Of all of these, the most familiar is probably the Map View.  This is a generally 2-dimensional view of the surface of the Earth.  In the one shown at left, you can see 3 rivers, and you might also (in other maps) see roads, geographic boundaries, rock types, and really, anything that can be depicted at the surface of the Earth. The problem is that we can't see anything about changes in elevation on that map view.  If, for instance, you walked from the lower left to the lower right of that map, you'd walk down to the bottom of a river valley, and then back out.  We show that, by using a Profile, which shows elevation changes along a line.  If you've ever gone on a long hike using a trail map, you might see a profile printed on the side of the map which shows not only changes in elevation, but changes in steepness, to help you decide whether you need sandals, sneakers, hiking boots, or climbing gear. Again, there's a problem with a profile:  We can't see what's going on below the surface.  To see that (or to show that you've figured that out), you'd use a Cross Section, which shows not only what's happening in the profile, but below the surface.  They are very useful.  For instance, in this case, we can see that there are 3 layers, that they've been folded (probably through a tectonic collision), and that there is a fault cutting through all of it.  Not only that, but you can get an idea of geologic history as well.  In this case, the orange, bottom, layer is oldest, then the green-blue (teal) layer, and finally the purple layer.  After that, the layers were folded or crumpled (because they originally get deposited in flat-lying layers), and then finally the fault cut through everything. But again, we're missing something - the surface.  Our last, most complex and most comprehensive way of looking at the Earth (although not always necessary or the best way) is a "Block Model".  In this we have one or more cross sections (in the one shown in the upper left, we have 2), as well as map view.  We can use this to get a very complete idea of what's happening in this view of the Earth. One last thing: It's not just geologists that use all of these.  Meteorologists, Oceanographers, even Astronomers all use these views, to look at the atmosphere, the ocean, even the interior of stars.  Even biologists use them.  You've probably seen cross sections of the body, and a photograph of an organism is a kind of a map.

For more information about these ways of looking at the Earth, click here

(Is this a map, profile, cross section, or block model?)

What is a plate?  It is more than just the crust, but a layer (the LITHOSPHERE) composed of crust (oceanic and/or continental) and uppermost mantle: (note: figure taken from USGS; click on figure to find website) This layer is about 150 km thick, and is rigid (hence the name litho = rock sphere), and rides on the soft layer below, the asthenosphere, which is capable of flowing.  The oceanic and/or continental crust that is on the top of the plate merely rides as a passive passenger on the plate, rather than having the continental crust forced through the oceanic crust as Wegener envisioned. Click here for a discussion of this diagram. Go to this link, and read it carefully before returning here.

1. "Plates" are rigid, spherical caps covering the earth's surface that move relative to one another.  A plate may be covered with only Oceanic Crust, only Continental Crust, or a combination of Continental and Oceanic Crust.  But within a Plate, all parts of the plate move together.
• "rigid" means that internally, plates do not deform.  All parts of a plate move together, and if 2 parts of the Earth's surface move in different directions, they are NOT on the same plate.
• "spherical" simply means that the plate conforms to the shape of the Earth.
• "cap" means that each plate covers a part of the surface of the Earth, fitting like jigsaw puzzle pieces.  It is not always easy to say how many plates there are, but most geologists agree that there are approximately 10 major plates (like the North America Plate or the Pacific Plate) and about 10 minor plates (like the Saudi Arabian Plate or the Caribbean Plate).
• Pacific Plate: all oceanic crust.
• Saudi Arabian Plate - almost all continental crust.
• North American Plate: includes western half of North Atlantic, so includes both continental and oceanic crust.
• This tells us that a boundary between continental and oceanic crust is not necessarily a plate boundary.  As we'll see, it can be, but there are many cases where it is not.
• Remember, there is no requirement that a plate boundary must be where the boundary between continental and oceanic crust occurs.  A Plate is defined as one section of the surface of the Earth in which all parts move together.  If a piece of oceanic crust and a piece of continental crust are moving together, then they are on the same plate.
2. Deformation of plates occurs only at their "Margins" (or "Boundaries").  What kinds of deformation? 
   
• Mountain building is often seen at some boundaries (but not all mountains are the result of plate boundaries).
• Volcanoes are commonly seen at some boundaries (but not all volcanoes are the result of plate boundaries).
• Earthquakes are seen at ALL plate boundaries and are almost exclusively found there.  A map of earthquake locations is a good way to identify plate boundaries.
• Click on this link for a world map of earthquake locations.  It is zoomed in on one location, so zoom out to see the entire world.
  
3. There are ONLY 3 types of motion between plates: towards each other, away from each other, and sliding side to side.
   Because of this, there are 3 basic types of Plate Tectonic Margins, based on the type of relative motion between the plates.
   
   Make sure you know how to draw them.  Doing so will help you understand the processes and features at each.
   Click on the drawing link to each boundary type to see how to do this.  And when you click on the "how to draw" links below, you'll be able to download a blank diagram that you can use to practice drawing the different margin types, which will help you understand them.
   
