Explain the evidence for plate tectonics and how each boundary type moves and behaves

Learning goal: Describe the historical evidence that led to the theory of plate tectonics; explain that Earth's lithosphere consists of major and minor tectonic plates that move relative to each other, driven by convection in the asthenosphere; and identify the three main types of plate boundaries by their motion and by the events and features each one produces.

The lithosphere and its plates. Earth's lithosphere is the rigid outer shell: the crust plus the uppermost, coolest part of the mantle, together about 100 km thick on average (thinner under the oceans, thicker under old continents). It is not one continuous shell. It is broken into tectonic plates: seven or eight major plates, depending on how they are counted, such as the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian and South American plates, plus a number of minor plates such as the Juan de Fuca, Caribbean, Nazca and Arabian plates. (Memorizing plate names is not the point; what matters is why the plates move, which way, and what results.) The plates fit together like the pieces of a cracked eggshell, and they move relative to each other, typically at a few centimeters per year, roughly the rate at which fingernails grow. A plate can carry both continental and oceanic crust; plate boundaries and coastlines are different things.

Evidence that built the theory. The theory was not accepted all at once; it grew as several independent lines of evidence pointed the same way.

Continental drift. Early in the twentieth century, Alfred Wegener noticed that the coastlines of South America and Africa fit together, that matching rock formations and mountain belts continue from one continent to the other, and that the same land fossils occur on continents now separated by oceans. He proposed that the continents had once been joined in a single landmass and had drifted apart. Most geologists rejected the idea because he could not explain what force moved continents through solid ocean floor.

Fossil distribution. Fossils of Mesosaurus, a freshwater reptile, are found in both southern Africa and South America, on coasts that now face each other across the Atlantic. A freshwater animal could not have crossed thousands of kilometers of salty open ocean. Fossils of the seed fern Glossopteris are found on all the southern continents: South America, Africa, India, Antarctica and Australia. Land plants and freshwater animals with the same distribution are best explained by the continents having been joined when those organisms lived.

Seafloor spreading. After 1950, mapping of the ocean floors revealed a continuous system of mid-ocean ridges with a rift along the crest, and sampling showed the ocean floor is youngest at the ridge and gets steadily older with distance from it. Very little of the ocean floor is older than about 150 million years (one patch beneath the eastern Mediterranean has been interpreted as about 340 million years old, a rare exception), far younger than the continents. The explanation is that new ocean floor forms at the ridge as magma rises and solidifies, then moves away on both sides.

Paleomagnetism. When lava cools, iron-bearing minerals in it record the direction of Earth's magnetic field at that moment. Earth's field has reversed its direction many times in the past. Surveys across mid-ocean ridges found stripes of normal and reversed magnetism arranged symmetrically on either side of the ridge, like a tape recording played out in both directions. This was the decisive evidence that the seafloor spreads, and it supplied the moving conveyor that Wegener had lacked. Together these observations became the theory of plate tectonics in the 1960s.

Present-day measurements. The motion is no longer only inferred. Satellite positioning (GPS) receivers fixed to the ground on different plates record their positions changing by a few centimeters a year, in the directions and at the rates the theory predicts, and networks of seismometers locate earthquakes so precisely that a sinking plate at a subduction zone can be traced hundreds of kilometers down into the mantle.

What moves the plates. The energy comes from deep within Earth: heat left from planetary formation and heat continuously released by decaying atoms, as the previous lesson explained. That thermal energy drives convection currents in the asthenosphere, the hot, weak, mostly solid part of the upper mantle beneath the lithosphere. Hotter, less dense rock rises slowly; cooler, denser rock sinks; and the rigid plates above move with the slowly flowing, mostly solid rock beneath them. Two further effects work with convection. At a mid-ocean ridge the raised, hot plate edge slides downhill off the ridge (ridge push). Where an old, cold, dense plate edge sinks into the mantle, its weight drags the rest of the plate behind it (slab pull), and many geologists consider slab pull the largest single contribution. All three are parts of the same convecting system, not competing explanations.

Three types of boundary. Plate boundaries are of three main types, defined by relative motion (some boundaries combine two kinds of motion), and each kind leaves its own signature of events and features.

Divergent boundaries: plates move apart. Magma rises to fill the gap, producing new lithosphere. Features: mid-ocean ridges, often with a central rift valley; on land, rift valleys such as the East African Rift. Events: frequent but mostly shallow, moderate earthquakes and volcanic activity. Example: the Mid-Atlantic Ridge, which runs through Iceland.

Convergent boundaries: plates move toward each other. What happens depends on what collides. Where dense oceanic lithosphere meets less dense continental lithosphere, the oceanic plate sinks beneath the continent (subduction), forming a deep ocean trench and a chain of volcanic mountains, like the Andes. Where two oceanic plates converge, one subducts under the other, producing a trench and a curved chain of volcanic islands, an island arc, like the Aleutian Islands or Japan. Where two continents converge, neither is dense enough to sink deep into the mantle the way oceanic lithosphere does, so instead of a subduction zone with a trench the crust crumples and thickens into high mountains with little or no volcanism; the Himalaya are forming this way as India pushes into Asia. Convergent boundaries produce the largest earthquakes and the deepest, and subduction zones are where most explosive volcanoes occur.

