Plate Boundaries  

Convergent Boundaries

Ocean to Continental Convergence:

When an oceanic plate meets a plate with a continent on its leading edge, the continental plate is less dense so the Oceanic plate starts to sink. This process of on plate sliding underneath another is also called subduction. The subduction of the oceanic plate pushes up and folds the continental crust on the edge of the continental plate. This forms mountain ranges. As the ocean plate moves downward into the mantle, it starts to melt. Some of the melted rocks come up through another plates, which causes volcanoes. You can see these volcanic mountain ranges near or on the edge of the continent.

Ocean to Ocean Convergence:

Oceanic plates are born at mid-ocean ridges where molten rock rises from the mantle, cools and sets. As new molten rock erupts at the mid-ocean ridge, the newly created oceanic plate moves away from the ridge where it was created.

The farther the plate gets from the ridge that created it, the colder and denser it gets. When two oceanic plates collide, the plate that is older is the one that will sink. What comes next is similar to the continental vs. oceanic plate collision. A subduction zone forms and a curved volcanic mountain chain forms above the subducting plate. This time the volcanoes rise out of the ocean, so we call these volcanic mountain chains, island arcs.

Continental to Continental Convergence:

Because the continental plates have a low density and greater thickness, they act differently compared to oceanic plates. At continental to continental convergence neither plate subducts, sinks because it is too light to be carried down into a trench.The two plates converge, buckle, fold, and fault to form complex mountains ranges of great height. Continental to continental convergence produced the Himalayas when the Indian-Australian plate collided with the Eurasian plate.

Divergent Boundaries

Ridge (Oceanic):

When a divergent boundary occurs beneath oceanic lithosphere, the rising convection current below lifts the lithosphere producing a mid-ocean ridge.

Effects that are found at a divergent boundary between oceanic plates include:

  • A submarine mountain range such as the Mid-Atlantic Ridge
  • Volcanic activity in the form of fissure eruptions
  • Shallow earthquake activity
  • Creation of new seafloor and a widening ocean basin.

Rift (continental):

When two continental plates diverge, a rift forms. A rift is a dropped zone where the plates are pulling apart. As the crust widens and thins, valleys form in and around the area, as do volcanoes, which may become increasingly active. When the rift is first forming, streams and rivers flow into the low valleys, and can form lakes. These lakes can widen the crust along the boundary, thinning it down so that it can break off and form a new tectonic plate. Now water from the ocean will fill in the space making new seas.

  • Diverge: To split or move apart

Transform Boundary

Transform Boundary:

Transform Boundaries are places where tectonic plates slide past each other.Transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like stream beds that have been split in half and the two halves have moved in opposite directions.The most famous transform boundary in the world is the San Andreas fault.

The Transform Boundary may not be significant compared to the others, but their sliding motion causes lots of earthquakes. The strongest and most famous earthquake along the San Andreas fault hit San Francisco in 1906. Many buildings were shaken to pieces by the quake, and much of the rest of the city was destroyed by the fires that followed. More than 600 people died as a result of the quake and fires.