Under the Sea
The Earth's crust covers the entire planet. The continental crust, containing the land masses, covers
roughly 1/3 of the Earth. It is about 29-40 km (18-25 mi) thick and composed largely of granite. The
oceanic crust, underneath water, covers 2/3 of the Earth. It is only about 5-10 km (3-6 mi) thick and
made mostly of basalt.
The most noticeable feature on the ocean floor is a 50,000 km (31,000 mi) mountain range that winds
through the Earth's oceans like the seam on a baseball. It rises 3,000 meters or more (almost 10,000 ft)
above the ocean floor, taller than many mountains on the continental crust.
Scientists believe the mid-ocean ridge is evidence that the Earth's crust is split into large plates. The
ridge defines some of the plates' edges. Earthquake and volcanic activity along the ridge add to this
evidence. Past and recent volcanic eruptions in Iceland, which lies on the ridge, are just one example.
The plates at the mid-ocean ridge are spreading apart, about 2.5 cm per year along the Mid-Atlantic
Ridge. Seafloor spreading can form a rift valley in the crest of the ridge, as the
profile in this activity shows. Scientists think molten rock from the mantle is
oozing upward or erupting into the widening split between the plates, forming
new crust.
The Puerto Rico Trench is the deepest place in the Atlantic Ocean, 8,600 meters
below sea level. However, the Marianas Trench in the Pacific Ocean is even
deeper, just over 11,000 meters below sea level.
Columbus traveled from
Spain to the Dominican
Republic in search of the
New World, though his
route was not the straight
line we are mapping. Little
did he know what was
underneath his ship. Use
the table below to plot the
points. Connect the points.
Label the Puerto Rico
Trench and the Mid-Atlantic
Ridge.
Color the part that is water, blue and the part that is land, brown.
Point
Plot the points on your graph paper with your group.
Depth
1
2
3
4
5
6
7
8
9
10
11
12
13
Ocean Floor
sea level
-0.2 km
-2.0 km
-3.0 km
-8.6 km
-8.5 km
-5.5 km
-4.5 km
-3.5 km
-2.7 km
-3.1 km
-2.6 km
-3.3 km
Under the Sea
Answer these questions on a separate sheet of paper with your group.
1)
Rotate the original topography between Dominican Republic and Spain 90 degrees clockwise
about the origin. Label the new coordinates of the Puerto Rico Trench.
2)
Reflect the original topography across vertical line 11. Label the new coordinates of the Puerto
Rico Trench.
3)
Translate the original topography three units to the right. Label the new coordinates of the
Puerto Rico Trench.
4)
Dilate the original topography by a scale factor of 2. Label the new coordinates of the Puerto
Rico Trench.
5)
Dilate the original topography by a scale factor of 1/2. Label the new coordinates of the Puerto
Rico Trench.
6)
Which of the dilations could actually occur?
7)
What do you notice about the transformations you did in numbers 1-3 compared to the
original?
8)
What do you notice about the transformations you did in numbers 4-5 compared to the
original?
9)
Point out the midpoint of the Mid-Atlantic Ridge and circle it.
10)
Use transformations to describe or model sea floor spreading. What might the topography of
the region look like in the future?
11)
Estimate the area of the cross section.