2.3

 

Theory of Plate Tectonics- “A theory which proposes that Earth’s outer shell consists of individual plates that interact in various ways and thereby produce earthquakes, volcanoes, mountains, and the crust itself.” (Tarbuck, Lutgens, and Linneman 44)

 

According to plate tectonics, Earth’s rigid outer shell—the lithosphere—is made up of the crust and the uppermost, coolest part of the mantle. Its thickness and density differ depending on whether the lithosphere is oceanic or continental. The asthenosphere is a hot, weak layer beneath the lithosphere where high temperatures and pressures bring rock close to melting, allowing it to flow slowly. In contrast, the cooler, rigid lithosphere bends or breaks rather than flows. Because the lithosphere rests on this softer, more ductile layer, it can move independently above the asthenosphere. 

 

The lithosphere is broken into ununiform segments called lithospheric plates which are in constant motion in respect to one another. There are seven major lithospheric plates that are recognized and account for 94 percent of Earth’s surface area: the North American, South American, Pacific, African, Eurasian, Australian-Indian, and Antarctic plates. Because Earth’s plates are always moving, most geological activity happens where they meet, making plate boundaries the primary zones of crustal deformation. Scientists originally identified these boundaries by mapping earthquake and volcano locations, which clearly outlined the edges of the plates. There are three main types of boundaries: divergent boundaries, where plates pull apart and new seafloor forms; convergent boundaries, where plates move together and one may sink into the mantle or two continents may collide to form mountains; and transform boundaries, where plates slide past one another without creating or destroying crust.

 

2.4

 

Most divergent boundaries lie along oceanic ridges, where new seafloor is created, making them constructive plate margins. These ridges are elevated areas marked by high heat flow and volcanism, and together they form Earth’s longest continuous topographic feature. Along the crest of some ridge segments, a deep canyonlike rift valley appears, showing that tensional forces are actively pulling the oceanic crust apart at the ridge crest.

Seafloor Spreading- “a hypothesis, first proposed in the 1960s by Harry Hess, which suggests that new oceanic crust is produced at the crests of mid-ocean ridges, which are the sites of divergence.” (Tarbuck, Lutgens, and Linneman 47)

 

Continental rifting starts when tensional forces pull the lithosphere apart, causing it to stretch and allowing hot mantle material to rise and push the surface upward. As the lithosphere thins, the brittle crust fractures into large blocks. Continued pulling causes these blocks to sink, forming a long, narrow depression known as a continental rift. Over time, this rift can widen into a narrow sea and eventually develop into a new ocean basin.

 

2.5

 

Although a new lithosphere is being created, the net lithosphere does not change, and this is because of convergent plate boundaries and subduction. Convergent boundaries are sometimes called subduction zones because the lithosphere descends/subducts into the mantle. Deep-ocean trenches are created by subduction zones. Oceanic lithosphere bends as it it descends toward the mantle and creates these trenches.

 

Oceanic-Continental Convergence

 

When an oceanic plate and a continental plate collide, the denser oceanic plate is forced beneath the lighter continental plate in a process called subduction. As the oceanic plate sinks into the mantle, it carries water trapped in minerals and sediments. This water lowers the melting point of the surrounding mantle rock, causing partial melting. The molten material rises toward the surface, feeding volcanoes on the edge of the continent. These chains of volcanoes are called continental volcanic arcs, and they form parallel to the subduction zone.

 

Oceanic-Oceanic Convergence

 

When two oceanic plates converge, one of them is usually older, colder, and denser, so it gets forced beneath the other in a subduction zone. As the subducting plate sinks into the mantle, it carries water trapped in its minerals and sediments. This water lowers the melting point of the mantle above, causing partial melting. The resulting magma rises through the overriding oceanic crust and erupts to form a chain of volcanoes on the ocean floor. Over time, these volcanoes build up into islands, creating what is called a volcanic island arc.

 

Continental-Continental Convergence

 

When two continental plates converge, neither one readily subducts because both are thick and made of relatively low‑density rock compared to oceanic crust. Instead of one plate sinking beneath the other, the collision causes the crust to crumple, fold, and thicken. This process pushes up massive mountain ranges, like the Himalayas where India collided with Eurasia.

 

2.6

 

At transform plate boundary, plates slide horizontally past each other and don’t destroy or create lithosphere. J. Tuzo Wilson was a Canadian geophysicist who made major contributions to plate tectonics. He introduced the concept of transform faults, another term for transform plate boundaries. 

 

Fracture zones are long, linear features on the ocean floor that extend outward from mid‑ocean ridges. They mark areas where the crust has been broken and displaced by past transform fault activity. Unlike active transform faults, fracture zones lie beyond the ridge segments and are no longer sites of plate motion, but they still record the offset between ridge sections.

 

2.8

 

Evidence of Seafloor Spreading: Ocean Drilling

 

The Deep Sea Drilling Project (1966–1983) provided some of the strongest evidence for seafloor spreading by collecting sediment and rock samples from the ocean floor. Instead of relying on radiometric dating, scientists determined ages by studying microscopic fossils in sediments directly above the crust. They discovered that seafloor sediments become progressively older with increasing distance from the ridge crest, exactly as seafloor‑spreading predicts. Additional drilling showed that sediments are nearly absent at the ridge crest and grow thicker farther away, further confirming that new oceanic crust forms at ridges and moves outward over time.

 

Mantle Plumes, Hot Spots, and Island Chains as Evidence

 

Mapping volcanic islands and seamounts in the Pacific revealed long, linear chains of volcanoes, including the Hawaiian Island–Emperor Seamount chain, which stretches from Hawaii to the Aleutian trench. Radiometric dating shows that volcanoes become progressively older with increasing distance from the Big Island, supporting the idea that a stationary mantle plume lies beneath Hawaii. As the Pacific plate moves over this hot spot, rising mantle material melts and forms a trail of volcanic islands and seamounts whose ages record the plate’s motion. Hawaii is the youngest island in the chain, while older islands like Kauai show deeply eroded, inactive volcanoes, in contrast to the fresh lava flows and active volcanism on the Big Island today.

 

Evidence: Paleomagnetism

 

Earth generates a magnetic field with invisible lines of force that run between its magnetic poles and extend into space. Although we can’t feel this field the way we feel gravity, a compass reveals its presence: the needle, acting as a tiny magnet, aligns itself with these magnetic lines. When held flat, one end of the needle points toward magnetic north and the other toward magnetic south, which today lie close to Earth’s geographic poles.

 

Some minerals found in nature can become magnetic and respond to Earth’s magnetic field. A common example is magnetite, an iron-rich mineral often found in basaltic lava flows. When basaltic lava erupts, it is hotter than 1000°C (1800°F), which is above the Curie point—the temperature (about 585°C or 1085°F) at which magnetite cannot hold a magnetic charge. While the lava is still molten, the magnetite grains are not magnetic. But as the lava cools below the Curie point, these grains become magnetized and line up with Earth’s magnetic field at that time. Once the rock fully solidifies, this magnetic alignment becomes locked in place. In this way, the minerals act like tiny compass needles that “point” toward the magnetic poles as they existed when the rock formed. Rocks that preserve this ancient magnetic direction are said to have paleomagnetism, meaning “ancient magnetism.” 

 

Studies of paleomagnetism in ancient European lava flows revealed that the magnetic north pole appeared to move from near Hawaii to the Arctic Ocean over the past 500 million years (according to uniformitarian interpretations). This suggested either that the pole itself had wandered or that Europe had shifted position relative to a mostly fixed magnetic pole. Although Earth’s magnetic poles do shift somewhat, long-term averages show they stay close to the geographic poles. This supports Wegener’s idea that the continents, not the poles, are what moved.

 

Additional support came when scientists mapped a similar “polar‑wandering” path for North America. For the first 200 million years, according to uniformitarian interpretations, Europe and North America showed parallel paths but were offset by about 5000 kilometers. Around 180 million years ago, according to uniformitarian interpretations, the paths began to converge, matching the time when the Atlantic Ocean started to open. When the continents are repositioned to where they were before drifting apart, the two paths line up perfectly. This alignment shows that Europe and North America were once connected and moved together as part of the same landmass.

 

2.9

 

Scientists measure plate motion using several methods that reveal both the speed and direction of movement. By dating rocks drilled from the ocean floor and comparing their distance from mid‑ocean ridges, researchers can calculate spreading rates. Maps of ocean‑floor age and the orientation of transform faults also show how plates have moved in the past.

 

GPS technology provides another precise way to track plate motion over time. Satellites measure the exact position of points on Earth to within a few millimeters, allowing scientists to detect even small shifts. These measurements confirm motions such as Hawaii drifting northwest at 8.3 centimeters per year and the Atlantic widening at nearly 2 centimeters per year.

 

2.10

 

Geologists have learned that lithospheric plates move because they are part of a larger convection system within Earth’s mantle. Convection is a type of heat transfer in which warm, less‑dense material rises and cooler, denser material sinks. Just like heated water in a beaker circulates as it warms and cools, hot mantle material rises and cooler mantle material sinks, creating convection currents that help drive plate motion—though the real system inside Earth is far more complex.

 

Geologists agree that the main force driving plate motion is the sinking of cold, dense oceanic lithosphere at subduction zones. This process, called slab pull, happens because the cold oceanic plate is denser than the warm asthenosphere beneath it, so it sinks into the mantle like a heavy anchor pulled by gravity. Even though the slab warms as it descends, it is constantly replaced by cooler, denser lithosphere from above, keeping it much colder—and therefore denser—than the surrounding mantle, which allows gravity to keep pulling it downward.

 

Another force that helps move plates occurs at ocean ridges, where newly formed oceanic lithosphere is pushed away from the ridge axis. This process, called ridge push, happens because oceanic ridges sit higher than the surrounding seafloor, causing the lithosphere to slowly “slide” downhill under the pull of gravity. Even though the slopes are gentle, the huge amount of material involved makes the ridge push a significant driving force in plate motion.