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3: Plate Tectonics

Last updated

Mar 28, 2025
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- 2.8: Detailed Figure Descriptions
- 3.1: Earth's Interior

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Introduction to Plate Tectonics

Plate tectonics is a theory that best explains all available observations concerning the Earth's geological features and their changes over time. As a theory it is constantly being evaluated and refined. More specifically, it is a model that explains the origins of continents and oceans, folded rocks and mountain ranges, igneous and metamorphic rocks, earthquakes and volcanoes, and the movements of the continents. Plate tectonics was first proposed in 1915, but did not become an accepted part of geology until 1965. It took 50 years for this theory to be accepted for a few reasons.

First, plate tectonics was a paradigm shift in thinking about Earth and it was difficult for many established geologists to accept. While observations made by Alfred Wegener concerning the movement of continents were complete and his hypothesis of ‘drifting continents’ was very astute, he could not explain a driving mechanism that would account for the perceived motion of the continents through time. Many geoscientists would not accept his ideas without an explanation of a plausible driving mechanism.

Second, many of Wegener’s observations were from studies of rocks, fossils and landforms in the southern hemisphere and most geologists of the time were in the northern hemisphere (Europe and North America). These Northern Hemisphere geologists were unfamiliar with the rocks and structures in the southern hemisphere. It was difficult for them to independently verify Wegener’s observations and therefore easier to discount them.

Third, the technology necessary to collect the data to develop the concept of plate tectonics simply did not exist until the middle of the 20th century. Though not covered in depth in this resource, we recommend exploring the development of Plate Tectonics theory in Physical Geology – H5P Edition.

Map of major tectonic plates and their boundaries with arrows indicating direction of plate motion. Described in caption. — Figure $\PageIndex{1}$ : World map showing the boundaries of the tectonic plates. The North American Continent takes up only part of the North American plate (brown). The Mid-Atlantic Ridge, a spreading center (in other words, a place where plates are moving away from one another) is also labeled. The arrows indicate the directions of plate movement. Convergent boundaries are indicated by arrows pointing toward each other, divergent boundaries are indicated by arrows pointing away from each other, and transform boundaries are indicated by arrows pointed in opposite directions parallel to the plate margin. "Global Plate Boundaries and Plate Motion" by Earth@Home, a derivative of the original work, is in the public domain. Access a detailed description.

Learning Objectives

By the end of this chapter, you should be able to:

Describe the geological processes that take place at divergent, convergent, and transform plate boundaries.
Describe the physical mechanisms that drive plate movement.
Locate and describe California’s major plate boundaries.

3.1: Earth's Interior
This page provides an overview of Earth's internal structure, outlining its three main layers: the crust, mantle, and core, alongside their compositions and physical properties. It discusses the significance of features like xenoliths and the Mohorovičić Discontinuity, and highlights the dynamics of tectonic plates, buoyancy effects on mountains, and the generation of the magnetic field by the outer core.
3.2: Tectonic Plates, Plate Motions, and Plate Boundaries
The page provides an overview of Earth's tectonic plates, describing the major and minor plates and their movement patterns. It explains the types of plate boundaries: divergent, where plates move apart; convergent, where they move together; and transform, where they slide past each other. The page details how these movements lead to geological formations like volcanic arcs and mountain ranges, and touches on the historical breakup of the supercontinent Pangea.
3.3: Mechanisms for Plate Motion
This page discusses the current understanding of plate motion, highlighting three key factors: mantle convection, ridge push, and slab pull. Mantle convection allows lithospheric plates to move by circulating materials, while ridge push results from gravity causing plates to slide from mid-ocean ridges. Slab pull refers to the force exerted by a subducting slab dragging its plate downward.
3.4: California's Plates and Plate Boundaries
This page discusses California's three types of plate boundaries: the San Andreas Fault as a significant transform boundary, the Mendocino Transform offshore linked to large earthquakes, and the Cascadia subduction zone causing volcanic activity. It also mentions the Gulf of California as a divergent boundary connected to the San Andreas system, highlighting the region's tectonic complexity and earthquake risk.
3.5: Chapter Summary
A summary of the Plate Tectonics chapter.
3.6: Detailed Figure Descriptions

Thumbnail: Western North America and its major plate boundaries including the San Andreas Fault. "San Andreas Transform Plate Boundary" by the USGS is in the public domain.

Introduction to Plate Tectonics

Learning Objectives

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