The oceans
The oceans

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The oceans

2.1.2 Bathymetric data online

Today some of the very best bathymetric data are freely available online, as you will see from Activity 1. Here, you will use Google Earth to investigate some of the features of the ocean floor and to navigate around using latitude and longitude.

Activity 1 The ocean depths

Timing: Time guide: 1 hour

Part 1: Downloading and exploring a layered data file

First watch Video 1, which demonstrates how to use Google Earth to look at the ocean. (You need to view this video in 'Full Screen' mode to see the details.)

Download this video clip.Video player: Video 1
Skip transcript: Video 1 Using Google Earth to look at the ocean.

Transcript: Video 1 Using Google Earth to look at the ocean.

INSTRUCTOR
When I open the Google Earth application, slowly the Earth rotates into view. Now, you're most likely used to using Google Earth to look at land, but here we're going to use it to look at the ocean.
And the first thing I want you to do is to select the Search Box on the top left hand side and enter the coordinates 47, 34 degrees north and 7 degrees 34 west. Now, these are latitude and longitude coordinates. Now when I click Search, now the Earth rotates and it zooms into a particular area of the ocean.
Looking at the surface of the ocean, there are some things that we need to take notice of. The first is the bit in yellow is the data source. And the second bit is that when I put my mouse over the surface, you can see the latitude and longitude moving but also the elevation changing, as well. So the elevation is the water depth, and so what those numbers are showing us are the water depth at particular latitudes and particular longitudes.
Going back to my original view now, what I'm going to do is zoom out so you can see where the point is. Now I zoom out more and more and more, and in the bottom right hand corner, you can see the altitude above the ground.
And when I get to about 1000 kilometres, I'm going to stop. And you can see the coast of France come into view, Cornwall, Wales, and Ireland.
And different shades of blue in the ocean represent different water depths. Light blue is shallow, dark blue deep.
End transcript: Video 1 Using Google Earth to look at the ocean.
Video 1 Using Google Earth to look at the ocean.
Interactive feature not available in single page view (see it in standard view).

Question 1

Using Google Earth find out the water depth at ~47° 34′ N, 7° 33′ 30″ W.

Answer

The most straightforward way is to type 47 34 N 7 34 W in the 'Search' box, then use your mouse and the latitude and longitude information on the bottom right to move your cursor to the correct point and so find the water depth at 47°34′ N, 7° 33′ 30″ W.

The water depth is approximately 1103 m.

Question 2

By moving your pointer, find out the approximate water depth off the coast of France and the depth of the darker (and so deeper) region to the west.

Answer

The water depth off the west coast of France is approximately 120 m and the depth in the dark blue region to the west is ~4600 m.

The light blue region is called the continental shelf and the deeper region is called the abyssal plain. The area between these two regions is the continental slope.

Task 2

Now download the Activity 1 data file [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] to your computer and then watch Video 2, which demonstrates how to use Google Earth with a file containing information in layers.

Instructions for downloading and opening the data file

Note: the file for this activity may download as a '.zip' file. If so, you will need to change the file extension to '.kmz' in order to open it easily in Google Earth. You can do this selecting the 'Save as' option when you download the file and changing the extension so that the file name ends in '.kmz' (i.e. 's206_1_activity_1.kmz'), then saving the file to your computer.

You may find that double-clicking on your saved data file automatically causes Google Earth to open. However, if this does not happen, you can open the file manually from the 'File' menu in Google Earth by clicking on 'Open…' and then using the resulting dialogue box to navigate to where you saved the data file, selecting it, and clicking 'Open'.

Download this video clip.Video player: Video 2
Skip transcript: Video 2 Using Google Earth with a file containing information in layers.

Transcript: Video 2 Using Google Earth with a file containing information in layers.

INSTRUCTOR
Open the file that you just downloaded from the course website, and Google Earth will open. Now, the picture you get of the screen may be different to the one that you're used to seeing. On the top left-hand side, there's a box, and that box contains Google Earth layers. And these layers are information which we're going to use.
What I would like you to do is to left-click and so select the one which says Depth Section Across. And you can't see the rest of it, but you will in a moment. Then right-click and go to the drop-down menu and say Show Elevation Profile. Now when that happens, you can see that the picture in the main screen reorientates itself. The top of the screen has a yellow line, and the bottom has a graph.
Now I would like to make sure that you've got turned off the water surface effect. So go up to the View menu, left-click, and you can see I've got it selected as a tick. Click with your left mouse button, and you can see that the picture in the main window has now got much, much clearer.
Now that yellow line is a transect which runs from shallow water, light blue, across in the region in which we call the continental slope into deep water. The graph at the bottom represents the water depth along that profile.
And now the really good bit. We can use that transect and that graph to work out the water depth along the line in yellow, and also to work out the average water depth at particular locations. So if you put your mouse on that bottom plot, you can see that straightaway in the view picture we have a red arrow.
Now the minus 124 metres is the water depth. So the water depth is 124 metres at that point. The 2.72 kilometres is the distance from the beginning of the transect, the yellow line. And the minus 0.2% is the gradient of the sea floor at that location.
Now as I move my mouse along this plot in the bottom, you can see that the arrow actually moves water depths, the distance along the transect, and the slope for those particular locations. You can see now that as I go over the continental slope region, it gets much, much steeper. And the water depth's increasing.
And then finally we get to the bottom section. And you can see that the water is much deeper. At that location, 185 kilometres from the start of the section, it's 4563 metres deep. Really quite deep.
And the last thing we can do with this elevation plot is we can actually use it to find the average water depth at a particular location. So if we go back over the continental shelf region with our arrow by moving our mouse, now if I left-click with my mouse button and drag the mouse across, you can see that I've got now a red line on either side of my yellow line in the top picture. That means that I've got the average depth along that bit that's shaded in red.
If you look at the numbers in the bottom window, you can see that the area of shading, it gives us the minimum, the average, and the maximum elevation. So the first thing to look at is the minimum elevation. That's minus 167 metres. Then the average water depth along that section is minus 147 metres. And the maximum is minus 127 metres.
So along that bit that's shaded, the shallowest water depth is 127 metres. The deepest water depth is 167 metres. And the average water depth is 147 metres. And finally, when you look at the range totals just underneath those numbers, you can see that the length that I highlighted was 45.9 kilometres.
End transcript: Video 2 Using Google Earth with a file containing information in layers.
Video 2 Using Google Earth with a file containing information in layers.
Interactive feature not available in single page view (see it in standard view).

