Geology Cafe

Introduction to Geology

Chapter 5 - Plate Tectonics

Plate Tectonics, a unifying theory

Modern understanding of the evolution of the Earth through time is the culmination of hundreds of years of world-wide exploration and observation. Maps compiled by early global explorations resulted in the observation of the similarities of coastline patterns on opposite sides of the Atlantic Ocean (noted from early maps by Abraham Ortelius in 1596). Also, early exploration brought awareness of the unusual abundance of volcanoes, mountain ranges, and earthquakes in the region bordering the Pacific Ocean, a region described as the "Ring of Fire."

Today, plate tectonics theory explains the large-scale motions of Earth's lithosphere. Plate tectonics theory builds on concepts of "continental drift," an early theory based on limited land-based geologic mapping and paleontological data gathered in the late 19th to early 20th century. Global seafloor exploration efforts following World War II resulted in the development of seafloor spreading theories in the late 1950s and early 1960s. This exploration effort involved perhaps thousands of scientists within the "global geoscience community" (geologists, oceanographers, paleontologists, and geophysicists) who systematically gathered information and mapped the world. Seafloor mapping, along with the study of volcanoes and earthquakes lead to the development of modern plate tectonics theory.

Learn more at these online resources by the Smithsonian Institution and U.S. Geological Survey:

This Dynamic Planet (World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics (USGS) )
This Dynamic Earth (The Story of Plate Tectonic (USGS))

1. Explain the theory of continental drift, and list the evidence supporting this theory.

2. Illustrate how the Earth's magnetic field affects rocks on the Earth's surface, and explain how this relates to continental drift.
3. Define seafloor spreading, and describe how reversals in the Earth's magnetic field appear on the seafloor.
4. Cite recent evidence that supports the theory of plate tectonics.
4. Illustrate the types of plate boundaries and their features.
6. Describe the possible mechanisms driving plate tectonics.
7. Define orogenesis and locate major orogenic belts on the Earth today.
8. Explain the ideas of isostacy and crustal uplift.
9. Correlate the types of mountain building with the types of plate boundaries.
10. Explain how material from the asthenosphere is transformed into continental crust.
11. Illustrate how continental crust is accreted to form a large continent.

Keywords and Essential Concepts
1. Explain the theory of continental drift, and list the evidence supporting this theory.

Continental Drift was an early hypothesis that attempted to explain the arrangement of continents on the Earth's surface. Although continental drift was supported by many observations, many things about it remained unexplained until advances in new scientific methods leading to new discoveries about the nature of ocean basins and the internal structures of the Earth's lithosphere.

"continental drift"—theory that continents were once assembled. Promoted as a plausible theory by Alfred Wegener proposed in 1912, however the theory criticized for lack of a mechanism to explain the processes.

Ring of Fire
—an extensive zone of volcanic and seismic activity that coincides roughly with the borders of the Pacific Ocean basin. Continental drift could not explain the complex geology and the abundance of earthquakes and volcanoes around the Pacific Basin and elsewhere around the world.

Ring of Fire Pangaea supercontinent Pangaea fossils
"Ring of Fire" in Pacific Basin (USGS) Continental Drift
"Breakup of Pangaea"
Fossils connecting lands of Pangaea (USGS)
(~200 million years)

Seismic waves passing through the earth reveal earth's structure Extensive study of shock waves of earthquakes and the global monitoring of underground nuclear bomb testing revealed information about the internal structure of the Earth. Zones of "seismic wave shadows" in the regions between about 105° to 140° on the opposite side of the globe from a seismic shock source revealed that part of the Earth's inner core is liquid material (magma). In contrast, an inner core is believed to consist of solid metal, possibly similar in composition of iron meteorites. Earth's magnetic field is believed to be associated with this inner metallic core. Between the core and the solid surface crust is the mantle, a zone of mostly solid rock under high pressure and temperature conditions. Earth cross section
Seismic shock wave provide information about the structure of the Earth Internal structure of the Earth

2. Illustrate how the Earth's magnetic field affects rocks on the Earth's surface, and explain how this relates to continental drift.

paleomagnetism—the study of the fixed orientation of a rock's magnetic minerals as originally aligned at the time of the rock's formation. Paleomagnetism is usually the result of thermoremanent magnetization (magnetization that occurs in igneous rocks as they cool below the Curie Point). Igneous rocks may keep their magnetic orientation they obtain at the time they form (if they are not altered). This magnetic signature is preserved, even if the landmass they are is moving.

