Geography 40
Global Environmental Change
Fall 2002



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Lecture 12 Solid Earth Circulation

CIRCULATION OF THE SOLID EARTH

Introduction:
- Wegener’s theory of Continental Drift – around 1920s: he noticed that continents looked like they could fit.
- Posited that they were all connected in the past as Super-continent: Pangea.
- Not accepted because no driving mechanism was proposed that could move continents.
- Later, seismic studies probed the earth’s surface and interior composition.
- In the 1960s geophysicists exploring the topography (elevation differences between high and low points) and magnetic features of the ocean floor. Maps had been made during and after WWII, showing long linear volcanic mountain chains along the ocean floor, called Mid-Ocean Ridges. Scientists proposed that these chains represent new sea floor that comes up through the ridges and spreads on either side of the rift in a process called Sea-floor Spreading. This was a critical bit of evidence for Continental Drift, because it postulated a driver for continental drift. Further evidence came from the magnetic orientation of the sea floor.
- Driving the study of Earth Science is the principle of Uniformitarianism: proposed in the late 18th century by James Hutton. This principle contends that the same processes we see at work today in shaping the earth’s environment have been operating throughout time and thus we can argue that the "present is the key to the past". Gradual processes taking place over long periods of time have shaped the mountains and valleys we see today. This principle is in contrast to the belief that Earth is very young and the features we see today are the result of catastrophes. This belief, Catastrophism, has relatively little support scientifically. However, while processes that shape the earth have been going on for a long time, geologic time is punctuated by extreme events, or catastrophes, such as meteors hitting the planet inducing extinctions; massive landslides, massive earthquakes and volcanic eruptions. These catastrophes are local in scope, and are relatively small in the overall shaping of the earth.

Structure of the Earth

Figure 8-2 (Elemental Geosystems Text) – cross section of earth’s interior.

From the outside in (i.e., surface to the core):
- The Crust. Not homogenous: varies in thickness and composition.
- Most pronounced differences are between the continental crust and the oceanic crust.
o Continental crust: underlies the continents; is thicker and less dense. Also is on average older. Oldest rocks are found on the continents – oldest rock known is found in NW Canada and is ca. 3.96 BY old.
o Oceanic crust. Underlies oceans; is thinner, denser and average age is younger (oldest rocks found on the ocean floor are only on order of 100s of million years.

- Classification of Crustal Rocks. All rocks are composed of minerals.
o Igneous
form by cooling and solidification of magma (from mantle).
- Intrusive (cools below surface) e.g., granite
- Extrusive (cools at surface) e.g., basalt
- Igneous rocks vary in composition, especially in amount of mineral quartz (SiO2).
- Felsic igneous rocks are light colored, less dense, quartz rich, common to continental crust.
- Mafic igneous rocks are darker, denser, less quartz; common tin oceanic crust.
o Sedimentary
any type of rocks exposed at surface weather (decompose) into finer materials called sediments – layers of unconsolidated mineral matter transported by water, wind or gravity.
- Process of lithification: sediments form cohesive, thick layered deposits called sedimentary rocks.
- Sandstone (original sediments were sand sized)
- Mudstone (original sediments were silt sized or finer)
o Layered mudstones called shales
- Limestones: bio-chemically formed rocks. When calcium carbonate (CaCO3) is precipitated into shells of living organisms (can have some that precipitate chemically, without living organisms). Shells deposited and then lithify.
o Metamorphic
when any type of rock is exposed to high temperatures and/or high pressures they are transformed both in mineralogical and chemical compositions. As long as no melting has occurred, then these altered rocks are metamorphosed, and called metamorphic rocks. (Melted rocks reform into igneous).
- Marble (from limestone)
- Schist (from shale)
- Gneiss (from granite)

- Sedimentary cover
o Sediments and sedimentary rocks overlay the igneous rocks of the oceanic and continental crust.
- In the oceans, sediments settle through the water column and are laid flat on the sea floor (relatively).
- On the continents, some accumulated in basins on the continents themselves.
- But most originally were laid on the ocean floor and then transported onto continents through tectonic activity. The relatively flat layering was then deformed through tectonic uplift. The oldest parts of continents were once sedimentary rocks that have significantly deformed and altered through many cycles of tectonic activity and metamorphism.

The Mantle
Lies below the crust, extending to the top of the fluid outer core.
- Figure 8-2 further divides the mantle into the uppermost mantle, the Asthenosphere, Upper mantle and lower mantle.
- The exact structure and composition of the mantle is still not known for sure
- Most of what is known is inferred from seismology
o Velocity structure of the mantle (derived from seismic studies) indicates that the mantle is relatively uniform in composition and formed of silicate minerals.
o But, as depth increases, so do pressure and temperature.
- Asthenosphere from ca. 80 km to ca 300 km - presence of some molten rock at this depth detected in seismic studies. Could be only about 1% molten, but that small amount of melt is all that is needed to allow the cust and the upper mantle to move relative to the underlying mantle. Critical to theory of Plate Tectonics.

