Week 1: When the earth moves: an
introduction to vertical and horizontal tectonics through the history
of geologic thought.
Reading for the week:
- USGS - The Dynamic Earth - This site provides an overview of plate tectonics. Some of this you will have likely encountered before, but it is a good place to start as a refresher.
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
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.
- Retreat of Niagara knickpoint. Niagara River started cutting into the Niagara escarpment
some 10,000 years ago, and has since cut a notch back some 10 km. Compute
the rate of retreat in cm/yr and comment on the rate. Info source
Bates, R. L., 1986, Laboratory Manual in Physical Geology, AGI,
NAGT; Merrill Publishing Company, 177 p.
- Uplift of marine terraces in a tectonic
environment, Fiordland, New Zealand.
Terraces at 370 m elevation above sea level are about 130,000
years old. Compute a rate in cms/yr and comment on this rate.
Info from: Keller, E. A. & Pinter,
N., 1996, Active Tectonics, Prentice Hall, p. 162.
- Sedimentation rates in Svalbard: Some 1900 m of strata were deposited in a Tertiary
basin from roughly 65-32 million years ago, during a time of
active tectonism. Some 380 m of strata were deposited from 265-252
million years ago, during a time of relative tectonic stability.
Compute rate of sedimentation in millimeters per year and comment
on them. Info from: Dallman, W. K. (ed.),
1999, Lithostratigraphic Lexicon of Svalbard, Norsk Polarinstitutt.
- Rate of movement on the San Andreas fault. Volcanic units of identical age, 23.5 million years,
and chemistry are separated by 315 km along the San Andreas fault.
Assuming they were once part of the same volcanic construct that
come up along the fault, what is there rate of separation in
cms/yr. Along another stretch of the San Andreas a stream channel
was offset 250 m from 5,900 to 3,900 years ago. What is the offset
rate? Info from: Mathews, V., 1976, Correlation
of Pinnacles and Neenach Volcanic formations and their bearing
on San Andreas Fault problem: AAPG Bulletin, v. 60. Keller, E.
A. & Pinter, N., 1996, Active Tectonics, Prentice Hall, p.
- Rate of sedimentation in Ridge Basin,
next to the San Andreas. In Late Miocene
to Pleistocene, some 10 million years, has a structural sediment thickness
of 4000 meters. What was the rate of subsidence in cm/yr. Info from: Crowell, J. C., 1984, Origin of Late Cenozoic
Basins in Southern California; in Sylvester, A. G., Wrench Fault
Tectonics; AAPG Reprint Series # 28.
- Speleothem growth from Northern Oman. Using U-Th techniques layers were date in speleothems.
The purpose was to also look at the oxygen-18 isotope values
and pull out a paleoclimate record, but it also allows computation
of how fast speleothems grow (longer term average). In 9,700
years some 320 mm of material was added to one stalagmite, and
in 101,000 years some 2,200 mm were added. Calculate the rate
of speleothem growth in mm/yr. Info from
Burns, S. J., Matter, A., Frank, N., Mangini, A., 1998, Speleothem-based
paleoclimate record from northern Oman; Geology, 26, 499-502.
- Beach ridges on Spitsbergen, glacio-isostatic
rebound: On Kong Karls Land raised
beaches about 10,000 years old are now at 130 m elevation above
sea level. Another beach some 6500 years old in central Spitsbergen
is at an elevation of 11 m above sea level. Calculate the rate
of relative uplift in cms/yr. Info from:
Hjelle, A., 1993, Geology of Svalbard, Norsk Polarinstitutt,
Oslo, Norway, 163 p..
This is a
photograph of the Festningen section area along the west coast
Spitsbergen. Note the modern beach deposits and bench in lower
right. Several geologists are standing on an old marine platform
with a thin veneer of beach deposits. Above them is yet another of these wave cut terraces. Altogether there
may be 5 distinct paleoshorelines preserved here. These resulted from
glacio-isostatic rebound, or the uplift of the crust after ice
has been removed by melting processes. Organic matter (e.g. driftwood,
whale bones) that is found in the beach deposits can be dated
and from this information a history of uplift obtained.