3. Define seafloor spreading, and describe how reversals in the Earth's magnetic field appear on the seafloor.

seafloor spreading—the formation of new areas of oceanic crust, which occurs through the upwelling of magma at mid-ocean ridges and its subsequent outward movement on either side.

Paleomagnetic studies of the world ocean basin resulted in the discovery of mid-ocean ridges and spreading centers. Seafloor spreading became a mechanism to explain "continental drift". However, seafloor spreading alone does not explain the formation of continental landmasses through geologic time.

A global geologic paradox?
  • The oldest ocean crust is about 200 million years.
  • Continental crust is made up of rocks measured into the billions of years, especially in the stable "craton" cores of continental shields (centers).

Mid Ocean Ridges Magnetic reversals Magnetic ridge, Juan de Fuca
Mid Ocean Ridges (USGS) Magnetic reversals (USGS)
Map of the Atlantic Basin Sea Floor Crust
Seafloor of the Atlantic Basin (USGS) Age of Oceanic Crust (USGS) West Coast reversals (USGS)

4. Cite recent evidence that supports the theory of plate tectonics.

plate tectonics—the modern scientific theory that the Earth’s outer shell (lithosphere) is composed of several large, thin, relatively strong “plates” that move relative to one another. Movements on the faults that define plate boundaries produce most earthquakes. Plate Tectonic Theory evolved in the 1960 after global observation data first was assembled.

Plate Tectonics Plate Tectonics
Plate tectonic model Plate tectonic model (USGS)

divergent boundary—when plates diverge, spreading centers form creating new oceanic crust. Examples include mid-ocean ridges in world's ocean basins. Spreading centers occur where continents are pulling apart. Examples include the Africa rift zones, Red Sea basin, Iceland, and North America's Great Basin region including the Gulf of California.

spreading center— A linear area where new crust forms where two crustal plates are moving apart, such as along a mid-oceanic ridge. Spreading centers are typically seismically active regions in ocean basins and may be regions of active or frequent volcanism.

convergent boundary—when continents collide... mountains belts form - examples include the Himalayas, Alps, and ancient Appalachian Mountains when the ancient continent of Pangaea formed. When continents collide with ocean crust... subduction zones with deep ocean trenches and volcanic arcs form - examples include the Andes Mountains, Aleutian Islands, Japan, Philippines, Indonesia, the ancient Sierra Nevada and modern Cascades Range.

subduction zone
—a plate boundary along which one plate of the Earth’s outer shell descends (subducts) at an angle beneath another. A subduction zone is usually marked by a deep trench on the sea floor. An example is the Cascadia Subduction Zone offshore of Washington, Oregon, and northern California. Most tsunamis are generated by subduction-zone-related earthquakes.

transform boundary
—when plates slide past each other creating fault systems along plate margins. Examples include the San Andreas Fault and major faults in Pakistan, Turkey, and along the Jordan River/Dead Sea.

5. Illustrate the types of plate boundaries and their features.

Types of Continental Boundaries
Convergent boundaries Divergent boundaries Transform boundaries
When continents collide mountains belts form. Examples:
  • Himalayas
  • Alps
  • ancient Appalachian Mountains
When plates diverge, spreading centers form creating new oceanic crust. Examples:

  • mid ocean ridges in world's ocean basins
When plates slide past each other creating fault systems along plate margins. Examples:
  • San Andreas Fault
  • Pakistan
  • Turkey
  • Jordan River/Dead Sea
Convergent boundary along Hinalayan Mountains Divergent boundary of Mid Ocean  Ridge in Iceland San Andreas Fault in Central California
Himalayan convergent boundary Mid Ocean Ridge in Iceland San Andreas Fault system is a transform plate boundary
When continents collide with ocean crust
trenches with subduction zones and volcanic arcs form - examples:
  • Andes Mountains
  • Aleutian Islands
  • Japan, Philippines, Indonesia, etc.
  • Ancient Sierra and modern Cascades
Spreading centers occur where continents are pulling apart. Examples:
  • Africa rift zones
  • Red Sea
  • Iceland
  • North America's Great Basin
Transform faults also occur within plates, but are related to moves that shape the seafloor.
South America plate boundar Divergent boundary forming in the Red Sea fransform faults on the seafloor; North American western plate boundary
Convergent boundary along the west coast of South America A divergent boundary forming in the Red Sea area Transform fracture zones offshore of California are within the Pacific Plate (USGS)

