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.

A map illustrating the major tectonic plates of Earth. — **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. (Public Domain; USGS).

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 (CC-BY 4.0, Chloe Branciforte, own work).
Crustal Properties	Continental Crust	Oceanic Crust
Composition	Granite	Basalt
Relative thickness	Thick	Thin
Relative age	Old	Young
Relative density	Less dense	More dense
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).

Colorful map illustrating age of the ocean floor. Warmer tones represent newer crustal areas and cooler tones are older crustal areas. Lines bisecting ocean basins like seams on a baseball are mid-ocean ridges (MORs). — **Figure 5.3**: Age, spreading rates, and spreading symmetry of the world's oceanic crust. (CC-BY 3.0; Müller, R.D., M. Sdrolias, C. Gaina, and W.R. Roest).