   
   
1. Divergent - where the plates are moving apart at that margin (see figure below).
• This is how to draw a Divergent Margin.
  
• These boundaries are usually found in the ocean, at Mid-Ocean Ridges, and are responsible for the generation of oceanic crust.
• The Mid-Ocean Ridge system is the longest continuous feature on the planet - ~80,000 km long, enough to wrap around the earth twice.
  
2. Transform - where the plates are moving in opposite directions along the direction of the margin (see figure below).
• These boundaries are usually found in the ocean, connecting short segments of Mid-Ocean Ridge, but are occasionally found on land as well.
  
• (modified from USGS - click on image) --> Zooming in on Mid Ocean Ridge -->
  
• Key Divergent features:
• Mid-Ocean Ridge or Rise
• Rift
• Magma chamber below ridge or rise
• shallow earthquakes
  
• Key Transform features:
• Mostly between Divergent or other plate boundaries
• shallow earthquakes
• Why and where does a Transform Boundary form?  Click here to see.
  
3. Convergent - where the plates are moving towards one another at that margin (see figures below).
• There are 3 types of convergent margins, based on the type of crust overlying the plates:
1. Ocean-Continent.
• (modified from USGS)
• Key features:
• Subduction zone
• Deep Ocean Trench
• Accretionary Prism or Wedge
• Earthquakes
• Magma formation on downgoing oceanic plate
• Continental Volcanic Arc
• This is how to draw an Ocean-Continent Convergent Margin.
  
2. Ocean-Ocean.
• (modified from USGS)
• Key features:
• Subduction zone
• Deep Ocean Trench
• Accretionary Prism or Wedge
• Earthquakes
• Magma formation on downgoing oceanic plate
• Volcanic Island Arc
• This is how to draw an Ocean-Ocean Convergent Margin.
  
3. Continent-Continent.
• (modified from USGS)
• Key features:
• Mid-Continent Mountains
• Greatly thickened crust
  
• Earthquakes
• This is what happens and how to draw an Continent-Continent Convergent Margin.
  
• The features found at each type of convergent margin are somewhat different.  What are the differences and why do we find these differences?

1. Through a number of different ways, we can find the age of different parts of oceanic crust (which is all formed at Mid-Ocean Ridges).
• For instance, in the North Atlantic, if you follow the direction of the fracture zones extending away from the Mid-Atlantic Ridge from Jacksonville, FL to its equivalent on Africa (Nouakchott, Mauritania), the width of the ocean is about 6000 km.
• The age of the ocean crust (in other words, the time when North America and Africa split from each other) next to the continents is about 165 million years.
• We can calculate the spreading rate (converted to cm/yr) as: 6000 km x 1000 m/km x 100 cm/m / 165,000,000 years = 3.6 cm/yr.
• This means EACH plate is moving about 1.8 cm/yr, one of the slower sets of plate motion.
  
• 
2. We can also calculate current plate movement rates by using high-precision GPS from one year to the next, on different continents, and watch how the distances between them change.  We get similar values as we calculate by using  multi-million year timescales.  The results tell us 2 things:
1. The year to year rates are pretty continuous for millions of years, and
2. Tha AVERAGE rate of plate motion worldwide is about 5 cm/yr (~2 inches/yr) and varies from 2 to 20 cm/yr.
3. How fast is 5 cm/yr?  If you measure how fast your fingernails grow (mark your thumbnail and watch it grow!), it's about 5 cm/yr.

Finally, click here to see a bunch of the features you will be answering questions about in the Canvas exercise on this lecture.

Suppose I asked you what kind of boundary a particular place is.  What would you look for?

•  The first thing you'd look for (assuming the information is available) is the direction of motion, which defines whether something is a convergent, divergent, or transform boundary.
• The other thing to look at are the features we find.  What is found at the different types of boundaries?  Go look at the boundaries we've covered here, and see which features are found at which boundary type(s) or mid-plate regions (i.e. NOT plate boundaries).  I expect that you will (1) be able to recognize what features are present at what boundary types, and (2) what boundary types contain what features:
• accretionary prism/wedge
• Benioff zone
• continental volcanic arc
• earthquakes
• fracture zone
• magma chambers
• marine sediment
• mid-continental mountain range
• mid ocean ridge
• mid ocean rift
• mid ocean rise
• mountain root
• subduction zone
• suture zone
• trench
• volcanic island arc
• We're not done.  We'll take another look at Plate Tectonics when we look at Ocean Basins.  When we do, think about how the following features do or do not relate to Plate Tectonics.  You don't need to worry about them until we cover Ocean Basins.
  
• active continental margin
• abyssal hills
• abyssal plains
• coastal plain
• continental cratons/platforms/shield areas
• continental margin
• continental rift zone
• continental rise
• continental shelf
• continental slope
• gabbro layer
• ocean basin floors
• ocean crust
• passive continental margin
• pillow basalt
• sheeted dike