All major interactions among individual plates are gonna occur after boundaries. That’s where they’re actually intersecting. Divergent plate boundaries two plates are gonna be moving apart.
So it’s constantly being pulled apart. This is what we refer to as seafloor spreading. These are what we refer to and call constructive merchants because new material is being created.
It’s constructing more now along divergent boundaries. This represents about 20% of all of Earth’s surface. We have rift valleys that may develop along the axis and typically these are spreading at five to nine centimeters a year.
Different types of plate boundaries
Divergent plate boundaries
We have two different kinds. We have oceanic rifting or seafloor spreading and you can see that magma is actually coming up. The asthenosphere is melting.
It’s pushing through the lithosphere and it’s creating this new seafloor. Now this section up here is higher than it is out here.
So once it becomes cool and dense it actually starts to sink whereas when it’s warm and hot and is much more buoyant. This is where the ocean is getting pulled apart.
Continental rifting that is going to happen on the continents. A great example is over in the East African Rift Valley. Again we have some upwelling and as it starts to up.
Well, it will come up, it will bust through and it causes warping. Now as this gets pulled apart we can create these little Lakes down.
It will eventually produce what’s called a linear sea. It will give way until it will eventually end up with an oceanic crust where we have water.
Right now Africa is being ripped apart where the East African Rift Valley is and we are experiencing continental rifting there.
Convergent plate boundaries
Now convergent plate boundaries are where plates collide and ocean trench form and the lithosphere is subducting into the mantle.
So this is going to be destructive. When the two collide only one typically survives and there are going to be three types of convergence, that we’re going to talk about.
We have oceanic continental convergence. This is where we have an oceanic plate that is converging with a continental plate. The denser oceanic slab sinks into this theta sphere.
We will have pockets of magma that develop and rise and then we’ll get continental volcanic arcs that form. So examples of where this is happening are the Andes, the Cascades, and the Sierra Nevada system.
This often forms volcanoes on our ocean floor. So volcanic island arcs form as volcanoes emerge up from the sea. We have Aleutian, the Mariana, and the Tonga islands. So again one oceanic, another one is actually being subducted.
The next type is going to be continental convergence. So subducting plates contain continental material. We have one continental plate going underneath another one and as it’s going down some of these materials are actually getting pushed up.
So instead of forming something, you know like our volcanoes and they’re forming more mountains.
Transform plate boundaries
These are where plates just slide past one another. This is consistent with transform faults and one of the largest ones that we have is gonna be the San Andreas Fault these are most common on the oceanic floor.
So this is where we have most or joining two segments of a mid-ocean ridge system. At the time they form, they’re roughly parallel to the direction that which the plate is moving. So this aids in the movement of the oceanic crust’s all material.
Evidence of plate tectonics
Paleomagnetism is a great set of evidence. This is probably the most persuasive evidence that there is for it. Ancient magnetism is preserved in the rocks.
So paleomagnetic records show that there’s polar wandering. Evidence that continents move shows Earth’s magnetic field or reversals.
It’s recorded in the rocks as they form on the oceanic ridges and it’s recorded reversals across oceanic ridges that confirmed that the seafloor is actually spreading.
So as magma is coming up through, it is aligning with the poles. Let’s say that it is normal magnetic polarity. Our magnetic pole is pointing to the north but at some point in the past, there was a reversal.
So as the magma was coming out it actually recorded something quite different and so as it’s coming out it’s getting cooler, denser. It’s moving out and this way from our seafloor spreading.
We’re gonna have different types of reversals and normal magnetism that are shown in the rocks.
Read: What is continental drift?
When the plates get stuck and the ground beneath our feet is always moving, we can see the results of the movement almost everywhere. The rit crack is what moved our planet’s outer shell. It is not one piece but instead is made of individual pieces of plates.
The continents are on plates that are covered with water are called oceanic plates.
They’re thinner and the rock is heavier. When continental plates collide, the folds can pile up and create huge mountain ranges up to eight kilometers high, and the oceanic plate pushes itself under a continental one.
The oceanic plate mounts but the liquid magma can return to the surface at another place. The volcano is born when two oceanic plates move apart.
The seam is called an ocean ridge. Magma from inside the earth is easier. This drives the plates further apart and makes their growth. Plates that glide past each other can get tangled up and stick when they separate.
On average it is 50 kilometers thick beneath the oceans and 70 to 100 kilometers thick under the continents. The continental lithosphere and the oceanic lithosphere also differ in composition.
The continental crust floats on the asthenosphere. The viscous upper part of the mantle lies beneath the lithosphere. The movement of the lithosphere which can be observed in a great number of places is the reason behind earthquakes and volcanic activity.
These movements do not take place randomly but along longitudinal belts. Volcanic activity and earthquakes mostly occur at the edge of continents. Oceanic island arcs oceanic trenches and mid-ocean ridges these motion processes mark the boundaries of tectonic plates.
The lithosphere is not uniform. It is broken up into tectonic plates of different sizes. Today we know of seven major tectonic plates and a number of other smaller ones which all move in relation to each other.
As it expands the mass of molten rock pulls the seafloor apart causing the tectonic plates on either side of the ridge to move away from each other. Thus the ocean basin grows wider.
A process called seafloor spreading is an example of the Atlantic ocean has been formed. However, since the surface of the earth cannot increase continuously at subduction zones volcanoes fold mountains and deep-sea trenches occur.
Examples of mountain ranges formed as a result of subduction are the Andes and the Himalayas. On rare occasions, two adjacent plates slip along. A fault resulting in an earthquake.
Oceanic lithosphere is formed continuously at mid-ocean ridges and disappears at oceanic trenches. Thus the size of plates and the location of dry land are continuously changing.
The pattern of earthquakes and deep-sea drilling earthquake patterns
These are going to be associated with our plate boundaries. Earthquakes that happen deep within the earth along trenches provide a method for tracking plates.
The deep-sea drilling, the age of the deepest sediments in the ocean was found; the youngest are typically near their ridges.
So that’s the higher point. Well, the older are at a distance from their Ridge. So ocean basins are geologically young.
Mechanisms for plate motion
No model can actually explain all the different facets of plate tectonics. The Earth’s heat is the main driving force for it.