- Glacial surge movement, Usherbreen, Svalbard: In 1978-1980 (3 year span) the glacier front moved forward
300 m. In 1985 it had surged a total of 1.5 km in that one year. Calculate and compare the glacier
movement rates in km/yr for these two different time periods. Info source:
Hagen, J. O., 1987, Glacier surge at Usherbreen, Svalbard; Polar
Research, 5, p. 239-252.
- Erosion in the Big Badlands of North Dakota: The picture to the right is of an elevation marker, and at the time it was installed, 1959, the curved cement surface at the top was flush with the ground. Since the ground surface has deflated, probably mainly by wind erosion. Estimating the ground surface at the time the image was taken (2013) is some 7 inches lower what is the erosion rate in cms/year. Also, is this likely representative of erosion rates in the area?
- Migration rate of Hawaiian volcanism: It is a well known fact that the Hawaiian islands were formed by volcanism and that the islands further north are progressively older, so that the site of volcanism migrated with time. The distance from Maui to Hawaii is 124 km, and the age difference is 1,000,000 years (1 Ma), with Hawaii presently active. Calculate in cm/year the rate of migration of the "spot" of volcanism that made the Hawaiian islands. Info source: http://www4.uwsp.edu/geo/faculty/ozsvath/Assignments/PlateTectonics.htm
- compare the various rates computed, and draw
some general conclusions, especially concerning rates related to tectonics.
Fundamental divisions of the earth to be familiar with
- atmosphere, hydrosphere, biosphere, cryosphere,
- continental crust vs. oceanic crust.
- continental crust:
- up to 3.9 billion years old
- granitoid and heterogeneous in composition.
Quartz and feldspar dominant minerals.
- 30-70 km thick.
- relatively weak, and exhibits considerable
- oceanic crust:
- 180 million years maximum in-place age.
- basaltic and relatively homogeneous in composition.
- 6-7 km thick with some 4 km average water
- relative strong, and distinctly layered.
- crust vs. mantle vs. core:
- image to right from USGS Dynamic Earth. Much has been learned since this was drawn which we will discuss as the course proceeds.
- planetary differentiation (gravity differentiation): crust is the planetary froth that has come
out of the convective turmoil and resultant partial melting of the mantle, and the core is what sunk lower.
- primary differentiate from very early accretionary
history of earth.
- ultramafic in composition; olivine and pyroxene
are dominant minerals at shallow levels, with higher P and T variants below.
- under tremendous range of P-T conditions
and can go through a myriad of phase changes.
- upper and lower mantle.
- outer core liquid iron; therefore convects
- a lot going on at core-mantle boundary that
we are just learning about.
Evidence for and ideas on
Some notable points in the history of thoughts regarding tectonism:
- Leonardo Da Vinci, 1400s - based on marine fossils in the Italian mountains: "From the two lines
of shells we are forced to say that the earth indignantly submerged
under the sea and so the first layer was made, and that the deluge
made the second."
- Nicolas Steno, 1660s - His cosmogeny included collapse of large
aqueous subterranean hollows to cause tectonism and the source of
waters for the deluge.
- James Hutton, 1770's - Uplift and subsidence very
important part of his rock cycle "machine" that provided a home for humanity, with an angular unconformity the marker of past tectonism.
- Charles Darwin, in Voyage of the Beagle: earthquakes and atolls.
- writing of an earthquake he experience in 1822 in western South
- "There can be no doubt that the land
round the Bay of Concepcion was upraised two or three feet ....
