Week 1: When the earth moves: an introduction to vertical and horizontal tectonics through the history of geologic thought.

Introductions.

Reading for the week:

Definition of tectonics: science of trying to understand the large scale movements and deformation of the earth's crust.

Definition of plate tectonics

The earth's outer shell, the lithosphere, is broken up into distinct semi-rigid plates that have moved large distances with respect to each other over geologic time spans. Average rates of motion are in the range of cms/year. Plate interiors are relatively stable. Geologic activity is concentrated at plate boundaries, and the type of relative motion and the type of crust and lithosphere involved are primary influences on the type of activity.

Caution: Note the qualifiers. We will get into an interesting discussion on what defines plate boundary vs. plate interior material later on.

Plate tectonics as a major paradigm in earth science. What does paradigm mean?

Plate motion video - dance of the continents. As we watch some of these videos, you can think of some questions one might ask. In one form or another, it is these questions that we will be addressing.


In class exercise: Computation and consideration of geologic rates.


Fundamental divisions of the earth to be familiar with


Evidence for and ideas on vertical tectonism

Some notable points in the history of thoughts regarding tectonism:

What are various indications of vertical movement, uplift and subsidence?


Geosynclinal theory


Significance of geologic mapping in 1800s. This mapping documented significant contraction and extension of crust. Glarus thrust in Switzerland - 50 km of movement on one sub-horizontal fault! Seuss, at turn of century recognized many more nappes and contractional structures in the Alps. How could this be explained in the world of vertical tectonics?

Image to right is a 1812 painting of Glarus Thrust by Hans C. Escher von der Linth showing the Glarus Thrust as the somewhat inconspicuous, sub-horizontal contact between the darker and lighter colored units. Since this contact could be traced for quite some distance and was a fault, it had important implications for the amount of thrusting that has occurred in mountain belts. Image source: http://en.wikipedia.org/wiki/File:Escher_Martinsloch.jpg.

Link to 2001 article regarding - "Why Plate Tectonics Was Not Invented in the Alps." The author is Trumpy, a well known geologist who focused his efforts on the Alps.

Image below is of a mountain side in French Alps that displays a large overturned fold. The layers are Mesozoic sediments. Such structures, with geometries that require a component of significant horizontal shortening, are abundant in the Alps. From the 1800s and on structures such as these presented a real challenge for those delving into tectonics to explain.

1881 Reverend Osmond Fisher, Physics of the earth's crust:

The importance of isostasy:

Alfred Wegener's contributions to continental drift theory.


Exercise: Evaluation of Wegener's arguments. (30 minutes)

Each group will read a select portion of Wegener's book, and report to the rest of the class on the following. Spend about 15 minutes reading the selection, and 5-10 minutes discussing the answer to the following questions. Summarize your main response on paper to hand in for participation points. Select a spokesperson to report on your findings.


The science of paleomagnetism

The magnetic field at any point is a combination of an internal core component, a rock component (NRM), and an "astronomical" component. The former usually dominates, which is lucky for us as it permits navigation. In our case we are interested in where the rock's magnetic field component comes from?

Two major ideas that come out of looking at NRM histories:

Image of recent polar wandering which contributes to the small yearly changes in magnetic declination and inclination. This recent polar wandering is different from the longer term polar wandering associated with plate motions (as we will see). Image source: http://www.nationalatlas.gov/articles/geology/a_geomag.html


Paleomagnetic field demonstration

Background: Usually a good magnetometer is needed to sense a rock's NRM, but for some rocks particularly rich in magnetic minerals the field the rock generates is strong enough to effect a standard compass. Banded iron formations with magnetite are a good example of such a rock.

Activity:

What can you conclude from this exercise?


Introduction to next week's topic - A geotechnical and geophysical revolution - seafloor spreading.


Bibliography for this week:

Course materials for Plate Tectonics, GEOL 3700, University of Nebraska at Omaha. Instructor: H. D. Maher Jr., copyright. This material may be used for non-profit educational purposes with appropriate attribution of authorship. Otherwise please contact author.