Show your work for partial credit. – Read questions carefully and completely. – Round calculations to two decimal places. – Answers should include appropriate units (km, cm/yr, etc.)
- Evidence for Plate Tectonics
The crude fit of present day shorelines of certain continents, matching fossils on landmasses separated by considerable distances, rock type and ages, and structural similarities and comparable mountain ranges on separate continents and paleoclimate evidence (glaciers, fossils from coal fields) provide support for the Theory of Plate Tectonics. Magnetic anomalies assist in the notion that tectonic plates move across Earth’s surface. Mountains, earthquakes and volcanoes are the result of interactions between plates at their boundaries.
Hot spots produce a very unique kind of evidence for plate tectonics. Hot spots are thought to be stationary plumes of hot rock rising from the mantle and melting. Because hot spots remain stationary for long periods of time, and are independent of plate movements, it is possible to use them to determine the rate and direction of movement of overriding plates at different periods of time.
Photo: Hawaiian-Emperor seamount chain collected from http://www.ngdc.noaa.gov/mgg/image/2minrelief.html
The Hawaiian-Emperor Seamount Chain consists of the Hawaiian Ridge of which the Hawaiian Islands are part as well as the Emperor Seamounts, composed of an approximately 3,6000 mile underwater island arc chain within the Pacific Ocean formed over the past 60 million years. This Hawaiian-Emperor Seamount Chain supports the theory that tectonic plates move. Listed below are the age of selected islands of the Hawaiian Ridge and the distance from the hotspot currently located under Hawaii, specifically the Kilauea.
| Island | Distance (km) | Ave. Age (million years) |
| Kilauea (A) | 0 | Present day |
| West Maui | 221 | 1.1 |
| East Molokai | 256 | 1.9 |
| Kauai (B) | 519 | 5.1 |
Data collected from the Hawaii Center for Volcanology website.
To determine the rate and direction of the Pacific Plate:
Useful unit conversions: 1 km=1000 m; 1 m=100 cm; 1 Ma = 1 million yrs ago = 1,000,000 yrs ago
Average rate of motion = distance traveled over a period of time.
- What was the rate (cm/year) and direction of plate motion of the Pacific Plate in the Hawaiian region from 5.1 to 1.9 million years ago? North is at the top border of the image.
- What was the rate (cm/year) and direction of plate motion from 1.9 million years ago to the present? North is at the top border of the image.
- How has the rate of plate motion changed over the past 5.1 million years?
Photo collected from http://pubs.usgs.gov/gip/dynamic/Hawaiian.html
- Locate the Hawaiian Island Ridge and the Emperor Seamount Chain (submerged volcanic islands) above. What does the bend in the chain of seamounts over the past 60 million years indicate? Include a mention of what direction(s) is/are the Pacific Plate traveling?
- What is responsible for the formation of the Hawaiian Island Ridge?
- What is responsible for the formation of the Emperor Seamount Chain?
- Is your answer to questions #5 and #6 one and the same?
- Isostasy
Earth’s crust consists of buoyant blocks that float in gravitational balance on top of the mantle.
Gravity pulls down on an object (a wooden block, an iceberg, a ship) in a fluid. The base of the object is submerged and displaces water. Buoyant force (fluid pressure that increases with depth) pushes up on the object. An equilibrium line separates the submerged base from the exposed top of the object.
When…
The density (r) of the object exceeds the density of the fluid (rob > rfl) … the object sinks
The density of the object is less than the density of the fluid (rob < rfl) … the object rises.
The object displaces a volume of fluid equal to its own mass (rob = rfl) … the object floats.
The crust is supported by the mantle in much the same way. Equilibrium exists between the force of gravity (pulling down) and the buoyant force (pushing up).
Imagine the crust (oceanic or continental) as a block floating in the mantle (rough schematic below). The total height (thickness) of the crust (Hcrust) is equal to the height of the crust above the mantle (Habove) plus the height of the crust within the mantle (Hbelow). In other words: Hcrust = Habove + Hbelow.
Habove
Equilibrium line
Hbelow
The amount of the crust submerged beneath the equilibrium line (Hbelow) depends on the density (r) of the object. The crust is in isostatic equilibrium (floating) when it displaces a volume of mantle equal to the mass of the entire crust. If the crust is 50% as dense as the mantle, then 50% of the crust will be submerged. If the crust is 80% as dense as the mantle, then 80% of the crust will be submerged. We can write this relationship as:
Hbelow = (rcrust/rmantle) x Hcrust.
If we want to determine the amount of the block that is above water, we can write:
Habove = Hcrust – [(rcrust/rmantle) x Hcrust]
If the total thickness of the crust somehow changes, or if the density of the crust changes, then the height of the crust above the equilibrium line will also change.
III. Isostasy and Global Topography
Earth’s continental and seafloor topography have been very accurately measured via satellite data. Calculations of this data illustrate the bimodal characteristic of the Earth’s surface, that is, there are two elevations that are most common. One elevation mode occurs at approximately 0-1 km above sea level and corresponds to continental crust for which the cumulative percentage of land (20.9%) is primarily lowlands. The other elevation mode occurs at 4-5 km below sea level, which corresponds to oceanic crust for which the cumulative percentage (23.2%) of seafloor is primarily abyssal plains. Only 29.2% of Earth’s total land surface is above water.
The average elevation of continents is approximately 0.84 km above sea level. The average elevation of seafloor is approximately 3.87 km below sea level. The difference in average topography is 4.71 km. What accounts for this difference?
Basalt forms most of the ocean crust. Basalt (a rock composed of pyroxene and some plagioclase) has an average density of 3.1 g/cm3. The ocean crust has an average thickness of 5.0 km.
Granite forms most of the continental crust. Granite (a rock composed mainly of quartz and potassium feldspar) has an average density of 2.8 g/cm3. The continental crust has an average thickness of 30.0 km.
Oceanic and continental crust both rest of top of the mantle, which is thought to be peridotite (a rock made entirely of olivine). Peridotite has an average density of 3.3 g/cm3.
- Use the average densities of granite and peridotite and the isostasy equation to determine how high (in km) the continental crust floats above the mantle.
- Use the average densities of basalt and peridotite and the isostasy equation (for Habove) to determine how high (in km) the ocean crust floats above the mantle.
- Why does Earth have a bimodal global topography?
- What would the Earth look like if there were no differences in types of crust?
- Global Examples of Plate Tectonics Divergent Plate Boundaries:
Divergent plate boundaries are areas where tectonic plates (crust and lithosphere) move away from each other. The most prevalent location for this movement is along the Mid-ocean Ridge where new ocean crust is consistently being created at a submerged mountain range. Here, seafloor spreading has led to the separation of Pangaea and the expanse of oceans around the world.
Straddling the Mid-Atlantic Ridge lays the country of Iceland. Iceland provides a perfect analogy of what is occurring during seafloor spreading. This continental crust parting is an example of continental rifting. As the North American plate moves westward relative to the Eurasian plate, Iceland is being torn apart. As a result, Iceland, despite its name and many glaciers, is a volcanically active area aptly represented by the Krafla Volcano and the many lava fountains that are part of the Thingvellir Fissure Zone.
There are several examples of divergent plate boundaries creating or expanding oceans, gulfs and seas. Choose either the East African Rift zone or the Gulf of California area and explore the plate tectonics in the region. Briefly but thoroughly answer the following questions.
- What location did you choose?
- Which plates are involved in the divergent plate boundary?
- Is there evidence of volcanism as a consequence of the divergence? If so, explain.
- Is there evidence of earthquakes as a consequence of the divergence? If so, explain
