layer that is broken into moving pieces called tectonic plates (Figure 5.1). This theory is supported by many lines of evidence including the shape of the continents, the distribution of fossils and rocks, the distribution of environmental indicators, as well as the location of mountains, volcanoes, trenches, and earthquakes. The movement of plates can be observed on human timescales and easily measured using GPS satellites.

Figure 5.1: Tectonic plate boundaries world map with their movement vectors and selected hotspots.

Plate tectonics is integral to the study of geology because it aids in reconstructing earth’s history. This theory helps to explain how the first continents were built, how oceans formed, and even helps inform hypotheses for the origin of life. The theory also helps explain the geographic distribution of geologic features such as mountains, volcanoes, rift valleys, and trenches. Finally, it helps us assess the potential risks of geologic catastrophes such as earthquakes and volcanoes across the earth. The power of this theory lies in its ability to create testable hypotheses regarding Earth’s history as well as predictions regarding its future.

Seismologists, geophysicists, volcanologists and other disciplines may all study plate tectonics. Each branch uniquely investigates the topic, and may explore current tectonics, or the tectonics of ancient (or future) Earth. Like many other geoscientists, working with other disciplines is common, with a heavy influence from both math and technology. Many of these geoscientists are employed by universities where they teach and/or do research, and state and federal agencies, including geological surveys, like the California Geological Survey or United State Geological Survey (USGS). Additional career pathways are available in the private sector including in mining and natural resource extraction. Many of these career options require a college degree and postgraduate work. If you are interested, talk to your geology instructor for advice. We recommend completing as many math and science courses as possible (chemistry is incredibly important for mineralogy). Also, visit National Parks, CA State Parks, museums, gem & mineral shows, or join a local rock and mineral club. Typically, natural history museums will have wonderful displays of rocks, including those from your local region. Here in California, there are a number of large collections, including the San Diego Natural History Museum, Natural History Museum of Los Angeles County, Santa Barbara Museum of Natural History, and Kimball Natural History Museum. Many colleges and universities also have their own collections/museums.

Evidence of Moving Continents

The idea that the continents appear to have been joined based on their shapes is not new; in fact, this idea first appeared in the writings of Sir Francis Bacon in 1620. The resulting hypothesis from this observation is rather straightforward: the shapes of the continents fit together because they were once connected and have since broken apart and moved. This hypothesis refers to a historical event and cannot be directly tested without a time machine (and we don’t have those yet). Therefore, geoscientists reframed the hypothesis by assuming the continents used to be connected and asking what other patterns we would expect to find.

This is exactly how turn of the century earth scientists, such as Alfred Wegener, addressed this important scientific question. Wegener compiled rock types, fossil occurrences, and environmental indicators within the rock record on different continents that appear to have been joined in the past, focusing mainly on Africa and South America; he found remarkable similarities across the continents! Other scientists followed suit and the scientific community was able to compile an extensive dataset that indicated that the continents were linked in the past in a supercontinent called Pangaea (named by Wegener) and have shifted to their current position over time. Dating these rocks using both relative and absolute methods allowed scientists to better understand the rate of motion, which has assisted in trying to determine the mechanisms that drive plate tectonics.

What Is a Tectonic Plate and What Are They Made From?

Many different lines of evidence suggest tectonic plates are moving. To build a theory, scientists need an explanation or a mechanism to explain the observed patterns. The theory of plate tectonics states that the outer rigid layer of the earth, the lithosphere, is broken into pieces called tectonic plates (Figure 5.2), and that these plates move independently above the flowing plastic-like portion of the mantle, the asthenosphere.

Figure 5.2: The tectonic plates divide the Earth's crust into distinct "plates" that are always slowly moving. Earthquakes are concentrated along these plate boundaries.

Tectonic plates are composed of the lithosphere: the crust and uppermost mantle that functions as a brittle solid. These plates are composed of oceanic crust, continental crust or a mixture of both. Oceanic crust is thinner and normally underlies the world’s oceans, while the continental crust is thicker and, as its name implies, consists of the continents. The interaction of these tectonic plates is at the root of many geologic events and features, such that we need to understand the structure of the plates to better understand how they interact. The interaction of these plates is controlled by the relative motion of two plates (moving together, apart, or sliding past) as well as the composition of the crustal portion of the plate (continental or oceanic crust).

Continental crust has an overall composition similar to the igneous rock granite, which is a solid, silica-rich crystalline rock typically consisting of a mixture of pink (feldspar), milky white (feldspar), clear (quartz), and black (biotite) minerals. Oceanic crust is primarily composed of the igneous rock basalt, which is a solid, iron and magnesium-rich crystalline rock consisting of a mixture of black and dark gray minerals (pyroxene and feldspar). The difference in rock composition results in distinctive physical properties that you will determine in the next set of questions (Table 5.1).

Table 5.1: Major differences between continental and oceanic crust.

