The surface of the Earth is divided into rigid blocks, called tectonic plates that are bounded by narrow regions of high deformation called plate boundaries. Tectonic plates are comprised of both crust (oceanic or continental) and mantle rock and owe their rigidity to the stiffness of mantle rock at low temperatures. Tectonic plates come in various shapes and sizes and are continuously changing shape, either through the addition of new crust and lithosphere at mid-ocean spreading centers, or through the loss of material at subduction zones. Tectonic plate shape can also change due to breaking of the plate or accretion of a piece of an adjacent plate.
Tectonic plates move at rates of a few to tens of centimeters per year. However, just as the shaped of the plates are continuously changing, so to are the motion of the plates - both in terms of speed and direction. The complex shapes of the tectonic plates, the geometric constraints of plates moving on the confined surface of a sphere and changes in the forces acting on the plates all lead to changes in plate motion over time. The motion of tectonic plates are primarily driven by gravitational buoyancy forces associated with sinking of cold material into the mantle at subduction zones and rising of hot material at mid-ocean spreading centers. This motion is resisted by viscous forces on the base of the plates, by the strength of the plate itself, which resists bending into the mantle at subduction zones, and by the frictional and viscous forces acting between adjacent plates.
Detailed observation of both the present and past motions of tectonic plates are essential for addressing many questions in the geosciences, such as understanding the forces driving plate tectonics, the origins of intra-plate deformation, how deformation at plate boundaries manifests as earthquakes, and the physical structure of the deep mantle. Therefore, it is important for geophysicists to be able to both use observations to determine plate motions, and conversely to use to plate motions to make predictions about how those motions are connected to other observations.
The goals of this section is to get you comfortable thinking about plate motions on the surface of a sphere, and to learn some of the fundamental quantitative tools for determining plate motions (i.e., speed and direction) or to use plate motions to calculate related observations (e.g., the relative motion between two points). First, we will consider plate motions on a 2-D plane, how they are related to plate boundaries, and what it means to choose a reference frame. Second, we will review some of the ways in which we can observe plate motions, such as using magnetic anomalies on the seafloor or GPS measurements. Third, we will move to defining plate motions on a sphere using Euler poles and how to determine an Euler pole from observations. Finally, we will learn how to use a model of present-day plate motion to answer questions related to relative plate motions (e.g., How long until San Diego is at the same latitude as Sacramento?) and changes in plate boundaries (e.g., How long until the Juan de Fuca ridge reaches the Cascadia subduction zone?).