6. Describe the possible mechanisms driving plate tectonics.
Boiling pot of soup The source of heat is inside the earth including heat left over from the formation of the planet, radioactive decay, tidal forces, possibly others. Heat convection drives plate tectonic motion... like a boiling pot of soup moves froth back and forth with rising (warm) and sinking (cooling) currents. Convection in the mantle drives plate tectonic motions
Currents in boiling soup demonstrates convection Convection currents in the mantle drive plate tectonics

7. Define orogenesis and locate major orogenic belts on the Earth today.

Mountain belts and stable cratons orogenesis—The process of mountain formation, especially by a folding and faulting of the earth's crust.

Areas where mountains are found on Earth today are typically regions of active tectonic uplift located in areas associated with plate boundaries, particularly regions where plate convergence is taking place actively or in the intermediate geologic past (within the last 300 million years). Ancient rocks, in the range of 1 to 3 billion years are preserved in the ancient interior portions of continents. These regions were likely mountain belts in the distant geologic past, but have long since worn down to become lowlands and plateaus.
Mountain belts and stable ancient cratons

8. Explain the ideas of isostacy and crustal uplift.

Isostacy—the state of balance, or equilibrium, which sections of the earth's lithosphere (whether continental or oceanic crust) are thought ultimately to achieve when the vertical forces upon them remain unchanged. The lithosphere floats upon the semifluid asthenosphere below. If a section of lithosphere is loaded, as by ice, it will slowly subside to a new equilibrium position; if a section of lithosphere is reduced in mass, as by erosion, it will slowly rise to a new equilibrium position.

Isostacy and the density of crustal rocks Himalayas and Tibet
Isostacy and the density of crustal rocks Crustal thickening in the Himalayan Mountains (NASA)

9. Correlate the types of mountain building with the types of plate boundaries.

Iceland's Rifts African rifts Tibetan Plateau Plate Margins
Iceland's spreading center (USGS) Africa rift zones
Migration of "India"
Plate boundaries today (USGS)

This Dynamic Planet Pacific northwest
Plate Tectonic Features Map
(This Dynamic Planet USGS)
Pacific Northwest
(This Dynamic Planet USGS)

10. Explain how material from the asthenosphere is transformed into continental crust.

Plate Tectonics
Plate tectonic model
Subduction introduces oceanic crustal rocks (including sediments) back into the Asthenosphere. Water and gas helps low-temperature minerals to melt and rise as, forming new continental crust (less dense than oceanic crust). Floating on the Asthenosphere, the continental crustal materials accumulate, forming continents.
asthenosphere—a semifluid layer of the earth, between about 40 to 80 miles (100-200 km) below the outer rigid lithosphere (oceanic and continental crust) forming part of the mantle and thought to be able to flow vertically and horizontally, enabling sections of lithosphere to subside, rise, and undergo lateral movement associated with plate tectonics.

ocean crust—
part of Earth's lithosphere that underlies ocean basins. Oceanic crust is primarily composed of mafic rocks (rich in iron and magnesium) and are less dense than rocks that underlie continents (continental crust is enriched in silica and aluminum). Ocean crust around the world is significantly younger (less than 200 million years) relative to continental crust which has typically accumulated through the natural refining processes associated with plate-tectonics over many hundreds of millions to several billion years.

continental crust—the relatively thick part of the earth's crust that forms the large landmasses. It is generally older and more complex than the oceanic crust, and dominantly composed of igneous and metamorphic of granitic or more felsic composition.

11. Illustrate how continental crust is accreted to form a large continent.

accretion—a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features, or blocks or pieces of continental crust split from other continental plates.

—A fault-bounded area or region with a distinctive stratigraphy, structure, and geological history. (Not to be confused with terrain—a stretch of land, particularly with regard to its physical features: "rough terrain.")

Cross section of the South Bay region, California
Cross section of the Santa Clara Valley showing fault-bounded terranes. Over time, California has formed (assembled) by the accretion of terranes (small crustal landmasses) carried in by plate-tectonic processes slowly over geologic time.

craton—the part of a continent that is stable and forms the central mass of the continent. The craton region of North America includes the region between the Rocky Mountains (to the west) and the Appalachian Mountains (to the east) and including the Canadian Shield.

shield—a large area of exposed Precambrian-age crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to 3.5 billion years.

