Plates - understanding the tectonic jigsaw pieces.

The Earth's crust is broken up into around seventeen major pieces that we call tectonic plates. These plates form a jigsaw pattern around the planet with their edges interacting to cause earthquakes, volcanoes, mountain ranges and deep ocean trenches. But what makes up these jigsaw pieces?

Plates of crust

The crustal plates are made up of the two types of crust - continental crust and oceanic crust. We classify plates into three groups depending on their size - Major, Minor, and Micro plates.

Major Plates

These are plates that are greater than 20 million square kilometers.

Pacific Plate – 103,300,000 km²
North American Plate – 75,900,000 km²
Eurasian Plate – 67,800,000 km²
African Plate – 61,300,000 km²
Antarctic Plate – 60,900,000 km²
Indo-Australian Plate – 58,900,000 km²
South American Plate – 43,600,000 km²

By the way, lots of maps divide the Indo-Australian plate into two separate plates...but I have not seen any evidence that they are separate. One day we will have evidence for one way or the other...but for now, the evidence does not show them being two plates.

Minor Plates

These are plates that are less than20 million square kilometers but greater than one million square kilometers.

Somali Plate – 16,700,000 km²
Nazca Plate – 15,600,000 km²
Philippine Sea Plate – 5,500,000 km²
Arabian Plate – 5,000,000 km²
Caribbean Plate – 3,300,000 km²
Cocos Plate – 2,900,000 km²
Caroline Plate – 1,700,000 km²
Scotia Plate – 1,600,000 km²
Burma Plate – 1,100,000 km²
New Hebrides Plate – 1,100,000 km²

Micro Plates

These are plates that are less than one million square kilometer in size. Currently, around 56 have been identified, but this changes as new information is discussed by scientists studying the Earth's plates.

Imagine a hard boiled egg with the shell cracked before you peel it. You will find large pieces, smaller pieces and in some places lots of tiny pieces. That is what the Earth's crustal plates are like.

One major misconception is that a plate can only be ONE type of crust. For example, a plate made up of just continental crust. But this is not the case. Most plates are made up of a combination of crustal types with oceanic crust welded onto continental crust. So the boundaries between the types of crust are NOT necessarily the boundaries between plates.

Also, you have to remember that sea level does not govern where continental crust ends. Continental crust can extend way out under the ocean.

One important difference between these two types of crust is their "buoyancy" dues to the density of the rocks. Because continental crust is made up of rocks of all types, including those rich in quartz, they are relatively less dense than the oceanic crust which is made up of silica-poor (ie no free quartz) basalt. Becuase of the 'buoyancy of continental crust, it has a survived many of the changes on the surface of our planet, and so here you find the very oldest rocks of the crust.

So now we have the jigsaw pieces. Next, we need to think about how those pieces interact with each other. But that's another story....

For more of our blogs on Plate Tectonics see these posts: