How to draw a cross section of a Divergent Boundary
First, click on this, and download and print the document.*  It contains the beginnings of each plate boundary you will need to know how to draw.
* - I'm not kidding about this.  Print out this document and practice the drawings, or you will regret it when you take the exam.

	When students think about a Divergent Boundary, they often think about a continent splitting (which DOES happen, and we'll discuss this when we discuss Ocean Basins).  This leads to some problems, though.
	First, students will often envision a continent (the average thickness of which is 30-35 km, or roughly 20 miles) splitting apart, leaving an enormous crevasse.  But NOWHERE on Earth is there a crevasse anywhere near that depth.
	Second, almost ALL divergent boundaries on Earth are in the ocean, not on land.  So this view of a Divergent Boundary is WRONG.  DO NOT USE THIS.
	So instead of a continent, let's look at the ocean.  The image to the left is what you should see in the document you downloaded and printed.
	The first factor you should be aware of is that a Divergent Boundary is ALWAYS a place where there is a heat source deep underground.  Remember that "temperature" refers to the rate at which atoms and molecules move.  Higher temperature means that atoms and molecules are vibrating faster, which means that they move farther apart, the material expands, its volume increases, its density (mass/volume) decreases, and what happens to low density materials?  Like a hot air balloon, they move up.
	Because of that, the crust over a heat and magma source is uplifted.  This creates a bump, and since the Divergent Boundary is not a point but a line between plates, this bump is an elongated, elevated feature: a "ridge" (or sometimes a "rise").
	Magma rises from the magma chamber to form new oceanic crust where the crust separates at the Divergent Boundary.  As the plates move apart, more magma rises to the surface, creating more crust, in a continual* process.  This is how new seafloor is created. *Eruptions occur every few years to every few thousand years.  On million-year timescales, the process is continuous and in conveyor belt fashion, seafloor is created at the Divergent boundary and moves away.
	At the very top, tension causes a narrow channel right on top. Why does the uplifted area drop as you move away from boundary on either side?  Remember that it was uplifted in the first place because it was warm.  As it cools off, it gets denser, and subsides.  The surrounding seafloor averages about 4 km deep, while the top of the ridge is about 2.5 km in average depth.
	This is what I want you to be able to draw - and label.  Important features: 1. Mid-Ocean Ridge 2. Mid-Ocean Rift 3. Magma chamber below the ridge. 4. Magma rising to surface from magma chamber 5. Subsidence away from ridge.
	This is a world physiographic map, on which I've marked the divergent boundaries in red.  Some of them may overlap with Transform Boundaries (which will be discussed shortly) and there are gaps which represent Transform Boundaries as well.  To see it, click on it to go to a new web page.  It is a very large image. Where can you "see" divergent boundaries?  Mostly, they are in the ocean, as mid-ocean ridges and mid-ocean rises.  At this point in geologic history, the only divergent boundary on land is the East Africa Rift System.  But as Pangaea and as Gondwana began to break up roughly 200-100 million years ago, there were divergent boundaries (as rift systems) between where the continents were breaking up. Now click your "back" button to go back to the Plate Tectonics page.