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Examines subduction zones, volcanic arcs, and the formation of massive mountain ranges.
Did you know that marine fossils—the remains of ancient sea creatures—are found at the summit of Mount Everest, the highest point on Earth? How does the floor of a prehistoric ocean travel five miles up into the sky?
Plate tectonics is driven by density. When two plates meet at a convergent boundary, their composition determines the outcome. Oceanic crust is thin but dense (approx. ), while continental crust is thick and buoyant (approx. ). In Oceanic-Continental convergence, the denser oceanic slab is forced down into the mantle in a process called subduction, creating a deep-sea trench. In Oceanic-Oceanic convergence, the older, colder (and thus denser) plate subducts. However, in Continental-Continental convergence, neither plate is dense enough to subduct. Instead, they collide and compress, thickening the crust upward to form massive mountain ranges like the Himalayas.
Quick Check
If an oceanic plate meets a continental plate, which one will stay on the surface and why?
Answer
The continental plate stays on the surface because it has a lower density (approx. ) compared to the oceanic plate (approx. ).
It seems counterintuitive, but the presence of water is what triggers volcanic eruptions at subduction zones. As the oceanic plate descends, it carries volatiles (like and ) trapped in marine sediments and hydrated minerals. At depths of about , the intense pressure squeezes this water out into the overlying mantle wedge. This water acts as a catalyst, lowering the melting temperature of the mantle rock—a process known as flux melting. This is similar to how salt lowers the melting point of ice on a road. The resulting magma is less dense than the surrounding rock, causing it to rise and form a volcanic arc.
Imagine dry peridotite (mantle rock) at a specific depth has a melting point of . 1. The ambient temperature at that depth is only , so the rock remains solid. 2. Subduction introduces into the rock. 3. The melting point drops from to due to the chemical change. 4. Because , the rock melts into magma.
Quick Check
Does flux melting occur because the subducting plate gets hotter as it goes deeper?
Answer
No, it occurs because the introduction of water lowers the melting temperature of the rock, not necessarily because of an increase in external heat.
Orogeny refers to the structural deformation of the Earth's lithosphere during mountain building. When two continental masses converge, the immense pressure causes the rock layers to undergo folding and faulting. Because the crust is too buoyant to subduct, it must go somewhere; it thickens both upward (forming peaks) and downward (forming a deep 'root'). These fold mountain belts are characterized by anticlines (upward folds) and synclines (downward folds). The crustal thickness in these regions can reach up to , nearly double the average continental thickness.
Geologists use the principle of isostasy to understand mountain height. If a mountain range has a height above sea level and a root extending into the mantle, the relationship is often modeled by the density ratio of the crust () and mantle ():
If , , and , the root would be:
This shows that for every kilometer of mountain we see, there is a much larger 'root' supporting it below.
Which process is responsible for the formation of the Andes Mountains?
What happens to the melting point of mantle rock when water is introduced at a subduction zone?
In a continental-continental collision, the higher density of the plates causes them to sink deep into the asthenosphere.
Review Tomorrow
In 24 hours, try to explain the 'salt on ice' analogy to a friend to reinforce how flux melting works.
Practice Activity
Look at a world map and identify three mountain ranges. Based on their location (coastal vs. mid-continent), predict if they were formed by subduction or continental collision.