The Core
Believed to be dominated by iron, a very dense mineral, and small amounts of other minerals like nickel and trace amounts of oxygen or sulfur. This composition of minerals is much denser than overlying mantle and seismic waves reflect off the core-mantle boundary.
- The core is ca 3000 km from the surface of the earth, but still affects conditions on the surface because it is the source of Earth’s magnetic field.

Lecture 13

Summary of points from lecture 12:
- The structure of the Earth extends from the Lithosphere, which comprises the crust and the uppermost mantle below which is the Asthenosphere, to the outer and inner core.
- For the most part, we are primarily concerned for this course with the top 250 km of earth (crust + asthenosphere).
- Note the material that has made up the earth has differentiated over the history of the earth, with the more dense material (like iron) making up the core, and increasingly less dense outward.
- The outer core is believed to be liquid molten iron and it is here that the Earth’s magnetic field is generated. Thus changes in the Earth’s magnetic field, particularly in its polarity originate here.

Lithosphere:
- continental crust and oceanic crust resting on top of the rigid uppermost mantle (combined = lithosphere), in turn resting on top of the plastic Asthenosphere.
- This figure (8.4) shows how the crust is constantly adjusting to changes in load: so mountain mass sinks, displacing the mantle below;
o Weathering and erosion lead to sediments building up and deforming the ocean crust below them.
o Rock samples\

Theory of plate tectonics
Check out website: http://jazz.wr.usgs.gov/atlas/sfbay/ ,

- Sea floor spreading. WWII mapping of sea floor and its magnetic patterns.
o First clues – evidence of chains of sub-sea volcanic mountains running down the centers of ocean basins – Mid-ocean ridges.
- A rift, narrow valley, runs down the center of the ridges
- Proposed that these linear volcanic chains were extruding new sea floor
- Hot young material comes out, expands near the spreading centers and as it spreads out to either side of boundary at ridge axis –
- Material cools and contracts, causing sea-floor to subside.
- Creates symmetrical undersea mountain belts 1000 – 4000 km wide that rise 2 to 3 km from the floor.
o Magnetic polarity of the rocks on the sea floor: reversals
- Rocks on the sea floor are basaltic, known to be of volcanic origin.
- Magnetic polarity reversals were discovered in volcanic rocks on land
- When lava cools, the rocks become magnetized in the direction of the Earth’s magnetic field (at time of cooling). Todays magnetic field is considered "normal" and opposite polarity is a "reversal".
- So sea floor spreading explains this pattern seen in the sediments, with older sediments as we move in either direction away from the spreading axis.
- Sea-floor spreading explains how continents could "drift" through the rigid sea floor: Answer was that the continents don’t – the continents and segments of ocean floor are connected into plates that continuously move away from one another at the mid-oceanic ridges.

Lithosphere is made up of about 20 (book says 14) plates that extend through the crust into the uppermost mantle.
Lithospheric plates:
- Oceanic lithospheric plate is topped only by oceanic crust.
- Continental lithospheric plate describes the part of a plate that is topped by continental crust.
- Plates move relative to each other at average speeds of a few centimeters per year.
- Friction between plates causes periods of stasis (when stress is building) and movement (stress is released)
- Plate Boundaries:
o Divergent margins – plates moving away from each other
o Convergent margins – plates moving towards each other
o Transform margins – slipping past each other
o Associated surface features: mid-ocean ridges, deep-sea trenches, transform faults.
- Divergent Margins: stresses pulling apart the lithosphere. Most divergent boundaries are mid-oceanic. Rift valleys on land represent sites of continental crust stretches (eg, East Africa)
- Convergent Margins: regions where 2 plates come together.
o The Earth is not increasing in size, so if new sea floor is produced at mid-ocean ridges, old sea floor must be removed somewhere.
- Deep-sea trenches are long narrow very deep basins that are the sites where plates converge.
- There are 3 types of convergent plate boundaries:
- Oceanic – Continental boundaries: denser oceanic plate forced to sink – subduction. The plate (slab) going down subsides into the mantle, bending and creating the deep linear trench.
o Friction between down-going plate and overriding plate generates seismic activity near the surface.
o Melting off the top of the slab produces igneous rock at the surface, resulting in volcanic arc.
- Oceanic – oceanic boundaries: one subducts beneath the other. A range of volcanic mountains forms on one side of the trench, rising up on the sea floor rather than on land. If they reach the surface, they are called Island arcs. E.g., Marianas Islands
- Continental – continental boundaries: continents collide abruptly. Too buoyant to subduct, so the crustal portion of plate separates from mantle portion. Mantle below may subduct, while the continental crust on both plates crumples. Consequence is tall mountain belts and high plateaus. E.g., India converging onto Asian plate starting 50 million years ago and continuing today.

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