Having watched the video, now answer the question below.

Question 3

Open the Google Earth layer 'Depth Section across the continental slope'. This yellow line represents a depth transect across the continental shelf, the continental slope and the abyssal plain.

Using the technique demonstrated in Video 2, what is the average depth and slope of the continental shelf, the abyssal plain and the continental slope? Also record the length of the section you highlighted.

Answer

The average depth and slope of the continental shelf, abyssal plain and the length of the transect length over which the average is generated is shown in Table 1.

Table 1 Average depth and slope of the continental shelf, the abyssal plain and the continental slope.
LocationAverage depth/mAverage slope/°Transect length/km
Continental shelf~1590.250
Abyssal plain~47270.2 and 1.027
Continental slope~24670.4 and -7.549

From your measurements you should be able to see that both the continental shelf and the abyssal plain are astonishingly flat. A slope of between 0.2° and -0.2° over 50 km is flatter than anything one would observe on land. And despite the continental slope looking steep, the average slope is only -7.5°. Features look abrupt and steep in Google Earth and in the way relative heights in charts are represented, when in fact they are not.

Task 3

Now watch Video 3 from the United States National Oceanic and Atmospheric Administration (NOAA), which shows some of the features of the sea floor using the same data you are using in Google Earth.

Download this video clip.Video player: Video 3
Video 3 Features of the sea floor. (There is no audio associated with this video.)
Interactive feature not available in single page view (see it in standard view).

Part 2: Examining other features

Exploring ridges and trenches

Now watch Video 4 which demonstrates how to measure some of the other layers in the Google Earth file you have downloaded.

Download this video clip.Video player: Video 4
Skip transcript: Video 4 Demonstration on how to use layers with Google Earth.

Transcript: Video 4 Demonstration on how to use layers with Google Earth.

INSTRUCTOR
Finally, let's look at some other layers in the Google Earth file you downloaded from the course website. The different layers have different information. If you left click in the North Atlantic Transect, you get a yellow line across the Atlantic Ocean. And Figure 3 in the course shows the water depth along this line.
So if you were to right click and select Show Elevation Profile, you'd get the same plot that comes out. And you can see down the centre of the Atlantic Ocean is the Mid-Atlantic Ridge. Now, if we scroll down, you can see there's Earth's Tectonic Plates and also another layer called the Vent InterRidge.
First we'll select the Tectonic Plates. And you can see I've got red lines, and that red line marks the Mid-Atlantic Ridge across the Atlantic Ocean. And if I now select the Vents and then zoom in a little bit, you can see the location of lots of deep-sea vents in the North Atlantic.
And the different colours represent different things. If I click on one, you can see that that is the Vent Field Southern Oceanographer. It's an active field. It gives the position, the water depth, and there is actually a web link in there as well so you can click on it.
Now, if we unselect those two and select the Scotia Sea and then double click on it, we'll go down to the Southern Hemisphere across an oceanic trench. There's our oceanic trench. And so if we right click our Scotia Sea layer, Show Elevation Profile, perhaps the water depth and on the yellow profile.
So how deep is the oceanic trench? If you put your mouse there, you can see the red arrow, or the deepest point, 7245 metres, 7458, 7446. So a remarkably deep part of the world's ocean.
End transcript: Video 4 Demonstration on how to use layers with Google Earth.
Video 4 Demonstration on how to use layers with Google Earth.
Interactive feature not available in single page view (see it in standard view).

The extremely large mid-ocean ridge and the trenches that you saw in Video 3 are part of the tectonic plate system of the planet. Using the Google Earth layers, explore some of the features you saw in Video 3: the mid-ocean ridge, and the sections across the Marianas Trench (the deepest known location), the Scotia Sea Trench (a more typical ocean trench) and the section shown in Figure 3. You should investigate and note the depths, gradients and general features you observe.

