Questions to ask yourself:
- Where is the deepest part of the ocean?
- Where are the youngest rocks in the ocean?
- Where are the longest mountains in the world?
During World War II, geologists hired by the military carried out studies of the sea floor, a part of the Earth never before studied. The purpose of these studies was to understand the topography of the sea floor to find hiding places for both Allied and enemy submarines. Data gathered by oceanographic surveys conducted by many nations led to the discovery that a great mountain range on the ocean floor virtually encircled the Earth. Called the global mid-ocean ridge, this immense submarine mountain chain -- more than 50,000 kilometers (km) long and, in places, more than 800 km across -- zigzags between the continents, winding its way around the globe like the seam on a baseball.
Rising an average of about 4,500 meters (m) above the sea floor, the mid-ocean ridge is bigger than all the mountains in the United States except for Mount McKinley in Alaska (6,194 m). Though hidden beneath the ocean surface, the global mid-ocean ridge system is the biggest topographic feature on the surface of our planet.
Mid Ocean Ridges are the longest mountain ranges in the world!
- The oldest rock found on the continents is about 4.5 billion years old.
- How old are the oldest ocean rocks?
- Which ridge system is moving faster and how do you know?
Evidence for Sea-floor Spreading
- Oceans are deepest near the continents in deep ocean trenches.
- Oceans have giant mountain chains in the middle
called mid-ocean ridges. - The crust is hotter near the ridges.
- The age of the ocean rocks is younger near the ridges.
- The farther from the ridge the older the oceanic rock.
- This suggests that ocean crust is formed at the ridges.
Paleomagnetism: Ancient magnetics of the earth
The Earth has a magnetic field that causes a compass needle to always point toward the North magnetic pole, currently located near the North rotation pole. The magnetic field is composed of lines of force as shown in the diagram.
Reversals of the Earth's Magnetic Field: Studying piles of lava flows on the continents geophysicists found that over short time scales the Earth's magnetic field undergoes polarity reversals (The north magnetic pole becomes the south magnetic pole).
- In the 1950's it was discovered that when magnetic minerals cool below a temperature called the Curie Temperature the minerals align with the Earth's magnetic field lines. We can therefore determine the orientation of the magnetic field present at the time the rock containing the mineral cooled, and determine the position of the magnetic poles at that time.
Magnetic Striping: Ships towed a magnetometer to detect submarines. These records also showed that there were magnetic anomalies on the sea floor. Rocks along the ridges were found to have magnetic stripes showing polar reversals on both sides of the ridge.
- New oceanic crust and lithosphere is created at the oceanic ridge by eruption and intrusion of magma. As this magma cools it takes on the magnetism of the magnetic field at the time. When the polarity of the field changes new crust and lithosphere created at the ridge would takes on the different polarity. This led to the theory of sea floor spreading.
Sea Floor Spreading in a nutshell: View animations!
- When igneous rocks cool they trap the earth's magnetic field orientation.
- The earth's magnetic poles have switched directions many times.
- Magnetic striping occurs along mid-ocean ridges.
- Evidence: Sea-floor is being created and is spreading apart at the ridges.
Did you know that some of the strangest life forms on earth live at mid-ocean ridges? Click on Dive and Discover and learn more about the new frontier that we have deep under the ocean.
Student Activity: As one student pulls the oceanic plate "paper" the other student colors the magnetic polarity.
- Chose two different color crayons
- Fold your paper and place it in between the desks
- One person coloring and the other slowly pulling the paper out between the desks color each polar reversal using the two colors
Label:
- Oldest and youngest colored layer
- Normal and reverse polarity
- Mid-ocean ridge
- Draw arrows representing north direction
- Compare different student examples and see which one has a faster seafloor spreading rate.
View Animations from WGBH... Compare it to the "Ring of Fire
Plate Tectonics Theory
Move the plates like a puzzle!
Read This Dynamic Earth from USGS
Video: Plate Tectonic Revolution
1. Combining the evidence of continental drift and sea-floor spreading leads to Plate Tectonics.
- Like an apple's skin the crust of the earth is comparably very thin.
- This skin is broken into large plates that are constantly moving due to forces within the Earth.
- These moving plates create earthquakes, volcanoes, mountain ranges, deep-sea trenches and mid-ocean ridges.
- Tectonic Plates are large sections of the crust and solid upper mantle.
- Together they make up what is called the Lithosphere (sphere of rock).
- There are 7 major Lithospheric Plates and a few minor plates.
- The continents and oceans ride on top of these Lithospheric plates
2. How do we know where the Tectonic Plates are located?
3. Why do the Tectonic Plates move?
- Isostasy: Like an iceberg floating on water, the continents and the ocean basins, are floating on the Earth's mantle.
- It's all due to Density!
Check this out: Buoyancy and Floating Continents - Heat trying to escape the inside of the earth create currents.
- These Convection Currents in the Mantle (Asthenosphere) move the tectonic plates.
- Hot liquid rises to the surface and cool sinks
The Tectonic Plates = Lithospheric Plates and are topped by either continental or oceanic crust.
Continental Crust: Thicker and less dense than oceanic crust, made mostly of igneous Granite rock.
Oceanic Crust: Thinner and more dense than continental crust, made mostly of igneous Basalt rock.
USGS diagram of the 4 Plate Boundary types
Animations of the boundaries!
Divergent boundaries -- where new crust is created as the plates pull away from each other (Dividing).
