(PT) Boundaries Lesson

Boundaries

There are three ways to classify where plates meet, the boundaries.

These are divergent boundaries, convergent boundaries and transform boundaries.

Divergent boundaries

At divergent boundaries, tectonic plates are moving away from each other. Almost all the Earth's new crust forms at divergent boundaries, but most are not well known because they lie deep beneath the oceans. These are zones where two plates move away from each other, allowing magma from the mantle to rise up and solidify as new crust.

Convection currents diverge where they approach the surface. The diverging currents exert a weak tension or "pull" on the plate above it. Tension and high heat weakens the floating plate and it begins to break apart. The two sides move away in opposite directions, creating a divergent plate boundary.

The weaknesses between the diverging plates fill with molten rock from below. Sea water cools the molten rock, which quickly solidifies, forming new oceanic lithosphere. This continuous process builds a chain of volcanoes and rift valleys called a mid-ocean ridge or spreading ridge.

Little by little, as each batch of molten rock erupts at the mid-ocean ridge, the newly created oceanic plate moves away from the ridge where it was created.

Seafloor Spreading

Divergent boundaries in the middle of the ocean contribute to seafloor spreading. As plates made of oceanic crust pull apart, a crack in the ocean floor appears. Magma then oozes up from the mantle to fill in the space between the plates, forming a raised ridge called a mid-ocean ridge. The magma also spreads outward, forming new ocean floor and new oceanic crust. 

Rifts

When two continental plates diverge, a valleylike rift develops. This 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. Early in the rift formation, streams and rivers flow into the low valleys and long, narrow lakes can be created. Eventually, the widening crust along the boundary may become thin enough that a piece of the continent breaks off, forming a new tectonic plate. At this point, water from the ocean will rush in, forming a new sea or ocean basin in the rift zone.

Convergent boundaries

Convergent plate boundaries are where the plates collide. When you are determining the behavior of the plates you will need to know if the plate is continental or oceanic.

Continental Plate colliding with Oceanic Plate

In a contest between a dense oceanic plate and a less dense, buoyant continental plate, guess which one will sink? The dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere and a subduction zone is born! Where the two plates intersect, a deep trench forms.

Geologists aren't sure how deep the oceanic plate sinks before it completely melts, but we do know that it remains solid far beyond depths of 100 km beneath the Earth's surface.

When the subducting oceanic plate sinks deeper than 100 kilometers, huge temperature and pressure increases make the plate 'sweat'. Well, not exactly, but the uncomfortable conditions force minerals in the subducting plate to release trapped water and other gasses. The gaseous sweat works its way upward, causing a chain of chemical reactions that melt the mantle above the subducting plate.

This hot, freshly melted liquid rock (magma) makes its way toward the surface. Most of the molten rock cools and solidifies in huge sponge-like magma chambers far below the Earth's surface. Large intrusive rock bodies that form the backbones of great mountain ranges such as the Sierra Nevada form by this process.

Some molten rock may break through the Earth's surface, instantly releasing the huge pressure built up in the gas-rich magma chambers below. Gasses, lava and ash explode out from the breached surface. Over time, layer upon layer of erupting lava and ash build volcanic mountain ranges above the simmering cauldrons below.

Oceanic Plate colliding with Oceanic Plate

In a contest between a dense oceanic plate and a less dense, buoyant continental plate, you know that it's the dense oceanic plate that sinks.

What happens when two dense oceanic plates collide? Once again, density is the key!

Remember that oceanic plates are born at midocean ridges where molten rock rises from the mantle, cools and solidifies. Little by little, 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 ('heavier') it gets.

When two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink.

The rest of the story is a lot like the continental vs. oceanic plate collision we described above. Once again, a subduction zone forms and a curved volcanic mountain chain forms above the subducting plate. Of course, this time the volcanoes rise out of the ocean, so we call these volcanic mountain chains island arcs. The Aleutian Peninsula of Alaska is an excellent example of a very volcanically-active island arc.

Continental Plate colliding with Continental Plate

The Himalayan mountain range provides a spectacular example of continent vs. continent collision. When two huge masses of continental lithosphere meet head-on, neither one can sink because both plates are too buoyant.

It is here that the highest mountains in the world grow. At these boundaries solid rock is crumpled and faulted. Huge slivers of rock, many kilometers wide are thrust on top of one another, forming a towering mountain range. The pressure here is so great that an enormous piece of Asia is being wedged sideways, slipping out of the way like a watermelon seed squeezed between your fingers.

Transform Boundaries

Where two plates are sliding horizontally past one another. These are also known as transform boundaries or more commonly as faults.

At transform boundaries the two tectonic plates grind past each other in a horizontal direction. This movement result in a crack or fracture in the Earth's crust and is called a fault.

Transform boundaries are responsible for Earthquakes. The edges of transform boundaries are jagged and as they slide past one another they can catch and stick, locking the plates at a particular point. When the plates become locked they cannot move and tremendous stresses build up at the fault line. When this stress is released the plates suddenly slip into a new position. It is this sudden movement of the plates that causes earthquakes.

Most transform boundaries are found on the ocean floors however a few occur on land. The San Andreas fault zone in California is an example of a transform boundary. It is approximately 1,300 kilometers long as is formed by the Pacific plate grinding past the North American plate. This has been taking place over the last 10 million years at an average rate of 5 centimeters per year.

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