This video examines the major features on Earth’s surface
relative to the distribution of important earth processes.
Any conceptual model of how Earth works, like plate tectonics, must be able to explain these patterns.
When we refer to features, we are mostly focusing on
the physical characteristics of the ocean floor and mountain ranges on the continents.
Processes in this case involve earthquakes and volcanic eruptions.
We'll start with this figure. Its a global relief model of Earth’s surface that illustrates the topography of the land
and the bathymetry of the ocean basins
and lets us see the contrasting elevations of features on the continents and on the ocean floor.
The darker brown colors indicate higher elevations on land.
Darker shades of blue signify the deepest parts of the ocean basins.
Lighter blue colors indicate shallow conditions.
The first feature we will highlight is a relatively wide oceanic ridge system
that can be traced through all the ocean basins.
While higher than the surrounding ocean floor, the crest of the ridge system
is still about two and a half kilometers (or about 8000 feet) below sea level.
As you learn more about plate tectonics,
you will discover how such a large feature could form on the ocean floor.
Elsewhere, in the interiors of the ocean basins, we can sometimes find narrow, linear chains of islands.
These represent submarine volcanoes that have grown tall enough to rise above the sea surface.
The most famous of these are the Hawaiian islands.
When we look more closely, we can recognize that this island chain has relatively few islands
and is mainly composed of seamounts or submarine volcanoes.
Here are a couple of other island chains in the Pacific Ocean.
As we will discover, these island chains form very differently from the oceanic ridges.
The very deepest parts of the oceans are found in narrow trenches,
typically along the margins of some ocean basins.
For example, we can identify a trench along the western edge of the South American continent.
Trenches are also associated with another feature we want to introduce, island arcs.
These are also chains of islands but they are crescent shaped rather than a linear shape,
so they form a curved line on Earth’s surface.
These are common in the western Pacific, and one of these trenches, the Mariana Trench,
represents the deepest part of the world’s oceans at 11 kilometers depth (or 36,000 feet).
This oblique view of the Mariana trench makes it clear that the island arc is set a couple of hundred kilometers
west of the trench and is actually made up of both islands (here in green)
and seamounts (in the brown colors).
Plate tectonics needs to explain this gap between the trench and the location of the arc
as well as the different processes that give us both curved chains of islands and linear island chains.
In contrast to the examples we just described, there are many oceanic margins that do not have trenches.
Instead, these locations feature shallow continental shelves that step down into the ocean basin.
This is what that looks like along the east coast of the North American continent.
The continental shelf is actually the true edge of the continent.
This gives us something else to explain,
why are some continents and oceans separated by trenches while others are not?
Talking about the continents, let’s take a closer look at where some of the tallest mountains are located.
There are only three places on Earth where we find
mountains of over 6 kilometers in height or around 20,000 feet or more.
There are hundreds of them in the Himalayas,
there are dozens in the Andes Mountains of South America,
and there is one in North America, Denali, in Alaska
What set of geological conditions created these apparently unusual settings?
If we take a closer look at North America,
we find that the elevations of the mountain ranges increase to the west
with the ten tallest peaks on the continent, all in Alaska.
How will plate tectonics help us explain this pattern?
Shifting gears from features to processes, let’s look at where we find earthquakes.
This map shows the distribution of moderate to large earthquakes over a recent 30 day period.
You may notice that this isn’t a random pattern
and that earthquakes seem to be concentrated near some of the features we discussed earlier.
Let’s do the same thing for recent volcanic eruptions.
And by recent, we are thinking about volcanoes that erupted in the last 10,000 years.
Again, how do these patterns compare to those we've seen earlier?
Let’s put the earthquakes and volcanoes side by side,
which features listed toward the bottom of the slide
best match up with the distribution of earthquakes and volcanoes?
(Pause the video here to think about it if you like for a few moments)
Both earthquakes and volcanoes are adjacent to trenches found near continental margins and island arcs
and they appear common in association with mountain ranges.
In contrast, oceanic ridges are outlined by a modest number of earthquakes, especially in the Atlantic Ocean.
But not too many volcanoes.
If we look at the wider continental shelves, these locations are much quieter,
hence they are often referred to as passive margins.
Geologists collectively label processes such as earthquakes, volcanic eruptions,
and mountain building as examples of tectonic activity
and divide the outer layers of Earth into tectonic plates based on these patterns.
As you will soon learn, the relative motions of these plates cause these processes,
cause the earthquakes and volcanoes, and create the features we described earlier.
The largest plates are typically named after an associated continent or ocean.
This lesson sets the scene for a more in depth discussion of plate tectonics.
By the time you have finished, you should be able to explain how features such as oceanic ridges
and island arcs form and how the distribution of these features is likely to change in Earth’s future.
Now, how confident are you that you could successfully address the learning objective for this video?