Plate Tectonics in a Nutshell
The theory of plate tectonics is a relatively new scientific concept. While its forerunner—the theory of continental drift—had its inception as early as the late 16th century, plate tectonics only emerged and matured as a widely accepted theory since the 1960s (see This Dynamic Earth booklet). In a nutshell, this theory states that the Earth’s outermost layer is fragmented into a dozen or more large and small solid slabs, called lithospheric plates or tectonic plates, that are moving relative to one another as they ride atop hotter, more mobile mantle material (called the asthenosphere). The average rates of motion of these restless plates—in the past as well as the present—range from less than 1 to more than 15 centimeters per year. With some notable exceptions, nearly all the world’s earthquake and volcanic activity occur along or near boundaries between plates.
Using the Diagram to Discuss How Plate Tectonics Works
To learn more about how plate tectonics work, start at the diagram (available as a pdf) and explanation labeled (1). Although this diagram shows the interaction between continental and oceanic plates, the processes illustrated generally apply for the interaction between two oceanic plates.
- There are two basic types of LITHOSPHERE: continental and oceanic. CONTINENTAL lithosphere has a low density because it is made of relatively light-weight minerals. OCEANIC lithosphere is denser than continental lithosphere because it is composed of heavier minerals. A plate may be made up entirely of oceanic or continental lithosphere, but most are partly oceanic and partly continental.
- Beneath the lithospheric plates lies the ASTHENOSPHERE, a layer of the mantle composed of denser semi-solid rock. Because the plates are less dense than the asthenosphere beneath them, they are floating on top of the asthenosphere.
- Deep within the asthenosphere the pressure and temperature are so high that the rock can soften and partly melt. The softened but dense rock can flow very slowly (think of Silly Putty) over geologic time. Where temperature instabilities exist near the core/mantle boundary, slowly moving convection currents may form within the semi-solid asthenosphere.
- Once formed, convection currents bring hot material from deeper within the mantle up toward the surface.
- As they rise and approach the surface, convection currents diverge at the base of the lithosphere. The diverging currents exert a weak tension or “pull” on the solid plate above it. Tension and high heat flow weakens the floating, solid plate, causing it to break apart. The two sides of the now-split plate then move away from each other, forming a DIVERGENT PLATE BOUNDARY.
- The space between these diverging plates is filled with molten rocks (magma) from below. Contact with seawater cools the magma, which quickly solidifies, forming new oceanic lithosphere. This continuous process, operating over millions of years, builds a chain of submarine volcanoes and rift valleys called a MID-OCEAN RIDGE or an OCEANIC SPREADING RIDGE.
- As new molten rock continues to be extruded at the mid-ocean ridge and added to the oceanic plate (6), the older (earlier formed) part of the plate moves away from the ridge where it was originally created.
- As the oceanic plate moves farther and farther away from the active, hot spreading ridge, it gradually cools down. The colder the plate gets, the denser (“heavier”) it becomes. Eventually, the edge of the plate that is farthest from the spreading ridges cools so much that it becomes denser than the asthenosphere beneath it.
- As you know, denser materials sink, and that’s exactly what happens to the oceanic plate—it starts to sink into the asthenosphere! Where one plate sinks beneath another a subduction zone forms.
- The sinking lead edge of the oceanic plate actually “pulls” the rest of the plate behind it—evidence suggests this is the main driving force of subduction. Geologists are not sure how deep the oceanic plate sinks before it begins to melt and lose its identity as a rigid slab, but we do know that it remains solid far beyond depths of 100 km beneath the Earth’s surface.
- Subduction zones are one type of CONVERGENT PLATE BOUNDARY, the type of plate boundary that forms where two plates are moving toward one another. Notice that although the cool oceanic plate is sinking, the cool but less dense continental plate floats like a cork on top of the denser asthenosphere.
- When the subducting oceanic plate sinks deep below the Earth’s surface, the great temperature and pressure at depth cause the fluids to “sweat” from the sinking plate. The fluids sweated out percolate upward, helping to locally melt the overlying solid mantle above the subducting plate to form pockets of liquid rock (magma).
- The newly generated molten mantle (magma) is less dense than the surrounding rock, so it rises toward the surface. Most of the magma cools and solidifies as large bodies of plutonic (intrusive) rocks far below the Earth’s surface. These large bodies, when later exposed by erosion, commonly form cores of many great mountain ranges [such as the Sierra Nevada (California) or the Andes (South America)] that are created along the subduction zones where the plates converge.
- Some of the molten rock may reach the Earth’s surface to erupt as the pent-up gas pressure in the magma is suddenly released, forming volcanic (extrusive) rocks. Over time, lava and ash erupted each time magma reaches the surface will accumulate—layer upon layer—to construct volcanic mountain ranges and plateaus, such as the Cascade Range and the Columbia River Plateau (Pacific Northwest, U.S.A.).
TECTONIC TIDBITS: MISCELLANEOUS SALIENT FACTS
- Plate tectonics processes almost certainly have been operating since the formation of the Earth (~ 4.6 billions years ago). However, the evidence of such processes very early in Earth’s history have been masked or obliterated by younger geologic processes and deposits.
- Present-day continents are much older geologically than the seafloor of present-day ocean basins. Earliest recognized and dated continental rock (in Australia) was formed about 4.3 billion years ago. In contrast, the geologically oldest seafloor formed about 180 million years ago.
- Why this huge difference in geologic age between continental and oceanic rocks? Answer: the new crust formed along the ocean ridge crests is carried away by plate movement, and is ultimately “recycled” deep into the earth along subduction zones. But because continental crust is thicker and less dense than thinner, younger oceanic, most does not sink deep enough to be recycled and remains largely preserved on land.
- Present-day continents are fragments of a “supercontinent” (Pangaea) that broke up about 225 million years.
- There were a number of pre-Pangaea supercontinents, although the evidence becomes more and more obscure/problematic the farther back in geologic time. Pangaea itself was the product of accretion of fragments of pre-Pangaea supercontinent.
- More than 80% of the world’s earthquakes and volcanoes occur along or near boundaries of the tectonic plates.
- Discovery and mapping of the rugged topography (e.g., huge mountain ranges, deep canyons) and the “magnetic striping” of the ocean floor were important milestones in the development of the plate tectonics theory.
- Earth is the only planetary body in our solar system that exhibits plate tectonics in action—at present as well as in the geologic past. To date, space-based planetary geological studies have not discovered any evidence of extra-terrestrial plate tectonics.