Transform boundaries: plates slide past each other sideways. Lithosphere is neither created nor destroyed. Features: long, straight faults; offset streams and fences. Events: earthquakes, sometimes large, but little or no volcanism. Example: the San Andreas Fault in California, where the Pacific plate moves northwest past the North American plate.

Ohio's position. Ohio sits deep in the interior of the North American plate, far from any plate boundary. That is why Ohio has no volcanoes and why its earthquakes are infrequent and usually small compared with those of a boundary region. Ohio is not earthquake-free, though: old faults buried within the plate, such as those beneath the Anna area of western Ohio, do occasionally move, and a few Ohio earthquakes have been strong enough to be felt widely: the largest recorded, magnitude 5.4 near Anna in Shelby County in 1937, was felt in eight states, according to the Ohio Geological Survey. Being on a plate is not the same as being on its edge.

Misconceptions to avoid. Continents do not plow through the ocean floor; the whole plate moves, ocean floor included. Plates do not float on liquid magma; they move on hot, solid, slowly creeping rock. Earthquakes and volcanoes are not scattered at random; mapped together they outline the plate boundaries, which is itself one of the strongest pieces of evidence for the theory; the Ring of Fire around the Pacific is the clearest case. One exception is useful: the Hawaiian Islands are volcanoes in the middle of the Pacific plate, fed by a hot spot in the mantle beneath the plate. As the plate moves over the hot spot, a chain of islands forms, oldest farthest from the active volcano, which is a second, independent measure of the plate's motion.

Worked distance from rate: A plate moves at 5 cm per year. How far does it travel in one million years? 5 cm × 1,000,000 = 5,000,000 cm = 50,000 m = 50 km. Unit conversion is the step students skip: 100 cm in a meter and 1,000 m in a kilometer. Fifty kilometers in a million years is small on a map, but small rates add up. The Mid-Atlantic Ridge spreads at about 2.5 cm per year, and the Atlantic began to open about 200 million years ago: 2.5 cm × 200,000,000 = 500,000,000 cm = 5,000 km. A rate too slow to notice in a lifetime, kept up for long enough, opens an ocean thousands of kilometers wide.

Worked spreading rate from magnetic stripes (hypothetical figures): On each side of a hypothetical ridge, a magnetic stripe whose rocks are 4 million years old lies 100 km from the ridge crest. Each side has moved 100 km in 4 million years: 100 km ÷ 4,000,000 yr = 0.000025 km/yr = 2.5 cm/yr. That is the half-rate, the speed of one side. If the two sides spread equally, as the symmetry of the stripes indicates, they separate at twice that, 5 cm/yr. The symmetry of the stripes (same age at the same distance on both sides) is the evidence that new floor forms at the ridge and moves away equally in both directions.

Worked boundary identification: A region has a deep ocean trench just offshore, a line of active volcanoes on land parallel to the trench, and frequent earthquakes whose depth increases inland from the trench. Which boundary? Trench plus volcanic chain on a continent plus earthquakes deepening away from the trench is the signature of an ocean-continent convergent boundary: the oceanic plate is subducting beneath the continent, and the earthquakes trace the sinking plate. A transform boundary would show earthquakes but no trench or volcanoes; a divergent boundary would show a ridge or rift, not a trench.

Worked evidence evaluation: Which of these is stronger evidence that the seafloor spreads: (a) the coasts of Africa and South America fit, or (b) magnetic stripes are symmetrical about the ridge and seafloor age increases away from it? Answer (b). The fit of the coastlines is suggestive but could be coincidence and says nothing about the ocean floor. Symmetrical age and magnetism patterns are exactly what spreading predicts, no other proposed explanation has accounted for them, and they can be measured anywhere along the ridge system.

Representation task: On a blank world map, mark the approximate positions of the Mid-Atlantic Ridge, the Andes, the Himalaya and the San Andreas Fault, label each with its boundary type and the relative plate motion (arrows), and place a dot on Ohio with a note on which plate it sits and roughly how far it is from the nearest boundary.

Safe investigation, convection: Fill a clear rectangular container with room-temperature water and rest it on two blocks so there is space beneath for a mug to stand touching the base. Before the mugs go in, put one test drop of food coloring near the middle of the still, room-temperature water and note how it moves; food coloring is slightly denser than water and may sink a little on its own, so this is the comparison for what follows. Slide a mug of hot tap water (not boiling) under one end and a mug of ice under the other. After two minutes, use a long dropper to place one drop of food coloring near the bottom of the tank above the warm end and a second drop just below the water surface above the cold end. Record what each drop does over ten minutes, with a sketch every minute; the circulation can take several minutes to show. What result supports the claim: the coloring above the warm end rises and drifts along the top toward the cold end, while the coloring above the cold end sinks faster than the test drop did and drifts along the bottom toward the warm end, forming a loop. What the model cannot show: water is a liquid and the asthenosphere is mostly solid rock creeping over millions of years; the tank has no rigid plates on top; and the heat comes from a mug rather than from Earth's core.

Independent performance: Write a page titled "Why geologists accept that the plates move." Success criteria: it names at least three independent lines of evidence (fossil distribution, paleomagnetism, seafloor spreading, continental fit) and explains what each shows; it explains where the energy for plate motion comes from and how convection in the asthenosphere moves the lithosphere; it describes all three boundary types with the motion and one feature or event characteristic of each; and it explains, with a reason, why Ohio has earthquakes but no volcanoes.