Convergent Plate Boundary – boundary between two plates that are moving towards each other (converging,
colliding).
 Sometimes called a “slow collision” because it happens at about two to eight centimeters per year!
 Recycling of crust takes place at convergent plate boundaries.
 The Earth’s unchanging size means that old crust/lithosphere must be destroyed at about the same rate that
new crust is being created.
 Three types of convergent boundaries: (based on the type of crust involved)
1. Oceanic Crust vs. Oceanic Crust
2. Oceanic Crust vs. Continental Crust
3. Continental Crust vs. Continental Crust
 Battle of DENSITY!!!
 Remember your comparative adjectives:
continental crust = least dense
young, hot oceanic crust = more dense/denser
old, cold oceanic crust = most dense/densest
 The more dense plate will always be subducted beneath the less dense plate.
Oceanic-Oceanic Convergence
 When two oceanic plates collide, the OLDER, COLDER, DENSER plate will be SUBDUCTED beneath
the YOUNGER, WARMER, LESS DENSE plate, forming a topographical feature known as a DEEP-SEA
TRENCH.
 Trenches are long, narrow, deep-cutting canyons (8 to 10 km deep) formed by slab pull (negative
buoyancy) as the old, cold lithosphere descends into the hotter, less dense asthenosphere.
 The older, colder, denser the oceanic crust, the deeper the trench! (Basically, the mantle is pulling
downward on the ocean floor faster than it it’s being pushed.)
 The Challenger Deep, the deepest/lowest place on Earth, is part of a deep-sea trench. (It’s part of
the Marianas Trench in the Pacific Ocean.)
 As the more dense plate is subducted, some of the
sinking crust melts and seeps back up to the
surface forming volcanoes on the over-riding
plate.
 these volcanoes are usually strung out like
beads on a necklace forming a
VOLCANIC ISLAND ARC along the
subduction zone, beyond the trench, on the
OVER-RIDING plate.
Real World Examples:
1. Marianas Islands in the South
Pacific
2. Aleutian Islands near Alaska
Oceanic-Continental Convergence
 Continental Lithosphere is LESS dense than oceanic crust—it’s even less dense than young, hot oceanic
crust!
 The denser oceanic crust will be subducted beneath the less dense continental crust.
 As the denser, oceanic plate is subducted, millions of years of accumulated ocean sediments get “smushed”
onto the over-riding plate, forming “exotic” or “accreted” terranes.
 Some of the accumulated sediments get subducted into the asthenosphere, where they quickly melt and
begins to seep up to the surface. This magma forms intrusions of granite in the over-riding continental plate
(if it solidifies underground before reaching the surface) or volcanoes (if it makes it to the surface and
erupts).
 The result is a chain of volcanic mountains and igneous intrusions on the edge of the over-riding
continent that parallels the deep sea trench.
Real-world examples:
 Andes Mountains caused by subduction of Nazca Plate
beneath South American Plate.
 Cascade Mountains caused by subduction of Juan de
Fuca Plate beneath North American Plate.
Continental-Continental Convergence
 When two plates carrying continental crust collide,
neither plate gets subducted.
 Continental crust is relatively light and resists
downward motion.
Instead, the crust tends to buckle and fold up into mountains.
Mountain belts that are not volcanic are almost always formed by two continents colliding!
Real-World Example: 50 million years ago, the tectonic plate carrying India crashed into Asia (the
Eurasian Plate) leading to the formation of the Himalayan Mountains.
 The slow continuous convergence of the two plates over millions of years has pushed up the
Himalayas and the Tibetan Plateau.
 Most of this growth occurred during the past 10 million years.
 The Himalayas, towering as high as 8,854 m above sea level, form the highest continental
mountains in the world.
 The neighboring Tibetan Plateau, at an average
elevation of about 4,600 m, is well above the
summits of most mountains in the United
States.
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