At divergent plate boundaries, tectonic plates are moving apart and new sea floor is being created in undersea volcanic eruptions. In convergent boundaries sea floor is being destroyed.

Question 4

Which sort of tectonic boundary are the Mid-Atlantic Ridge, the Marianas Trench and the Scotia Sea Trench?

Answer

The Mid-Atlantic Ridge is a divergent boundary whereas the Marianas and the Scotia Sea Trenches are on convergent boundaries.

Exploring undersea vents

One of the most exciting discoveries in marine geology of recent years was the discovery of hydrothermal vents, where very hot water escapes from the Earth's crust into the oceans. If you click in the open box next to 'vents_InteRidge_2011_all.kml', all of the undersea vents known up to 2011 will be displayed on your map. The different coloured symbols indicate whether the vent has been directly or indirectly observed, and whether it is active or not.

Question 5

Are the vents typically found on divergent or convergent boundaries?

Answer

Overlaying the vent locations on the tectonic boundaries shows that the vents are typically found on divergent boundaries.

Task 4

To get an idea of the physical environment of a deep-sea vent, now watch Video 5 from the BBC series Planet Earth, and answer the questions that follow.

Download this video clip.Video player: Video 5
Skip transcript: Video 5 Physical environment of a deep-sea vent.

Transcript: Video 5 Physical environment of a deep-sea vent.

NARRATOR
The floor of the Atlantic Ocean is split in two by an immense volcanic mountain chain that winds, unbroken, for 45 000 miles around the globe. In places, it's riven by great fissures, from which superheated water loaded with dissolved minerals blasts into the icy depths. Clouds of sulphides solidify into towering chimneys as tall as a three-storey house.
At 400 degrees, this scalding cocktail of chemicals would be lethally toxic to most forms of life. But astoundingly, a particular kind of bacteria thrives here, and feeding on the bacteria, vast numbers of shrimps. So, beyond the farthest reach of the Sun's power, a rich, independent community exists that draws all its energy directly from the Earth's molten core.
On the other side of the planet, in the Western Pacific, bordering Japan, the dragon chimneys, another series of hot vents erupting in the darkness. Here, more but different bacteria thrive in a similar way. And here, too, more crustaceans, but quite different species from those around the hot vents in the Atlantic.
These are squat lobsters, clad in furry armour, jostling with one another beside the jets of superheated water for the best places from which to graze on bacteria. These vents, too, like those in the Atlantic, are isolated oases, so widely separated that each community is unique.
Cross to the other side of the Pacific to the deep, near the Galapagos Islands, and there are yet other fissures venting superheated water. One and one-half miles down, at a site known as 9 North, towering chimneys support a spectacular display of giant tube worms. These vents give off so much energy that some of the worms reach three metres in length. They're the fastest growing marine invertebrates known. All told, over 50 different species have so far been found living here.
The inhabitants of these bustling communities may grow at speed, but their existence can also be short, for the vents do not erupt indefinitely. Suddenly, unpredictably, they may become inactive.
Nine months have passed at 9 North. What were only recently chimneys teeming with life have turned into cold, sterile mineral monuments. Some eddy deep in the Earth's crust diverted the volcanic energy elsewhere, and an entire micro world was extinguished.
In places, volcanoes have erupted to build great submarine mountains. There are thought to be around 30 000 such volcanoes. Some, measured from the seafloor, are taller than Everest.
Sheer cliffs soaring to drowned volcanic peaks.
End transcript: Video 5 Physical environment of a deep-sea vent.
Video 5 Physical environment of a deep-sea vent.
Interactive feature not available in single page view (see it in standard view).

Question 6

Why is the water leaving the vents like billowing black smoke?

Answer

It is extremely hot and the water contains a chemical cocktail. The sulfides in the water solidify into chimneys and are presumably colouring the water black.

Question 7

How is life able to survive around the deep-sea vents?

Answer

A particular type of bacterium can use the chemicals in the hot water, and this in turn is eaten by shrimps and other animals.

Question 8

What are the fastest-growing marine invertebrates?

Answer

Giant tube worms are the fastest-growing invertebrates.

Question 9

Why do the animals on a deep-sea vent lead a tenuous existence?

Answer

Each community is unique and vents can start and stop rapidly. When they stop, the inhabitants will die.

Exploring Arctic gateways

For the final part of this activity, click in the open box next to the file called 'Arctic Gateways'. This file contains two transects: 'The Bering Strait' and 'The Greenland-Iceland-Scotland Gap'.

Question 10

Looking at the bathymetry across the two transects, which has the deepest depths from the Arctic to the rest of the global ocean? Is it the transect between the Pacific and the Arctic Ocean or the transect between the North Atlantic and the Arctic Ocean?

Answer

The deepest depths are between the North Atlantic and the Arctic Ocean along the Greenland to Scotland transect. Here water depths are in one region > 1000 m. The shallowest depths are between the Pacific and the Arctic Ocean across the Bering Strait, and water depths are only ~ 50 m.

You will see later in the course that the difference in maximum depths between the two ocean basins is critical for the circulation of the global oceans.

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