Crustal Properties	Continental Crust	Oceanic Crust
Composition	Granite	Basalt
Relative thickness	Thick (~25-70 km)	Thin (~3-10 km)
Relative age	Old	Young
Relative density	Less dense (2.75 g/cm3)	More dense (3.0 g/cm3)
Relative buoyancy	More buoyant	Less buoyant

The age of the oceanic crust has been determined by systematically mapping variations in the strength of the Earth’s magnetic field across the sea floor and comparing the results with our understanding of the record of Earth’s magnetic field reversal chronology for the past few hundred million years. The ages of different parts of the crust are shown in Figure 5.3. The oldest oceanic crust is around 340 Ma (read “million years”) in the eastern Mediterranean, and the oldest parts of the open ocean are around 180 Ma on either side of the North Atlantic. It may seem surprising, considering that parts of the continental crust are close to 4.0 Ga (read “billion years”) old, that the oldest seafloor is less than 400 Ma. Of course, the reason for this is that all older seafloor has been either subducted or pushed up to become part of the continental crust. As one would expect, the oceanic crust is very young near the spreading ridges, and there are obvious differences in the rate of sea-floor spreading along different ridges. The ridges in the Pacific and southeastern Indian Oceans have wide age bands, indicating rapid spreading (approaching 10 cm or 3.9 inches per year) on each side in some areas, while those in the Atlantic and western Indian Oceans are spreading much more slowly (less than 2.5 cm or less than an inch per year).

Figure 5.3: Age, spreading rates, and spreading symmetry of the world's oceanic crust.

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.

Figure 5.4: The major plate boundaries and their associated movement.

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).

Figure 5.5: Development of a mid-ocean ridge and continental rift valley.

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).

Figure 5.6: Mountains are produced when continental crustal slabs collide.

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.

Figure 5.7: Volcanic mountains are produced on a continent when oceanic crust subducts.

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).

Figure 5.8: Volcanic island mountains are produced when oceanic crust subducts

Transform Boundaries

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.

 

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. 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.

Table 5.2: Summary table of plate boundaries and their associated geologic phenomena.

Boundary Type	Plate Compositions	Earthquake Depth	Change in Crust	Identifying Features	Geologic Phenomena
Divergent continental rift	Continent- Continent	Shallow	Formation of rift valley	Rift valley and volcanoes	Volcanoes, Earthquakes
Divergent mid-ocean ridge	Ocean-Ocean	Shallow	Formation of new oceanic crust	Submarine mountains	Submarine Volcanic Activity, Earthquakes
Convergent collision zone	Continent- Continent	Shallow to intermediate	Metamorphism & folding of the crust	Mountains	Earthquakes
Convergent subduction zone	Continent- Ocean	Shallow to deep	Partial melting of the oceanic crust as it is subducted	Trenches, continental volcanic arcs	Volcanoes, Earthquakes, Tsunamis
Convergent subduction zone	Ocean-Ocean	Shallow to deep	Partial melting of the oceanic crust as it is subducted	Trenches, volcanic island arcs	Volcanoes, Earthquakes, Tsunamis
Transform	Continent- Continent	Shallow	No change	Offset rivers	Earthquakes
Transform	Ocean-Ocean	Shallow	No change	Associated with MORs	Earthquakes

Plate Tectonic Mechanisms

The question still remains, why do tectonic plates move? The answer comes down to gravity and mantle convection. The mantle flows through time creating convection currents. These convection currents flow underneath the plates and through friction pull them along at the surface as well as when they are subducted which is a force called slab suction. Related to this force is slab pull, which is a gravitational force pulling the cold subducting plate down into the mantle at a subduction zone. In addition, there is a force from potential energy at ocean ridges called ridge push. This is a gravitational force pushing down on the elevated ridge and because of the plate's curvature it results in a horizontal force pushing the plate along the earth’s surface. These forces all occur deep inside the Earth and operate on very large geographic scales making them difficult to measure. There are several competing models for the mechanisms behind plate motion, such that there are still some areas of debate surrounding the mechanics of plate tectonics which is why Plate Tectonics is a scientific theory. Documenting an event is much easier and more straightforward than explaining why it occurred. Watch this Minute Earth video on Plate Tectonic mechanisms.

Figure 5.10

Attributions

• Figure 5.1: Derivative of “Tectonic Plate Boundaries World Map” (CC-BY-SA 3.0; Eric Gaba (Sting) via Wikimedia Commons) by Chloe Branciforte
• Figure 5.2: “Tectonic Plates of the Earth” (Public Domain; USGS)
• Table 5.1: “Crustal Properties” (CC-BY 4.0; Chloe Branciforte, own work)​
• Figure 5.3: “Age of Oceanic Lithosphere” (CC-BY 3.0; Müller, R.D., M. Sdrolias, C. Gaina, and W.R. Roest via Wikimedia Commons)
• Figure 5.4: “General Plate Boundaries” (CC-BY 4.0; Chloe Branciforte, own work)
• Figure 5.5: “Rifting” (CC-BY 4.0; Chloe Branciforte, own work)
• Figure 5.6: “Convergence of the Continents” (CC-BY 4.0; Chloe Branciforte, own work)
• Figure 5.7: “Continental Volcanic Arc” (CC-BY 4.0; Chloe Branciforte, own work)
• Figure 5.8: “Volcanic Island Arc” (CC-BY 4.0; Chloe Branciforte, own work)
• Figure 5.9: “San Andreas Fault” (CC-BY 4.0; Chloe Branciforte using Google Earth, own work)
• Table 5.2: “Boundary Summary” (CC-BY 4.0; Chloe Branciforte, own work)