The elevation of this province is particularly interesting, from
its having been the theatre of several other violent earthquakes,
and from the vast numbers of sea shells scattered over the land,
up to a height of 600, and I believe 1000 feet ..... it is hardly
possible to doubt that this great elevation has been effected
by successive small uprisings, such as that which accompanied
or caused the earthquake this year, and likewise by an insensibly
slow rise, which is certainly in progress on some parts of the
- Below is show a picture of an atoll (source: http://en.wikipedia.org/wiki/File:Atafutrim.jpg). One of Darwin's earlier contributions was to realize that these were the end result of a volcanic island, that once volcanism ceased, started to sink, while the coral reef continued to build up on its own foundation.
various indications of vertical movement, uplift and subsidence?
- a major proponent was James Dana in 1890s.
- quite a few variants on the basic idea.
- three stage general history:
- development of linear trough of subsidence
with peripheral bulges, and infilling with sediment.
- sediments at depth get heated, producing
magma, and weakening roots.
- trough collapses and inverts to form a mountain
- two paired troughs - miogeosyncline and eugeosyncline, the later marked by a significant volcanic component, which is absent in the former.
- these terms have largely disappeared from the geologic literature.
- the focus was on vertical tectonics, with
subsidiary horizontal or contraction motions. Collapse and contraction
was due to earth's thermal cooling over time and associated shrinkage.
- many unsatisfactory aspects recognized at
the time. What might some
of these have been?
- used as a basic paradigm in Russia right
up to the 1980s (interesting history of science project right
- link to 1948 publication on geosynclinal theory by Knopf.
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:
- developed idea of isostatic compensation.
- postulated fluid interior with convection
- oceans expanding and continents contracting
- moon born from earth.
- a perhaps overlooked pioneer, but did not sway the scientific community of the time.
The importance of isostasy:
- what is isostasy ?
- will explore the significance of isostasy
shortly in another exercise.
Wegener's contributions to continental drift theory.
- 1929, Die Entstehung der Kontinente und Ozeane
(The Origin of Continents and Oceans).
- received a lot of attention, and hence worth
- preceded by Taylor, 1910.
- continental drift was generally rejected
by geologic community with a few minority community exceptions.
One has to wonder what the
reasons for rejection, given the present acceptance of plate
from du Toit (1921), but used by Wegener to show the geologic
similarity between South America and Africa.
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.
- What was the basic argument Wegener was presenting
in the section you read?
- To your knowledge is it correct?
- If not how is it incorrect?
- Considering only this one argument is there
anyway to try and counter it; e.g. is there an alternate explanation
to continental drift?
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
- polar wandering and drift.
- polarity reversals.
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.
- Note the geologic character of the specimen.
- Take the compass and find north direction
at a distance of at least several feet from the specimen. Note
the orientation with respect to the table or building walls.
- Note how the direction changes as you bring
the compass closer to the specimen.
- Note and draw a diagram of how the compass
changes as you circle it around the specimen.
What can you conclude from this exercise?
Introduction to next week's topic - A geotechnical
and geophysical revolution - seafloor spreading.
Bibliography for this week:
- Berkland, J. O., 1979 , Eilisee Reclus --
Neglected geologic pioneer and first (?) continental drift advocate:
Geology, 7, 189-192.
- Biram, J. - 1966 - Translation of Alfred
Wegener, 1929, The Origin of Continents and Oceans; Dover Press,
- Hallam, A., 1983, Great Geologic Controversies
- Chapt. 5, Continental Drift; Cambridge Press, p. 110-156.
- Holmes, A., 1944, The Machinery of Continental
Drift: the Search for a Mechanism; in Principles of Physical
geology, p. 505-509, Thomas Nelson and Sons Ltd.
- Lowman, P. D., Jr., 1983, faulting Continental
Drift, The Sciences, p. 34-39.
- Milnes, A. G., 1979, Albert Heim's general
theory of natural rock deformation (1878); Geology, 7, 99-103.
- Wegner, A. (translated by J. Biram from 4th edition, 1966 (translation date), The Origin of Continents and Oceans: Dover Press, N. Y.
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.