Mid-ocean ridges are created at divergent boundaries
Convergent boundaries -- where tectonic plates are coming together.
- Subduction: Where crust is destroyed as plates collide and one plate dives under another creating volcanic mountains and a deep sea trench.
- What do you notice in this photo?
- What would happen to any islands?
Accretion Terranes -- land added to continents through the process of collisions. Did you know that the entire west coast of the U.S.A. is new land?
- Accretion Terrains
- Collision Boundaries -- where two continental plates collide producing giant mountain ranges View from Space!
- Mount Everest was created by India colliding with Asia.
Transform boundaries -- where crust is neither produced nor destroyed as the plates slide horizontally past each other.
- Transform Faults produce a lot of earthquakes.
- The Great San Francisco Earthquake of1906
- San-Andreas Fault in California is a transform fault.
- Plate boundary zones -- broad belts in which boundaries are not well defined and the effects of plate interaction are unclear.
- Active continental margin -- when the edge of a continent is located along a tectonic plate boundary.
ex. West coast of South America (mountains and trenches)
- Passive continental margin -- When the edge of a continent is not located on a tectonic plate boundary.
ex. East coast of North America (no mountains or trenches) - Hot Spots: Stationary areas where molten magma continually breaks through the lithosphere.
- As a plate moves over a hot spot, volcanic island chains are created.
- Video: View the Hawaiian Islands in formation
- Google Earth Tour
Using colored pencils and your reference tables:
- Color the divergent boundaries red
- Color the transform boundaries blue
- Draw extra arrows showing plate movement
- Color all subducting plate arrows purple.
- Color all hot spots red
Lets put it all together!
Evidence in support of the Theory of Plate Tectonics:
- Shape of the coastlines
- Fossil evidence
a. Glossopteris flora (plant fossils)
b. Late Paleozoic seed ferns
c. Gondwanaland (India, Africa, Australia, S. America, Antarctica)
d. Mesosaurus (aquatic reptile)
- Rift Valleys of Africa - (continent breaking up)
- Geologic similarities between S. America and Africa
a. Same stratigraphic sequence (i.e. same sequence of layered sedimentary rocks)
b. Mountain belts and folded rocks line up
- Paleoclimatic evidence (ancient climatic zones match up)
a. Layers of glacial deposits are found at same place in sequence of rocks
*Note directions of glacial ice movement as indicated by striations or grooves in the rock
- Apparent youth of ocean basins and sea floor
a. Thin layer of sediment on basalt
b. Basalt dates to less than 200 million years (most less than 150 million years)
- Evidence for subsidence in oceans -
a. guyots - flat-topped sea mounts (erosion when at or above sea level)
b. Chains of volcanic islands that are older away from site of current volcanic activity - Hawaiian Islands and Emperor Sea Mounts
(also subsiding as they go away from site of current volcanic activity)
- Mid-ocean ridges located near ocean centers
a. High heat flow
b. Seismic wave velocity decreases due to high temperatures
c. Valley along center of ridge (graben)
d. Volcanoes along ridge
e. Earthquakes along ridge
- Benioff Zones - inclined zone of earthquake foci (plural of focus) near deep sea trenches
- Magnetic stripes on the sea floor
Yet there are many crazy explanations for Plate Tectonics!
The Expanding Earth?
Review Questions:
1. How does the composition of the oceanic crust compare to the continental crust?
a. The oceanic crust is mainly granite while the continental crust is mainly basalt.
b. The oceanic crust is mainly limestone while the continental crust is mainly granite.
c. The oceanic crust is mainly basalt while the continental crust is mainly sandstone.
d. The oceanic crust is mainly basalt while the continental crust is mainly granite.
2. As you travel from an ocean shore to the interior of a continent, the thickness of the Earth's crust generally:
a. decreases b. increases c. remains the same
3. The boundaries between all of the tectonic plates is best described as sites of:
a. frequent crustal activity
b. deep ocean depths
c. continental boundaries
d. magnetic age patterns
4. Which provides the best explanation of the mechanism that causes tectonic plates to move?
a. faulting of the lithosphere
b. convection currents in the mantle
c. the spin of the Earth on its axis
d. prevailing wind belts of the troposphere
5. According to the theory of plate tectonics, the distance between two continents on opposite sides of a mid-oceanic ridge will generally
a. increase b. decrease c. remain the same
6. Which feature is commonly formed at a plate boundary where oceanic crust converges with continental crust?
a. a mid-ocean ridge
b. an oceanic trench
c. a transform fault
d. new ocean crust
7. Coal beds found in sedimentary rocks of Alaska suggests:
a. Alaska's cold climate fossilized the coal
b. Coal usually forms in cold climates
c. Alaska once had a tropical environment
d. Plate tectonics formed the coal in a trench
8. Which is he best evidence supporting the concept of ocean floor spreading?
a. Earthquakes occur at greater depths beneath continents
b. Sandstones can be found both in North America and Europe
c. Volcanoes appear at random within the oceanic crust
d. Igneous rocks along the ridges are younger than those farther away from the ridges
9. Where are the deepest parts of the ocean found?
a. near the continents in subduction zones
b. in the middle of the oceans
c. near passive continental margins
d. near hotspots
10. Magnetic striping occurs along mid-ocean ridges due to:
a. subduction and convection
b. seafloor spreading and subduction
c. polar reversals and subduction
d. seafloor spreading and polar reversals
