<p>This is primarily an exercise in interpreting a minimalist diagram. Believe me, you will <em>have</em> to learn to read even sketchier diagrams. </p>

<p>That said, the test had the correct answer to the question. Unambiguously, there are only <strong><em>three</em></strong> plates.</p>

<p>The left and right plates each contain a piece of continental crust and a piece of oceanic crust connected by a <a href="https://en.wikipedia.org/wiki/Passive_margin" rel="nofollow noreferrer">passive continental margin</a>. In each case, the oceanic <a href="https://en.wikipedia.org/wiki/Terrane" rel="nofollow noreferrer">terrane</a> and the continental terrane move as a unit, so there's only one plate on the left and only one on the right. With the middle, oceanic-only plate, that makes three.</p>

<p>Although the diagram you gave us is a little more simplified than I'm comfortable with, you can tell that whoever drew the diagram meant for the left and right plates to contain passive margins. The elliptical loops of arrows are intended to be interpreted as <a href="https://en.wikipedia.org/wiki/Mantle_convection" rel="nofollow noreferrer">convection cells in the mantle</a>, and each cell drags the lithosphere above as a unit. So, anything above any one convection cell should be interpreted as one plate.</p>

<p>Here is a slightly more detailed diagram showing a passive margin:</p>

<p><img src="https://upload.wikimedia.org/wikipedia/commons/4/45/Passive_Contiental_Margin.jpg" alt="passive continental margin">
<em>(Source: Wikipedia, CC 3.0)</em></p>

CC BY-SA 4.0
2019-07-24T20:04:06.503Z
I'm unable to locate a good copyright-free image showing mantle convection cells, preferably one under a passive margin like [this one](http://www.geosci.usyd.edu.au/users/prey/ACSGT/EReports/eR.2003/GroupD/Report1/web%20pages/Convection.html)).
Spencer
CC BY-SA 4.0
2018-06-07T03:10:27.270Z
2019-07-24T20:02:08.427Z
2019-07-24T20:02:08.427Z
Spencer
<p>There are three plates, because there are two plate boundaries (marked in blue): a convergent margin to the left, and a divergent margin to the right.</p>

<p>Lithospheric <em>plate</em> #2 contains only oceanic <em>crust</em>. Lithospheric plates #1 and #3 contain both continental and oceanic <em>crusts</em>. Note the difference between the terms <em>plate</em> and <em>crust</em>.</p>

<p>The place where continental and oceanic crusts meet within a single lithospheric plate is called a passive margin.</p>

<p>It may have been a lithospheric plate margin in the past, but now it is a fossil margin and is not active.</p>

<p><a href="https://i.sstatic.net/uk5b1.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/uk5b1.png" alt="enter image description here"></a></p>

CC BY-SA 4.0
2018-06-07T09:14:06.157Z
2018-06-07T15:38:04.903Z
2018-06-07T15:38:04.903Z
Gimelist
<p>I came across a question asking what's the number of tectonic plates in this illustration:
<a href="https://i.sstatic.net/1BbCa.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/1BbCa.jpg" alt="enter image description here"></a></p>

<p>I don't know on which basis we count the tectonic plates.
And I'm confused because I think they are 5 but the answer is 3.
As the illustration shows that there are a mid oceanic ridge which is formed because of a divergent plate motion between two oceanic plates and an oceanic trench which is formed due to covergent plate motion between two oceanic plates, so I think that there are three oceanic plates plus two continental plates which are at the two edges of the illustration. Am I right?</p>

<plate-tectonics><homework>
Number of tectonic plates
Eman5
# Number of tectonic plates

(Asked by: username_0 on 2018-06-06)

I came across a question asking what's the number of tectonic plates in this illustration:
[![enter image description here](https://i.sstatic.net/1BbCa.jpg)](https://i.sstatic.net/1BbCa.jpg)

I don't know on which basis we count the tectonic plates.
And I'm confused because I think they are 5 but the answer is 3.
As the illustration shows that there are a mid oceanic ridge which is formed because of a divergent plate motion between two oceanic plates and an oceanic trench which is formed due to covergent plate motion between two oceanic plates, so I think that there are three oceanic plates plus two continental plates which are at the two edges of the illustration. Am I right?

**Tags:** plate-tectonics, homework


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## 2 answers

### Answer by username_1 on 2018-06-07. Score: 7

This is primarily an exercise in interpreting a minimalist diagram. Believe me, you will *have* to learn to read even sketchier diagrams.

That said, the test had the correct answer to the question. Unambiguously, there are only ***three*** plates.

The left and right plates each contain a piece of continental crust and a piece of oceanic crust connected by a [passive continental margin](https://en.wikipedia.org/wiki/Passive_margin). In each case, the oceanic [terrane](https://en.wikipedia.org/wiki/Terrane) and the continental terrane move as a unit, so there's only one plate on the left and only one on the right. With the middle, oceanic-only plate, that makes three.

Although the diagram you gave us is a little more simplified than I'm comfortable with, you can tell that whoever drew the diagram meant for the left and right plates to contain passive margins. The elliptical loops of arrows are intended to be interpreted as [convection cells in the mantle](https://en.wikipedia.org/wiki/Mantle_convection), and each cell drags the lithosphere above as a unit. So, anything above any one convection cell should be interpreted as one plate.

Here is a slightly more detailed diagram showing a passive margin:

![passive continental margin](https://upload.wikimedia.org/wikipedia/commons/4/45/Passive_Contiental_Margin.jpg)
*(Source: Wikipedia, CC 3.0)*

> **Comments:**
> **username_1**: I'm unable to locate a good copyright-free image showing mantle convection cells, preferably one under a passive margin like [this one](http://www.geosci.usyd.edu.au/users/prey/ACSGT/EReports/eR.2003/GroupD/Report1/web%20pages/Convection.html)).  

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### Answer by username_2 on 2018-06-07. Score: 5

There are three plates, because there are two plate boundaries (marked in blue): a convergent margin to the left, and a divergent margin to the right.

Lithospheric *plate* #2 contains only oceanic *crust*. Lithospheric plates #1 and #3 contain both continental and oceanic *crusts*. Note the difference between the terms *plate* and *crust*.

The place where continental and oceanic crusts meet within a single lithospheric plate is called a passive margin.

It may have been a lithospheric plate margin in the past, but now it is a fossil margin and is not active.

[![enter image description here](https://i.sstatic.net/uk5b1.png)](https://i.sstatic.net/uk5b1.png)


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