The most active geological activity on Earth occurs at boundaries between plates. The phenomena that occur differ greatly depending on the type of boundary.
Basic Knowledge
Plate boundaries are classified into three types.
①Divergent boundary (spreading boundary): A boundary where two plates move away from each other. A typical example is a mid-ocean ridge(such as the Mid-Atlantic Ridge). The mantle rises and new oceanic crust forms. Volcanic activity is active. When it occurs on a continent, a rift valley(a tectonic valley, such as the East African Rift Valley) forms.
②Convergent boundary: A boundary where two plates collide or one subducts.
・Oceanic plate+continental plate: The oceanic plate subducts, forming a trench(a deep depression). Volcanoes (volcanic arcs, such as the Japanese Archipelago) form on the continental side. Earthquakes occurring at plate boundaries take place.
・Continental plate+continental plate: Because their densities are similar, neither plate subducts, and large mountain ranges(the Himalayas and the Alps) form.
③Conservative boundary (transform boundary): A boundary where two plates slide horizontally past each other. A transform fault forms. A typical example is the San Andreas Fault (California).
Seafloor spreading hypothesis: The mantle rises at a mid-ocean ridge→new seafloor forms→it spreads to both sides. The oldest seafloor is only about 180 million years old (older seafloor subducts and disappears).
Divergent boundary（Plates move apart. Mid-ocean ridges and rift valleys form）
Convergent boundary（Plates approach each other. Trenches and mountain ranges form）
Conservative boundary（Plates slide horizontally past each other. Transform faults）
Mid-ocean ridge（A divergent boundary. A seafloor mountain range where new seafloor forms）
Trench（A convergent boundary. A deep depression where an oceanic plate subducts）
Seafloor spreading hypothesis（Hess・1960s. The hypothesis that seafloor is created and expands at mid-ocean ridges）
In Depth
One piece of evidence for the seafloor spreading hypothesis is the striped pattern of paleomagnetism. Alternating normal and reversed magnetic stripes (magnetic anomaly stripes) are arranged symmetrically on both sides of mid-ocean ridges. This shows that basalt magnetized in opposite directions was created each time geomagnetism reversed. The seafloor spreading rate can be calculated from the spacing of these stripes. The Atlantic Ocean is expanding at approximately 2〜3 cm per year, while the East Pacific Rise is expanding at approximately 15 cm per year.
The Japan Trench on the eastern side of Japan (maximum depth approximately 9000 m) is where the Pacific Plate subducts beneath the North American Plate. The plate-boundary earthquake whose focus was located there was the 2011 Tohoku earthquake and tsunami (2011・M9.0).
The Nankai Trough is where the Philippine Sea Plate subducts, and giant earthquakes of M8〜9 repeatedly occur there in cycles of 100〜200 years.
- Divergent=mid-ocean ridges (new seafloor)/rift valleys form
- Convergent=trenches (subduction)/mountain ranges form
- Conservative=transform faults (sliding past each other)
- Seafloor spreading=born at mid-ocean ridges→disappears at trenches
- Magnetic stripes on the seafloor=evidence of seafloor spreading
- Japan Trench=subduction of the Pacific Plate
Points to Note
① A mid-ocean ridge is a divergent boundary, and a trench is a convergent boundary. Do not reverse them. ② A transform fault is a conservative boundary. The crust only moves horizontally; no crust is created or destroyed. ③ Convergence between continents forms mountain ranges (neither plate subducts), while convergence between an oceanic plate and a continent forms a trench and a volcanic arc.
Practice
- Explain each of the three types of plate boundary.
- Explain the geomagnetic phenomenon that provides evidence for the seafloor spreading hypothesis.
- What landform is formed when continental plates collide?