GEO-113: Environmental Geology

Plate Tectonics Lecture Notes

I. Plate Tectonics - a theory that allows geologists to explain many geologic processes, particularly those at or near the earthπs surface (0-100 km). It is a unifying concept. Central to understanding how geologic processes operate - also a predictive model. Prior to this, many geologic processes were observed but their underlying causes were not known (for example, why do mountain belts occur where and when they do). Present theory gives a consistent picture by tying together many different processes.

II. History of the Idea

It had long been noted that the outlines of some continents fit together like pieces of a jigsaw puzzle.

1865 - Antonio Snider suggested they had actually been together at some previous time.

1910-1930 - Alfred Wegner promoted the idea that the continents had moved or "drifted" around the surface of the earth. Termed the idea as continental drift.

What was his evidence for all this?

1) Not only do the shapes of some continents match up, but also their geology. Similar rocks and structures now separated by oceans line up when the continents are put back together according to their shape.

2) Fossils of the same land animals are found on different continents now thousands of miles apart.

3) Glaciers occurred in areas now near equator. Tropical forests occurred in lands now near the poles. When continents reassembled glaciated areas now at poles and tropical forests now near equator

But Wegner had no causative mechanism. He though continents ≥plowed≤ through the ocean basins like a snowplow. His ideas were ridiculed. He was destroyed professionally, personally, and emotionally.

1940-1965 - Various independent lines of evidence showed that the earthπs outer shell was broken up into plates on which the continents rode (conveyor belt)- much of this came from Princeton- 5 miles down the road.

Our knowledge of the deep oceans was practically non-existent prior to the 1940s. Changed because of WW II and our desire to hide submarines. Wanted to know the age, composition, geology, shape, etc. of the world's oceans. Got a number of surprises.

1) Oceans are young (<200 m.y.) as compared to the continents. Rocks get older as you move away from mid-ocean ridges (MOR; these have 0 age) - continuous chain of undersea mountains.

2) Ocean Floor is 99% Basalt - very simple geology. Lab experiments show that this what you would get if you melted the underlying mantle. Therefore, this is where the melts that form the oceanic crust are coming from

3) Magnetic striping of sea floor - iron-bearing minerals in rocks become magnetically aligned when temperature falls below Curie point (500-800 degree C depending or mineral). Magnetic field reverses periodically (100,000 - 1 m.y.). Normal and reverse striping is symmetric across MOR indicating rocks form at MOR and then spread outward on either side

4) Apparent Polar Wander (APW) - Paleomagnetism on the continents shows either poles have moved or continents have moved relative to poles. Since not all APWs show the same motion, poles aren't moving, but continents are. They really should be called continent wander curves.

5) Earthquakes and volcanoes - concentrated in specific zones or belts. These mark the boundaries of the plates where all the relative motion is taking place. Most earthquakes are shallow (< 100km as at MOR). However, there are places where the earthquakes go very deep (~700km). Mark inclined planes, known as Benioff Zones, where the plates are moving back down (subducted) into the mantle.

All of this evidence leads to the following conclusions:

a) Sea floor created at MOR by melting of the underlying mantle. Melting due to depressurization melting as mantle material rises up beneath MOR.

b) Oceanic crust moves away from MOR - becomes magnetized - stripped due to periodic magnetic field reversals as sea floor spreading occurs.

c) Continents move with oceanic crust - not through it. They are passive passengers on a moving conveyor belt (like luggage and the baggage carousel.

d) Oceanic crust consumed at other locations known as subduction zones. Marked by a deep-sea trench and a zone of descending earthquakes into the mantle.

Continental Drift + Sea Floor Spreading = Plate Tectonics

III. Modern Plate Tectonic Theory

1. Lithosphere - solid rigid plates; form outer shell of the earth. Two types:

1) Continental - about 100-150 km thick

2) Oceanic - about 50 - 100 km thick

Both include uppermost mantle

2. Asthenosphere - from 50-150 km to about 500 km. Partially molten and, therefore, low in strength. Layer over which lithosphere slides. Oceanic lithosphere created at MORs

Melting takes place at MORs by decompression melting of upwelling asthenosphere. Lower pressure lowers melting temperature to about 1200 degrees C.

Continental lithosphere created at subduction zones - place where oceanic plates are consumed by being returned back into the mantle.

3. Plate Boundaries -There are three different types of plate boundaries. Sites of earthquakes and/or volcanoes.

1) Divergent - MORs, spreading centers, and continental rifts. Find earthquakes and basaltic volcanism.

2) Convergent - plates come together. Three types.

A) Ocean - continent: oceanic lithosphere subducted beneath continental lithosphere. Many earthquakes and andesitic-rhyolitic volcanoes. Example: Andes and Cascades. A very dangerous place!

Subducted material heated as it goes down. Eventually it melts, largely due to the presence of water in subducted lithosphere. High temperatures cause water to be released, lowering the melting temperatures in overlying mantle and causing it to melt. Melts rise toward surface and form a chain of volcanoes (island arcs and volcanic chains). Examples include Aleutians, Andes, Japan, and Cascades. Subduction zones are also the sites of intense metamorphism.

B) Ocean - ocean: Cold, denser oceanic lithosphere subducted beneath younger, hotter, and, therefore less dense oceanic lithosphere. Example: Marianas Trench

C) Continent - continent: Little or no subduction of low-density continental lithosphere. Usually just earthquakes. Example: Himalayas

3) Transform - plates slide past one another. Lots of earthquakes. Example: San Andreas.

Motion of plates is due largely to convection in mantle. Convection is due to internal heat of the earth. Also, dense, down-going subducted slabs drag along plates.

Plates move 1-10 cm/year (30 km/m.y. on average). This is as fast as your fingernails grow. Know rates form age of seafloor at various distances from MORs and from hot spots tracks.

Plate Tectonics occurs because interior of the earth is still hot. It does not occur on the moon or Mars because they are too small and have already cooled.

Volcanism continually replenishes atmosphere. Moon and Mars have none or thin atmospheres because little or no volcanism.

NO PLATE TECTONICS-NO VOLCANISM-NO ATMOSPHERE-NO COMPLEX LIFE-NO US

Volcanic action also provides many important minerals. No plate tectonics, no raw materials for a modern society. "Society exists by geologic consent!"

Volcanoes and earthquakes also cause great destruction of property and loss of life. "Subject to change without notice."

IV. Hot Spots -restricted areas of volcanic activity that are not associated with plate boundaries and that remain relatively stationary with respect to earth's rotation axis. Produce a line of volcanoes that increase in age away from the present hot spot.

Example: - Hawaii Midway Emperor Seamount

0 m.y. 25 m.y. 75 m.y.

Give a rate of 11cm/yr for the Pacific Plate.

Hot spots also occur on the continents, such as at Yellowstone National Park.

V. Plate Tectonics and the Rock Cycle

Plate tectonics is a tremendous recycling system. Material transported from one environment to another over 100s of millions of years.

Igneous rocks form at MORs >>> subduction zones: metamorphism occurs >>> melting >>>igneous rocks form continents >>> erosion of continents provides sediment >>> deposition in oceans >>> sedimentary rocks form >>> sedimentary rocks subducted >>> cycle repeats over and over perhaps many times.

Over billions of years the amount of continental crust increases.