Pangaea—a supercontinent comprising all the continental crust of the earth, theorized to have existed in late Paleozoic and through early Mesozoic times before the component continents separated and migrated into their current configuration.

World Physiographic Provinces Avalon Through geologic time new continental crust accumulates along the margins of continents. The "floating" continental crust eventually crashes into other land masses, and that may assemble into larger continental crustal plates. For instance, the formation of the ancient supercontinent Pangaea assembled through continental accretion. Pangaea later gradually split apart by continental rifting forming the world's continental landmasses that exist today. Plate Motion
Continental shields
Continental shields contain the oldest rocks preserved in the cores of continental landmasses. These regions formed by processes associated with continental accretion billions of years ago, long before the continents of the modern world existed.
Formation of Pangaea Breakup of Pangaea (USGS)

Formation of California's "Active" Continental Margin

Not all continental margins are plate boundaries. Over time, changes crust is created (along spreading centers) and is destroyed (along subduction zones where crustal material sinks into the mantle). As a result, plates move and continents move along with them. Continents can be split apart, moved, and crushed against other landmasses. Typical most continental landmasses will have a active plate margin (such as around the ring of fire) and have a trailing passive margin (such as the continental boundaries along the Altantic Ocean basin.

active continental margin—a continental margin that is characterized by mountain-building activity including earthquakes, volcanic activity, and tectonic motion resulting from movement of tectonic plates.

passive continental margin—a passive margin is the transition between oceanic and continental crust which is not an active plate margin. Examples of passive margins are the Atlantic and Gulf coastal regions which represent setting where thick accumulations of sedimentary materials have buried ancient rifted continental boundaries formed by the opening of the Atlantic Ocean basin.

North America motion Plate Tectonics Baltimore Canyon Trough Passive margin
Passive & active margins of the North American Plate are related to the dynamics of plate tectonic motion over time. New crust forms along the Mid Atlantic Ridge. Generalized diagram showing Western North America's active margin - note how subduction results in formation of the accreted terranes and and a volcanic arc. The East Coast passive margin in the region of the Baltimore Canyon Trough along the boundary between ocean crust and continental crust. Western North America's passive margin in Late Paleozoic time (before the opening of the Atlantic Ocean that began about 250 million years ago).

California Geology and Plate Tectonics History
California Faults California earthquakes Western North America Plate boundary
Geologic map of California shows the complexity of the different regions within the state. California earthquakes demonstrate that the region is an active margin (USGS) The San Andreas Fault system is a complex transform plate boundary along the West Coast (USGS)

Farollan Plate assembleing California Information about the geologic evolution of California:

Geologic History of Central California

A technical report about the San Andreas Fault System:

Wallace, Robert E., 1990, The San Andreas Fault System, California: U.S. Geological Survey, 283 p.

This historically significant report provides an overview of the history, geology, geomorphology, geophysics, and seismologyof the most well known plate-tectonic boundary in the world.


Assembling California:
California formed gradually over a billion years though processes involving subduction (forming island arcs) and by accretion (attachment of small land masses carried in for other parts of the Pacific Ocean basin). Before the opening of the Atlantic Ocean Basin, California was sometimes a passive margin.

General Summary of California Plate Tectonic History

No rocks older than ~1 billion years exist in CA - all materials in the CA region were subducted or moved elsewhere...

~1 billion to ~250 million: CA was a mostly a passive margin

~250 to ~30 million: subduction dominated the CA coast,
a great volcanic arc formed the core of the Sierra Nevada

~30 years ago to present: the San Andreas Fault System began to modify the coastline - transform faulting replaced subduction

Formation of the San Andreas Fault (USGS)

Supporting evidence of long-distance movement along the San Andreas Fault System.
Pinnacles volcano California Rocks California conglomerate
The Pinnacles Volcano originally formed near Los Angeles nearly 23 million years ago. The western half (Pinnacles Formation) is now about 215 miles north of the eastern half (Neenach Formation). Granitic basement rocks in the Coast Ranges originally formed as part of an volcanic arc complex in the Mesozoic Era. They were ripped off of SoCal and carried northward by plate tectonics motion. Cretaceous-age gravels deposited by an ancient river system in southern California were carried northward from their source area and are now scatter throughout the Coast Ranges.

Quiz Questions