Lecture 11
Late Paleozoic Mountain Building and Pangaea
Keep Reading Chapters 11 and 13
Focus Question: Why are there such large deposits of coal from one particular geologic period?
1. Following
the Devonian Period, the Mississippian and Pennsylvanian Periods included the
gradual formation of the supercontinent PANGAEA , composed of Gondwanaland to
the south (Africa, South America, Antarctica, India, Australia) and Laurentia
and Eurasia.
a.
Immense
forests were the sources of the great coal beds of these two periods.
b.
Additionally, extensive glaciation occurred,
which resulted in sea level drop.
Sea level rise and fall as a response to glacial-interglacial cycles at this
time produced facies changes in the sediments in the shallow seas of the flooded
mid-continents. These facies
packages are CYCLOTHEMS.
c.
CYCLOTHEMS have a standard sequence-first,
nonmarine sediments, then grading to shallow marine and then back to nonmarine
sediments. This is a facies change
.
d.
CYCLOTHEMS
are found in the Missippian and Pennsylvanian Periods worldwide.
2.
Mississippian
and Pennsylvanian time also was a time of the third and final orogenic event
that built the Appalachian Mountains.
This was the Alleghenian Orogeny.
See Ron Blakey’s map of the Late Pennsylvanian Period here:
http://jan.ucc.nau.edu/~rcb7/namPP300.jpg
a.
Africa
(part of Gondwanaland) collided with North America (as Pangaea formed) and the
continent-continent collision folded and faulted the coastline, creating the
Appalachian Mountains.
1.
Piedmont-region
closest to the impact-highly metamorphosed schists and marbles with igneous
intrusions, too.
2.
Blue Ridge – highly metamorphosed Precambrian
rocks exposed
3.
Valley and Ridge -highly folded anticlinal and
synclinal sedimentary rocks
a.
Ridges
are sandstone layers, very resistant to erosion
b.
Valleys are limestones and shales, very easily
eroded
4.
Plateau-region
far from impact, yet still marginally affected.
Relatively flat lying layers-eastern Ohio is included here.
3.
From
Pennsylvanian to Permian time, in approximately 70 million years, 90% of the
world’s coal resources were deposited.
Why? What happened in the
Pennsylvanian Period that led to such an astounding amount of coal?
a.
Pangaea
is forming-lots of land in the tropics and subtropics
b.
The coal is the result of burial and pressure on
immense amounts of plant remains, mostly trees, from immense forests.
c.
The trees were spore-bearing trees that grew in
wet soils, in lowland coastal areas, such as estuaries, streams, riverbanks,
swamps, marshes.
d. The trees were large-they grew from 60 feet (like Calamites) to 100 feet (like Lepidodendrons) high, with extensive root systems.
4.
Coal
forms from plant material that has been buried, has not undergone much decay,
and under elevated pressure and temperature loses its lighter elements, leaving
the carbon as coal. There are
grades of coal based on how much heat it will produce and how cleanly it will
burn:
Anthracite coal-produces the most heat and burns cleanly.
It is the most modified by heat and pressure, and is very hard and low in
dust. This is the most
valuable of the three grades of coal.
Bituminous coal-produces less heat and burns less cleanly.
It is not as modified by heat and pressure, and has lots of impurities
(especially sulfur).
Lignite coal-the least altered of the three grades-the distinct pieces of plant
material can still be seen. Lignite
burns inefficiently, and has many impurities, and produces the least amount of
heat of the three grades.
5.
Why
didn’t all those trees decay completely?
A key player in this problem is the plant polymer Lignin.
a.
Lignin is one of two important plant polymers
(the other is cellulose – what paper is made of).
Lignin adds strength to cell walls, allowing the height to be sustained
in trees.
b.
It appears in higher plants in late Devonian
time, and by Pennsylvanian time, is the key to tree height.
c.
Lignin is very tough for bacteria and fungi to
digest, and only a few kinds of bacteria and fungi break it down today.
The method for breaking it down (digesting it) did not develop until
Permian-Triassic time.
6.
The lack of a means of breaking down lignin led
to the storage of carbon (in the form of the trees) in the sediments.
If we think about the process of photosynthesis on a global scale, then,
we can see the problem:
Carbon dioxide
+ water
in the presence of sunlight
produces carbohydrates (for the plant) and oxygen (released)
a.
Carbon
dioxide is removed from the atmosphere, and isn’t returned-it is buried in the
form of the plants.
b.
Oxygen is released by the plants, but Carbon
dioxide is removed.
c.
The result is a rise in oxygen in the atmosphere,
a drop in carbon dioxide, and a tremendous store-house of carbon, in the form of
the plants…eventually to become coal.
7.
Effect of
rising Oxygen on animals?
a.
Gigantism
and the origin of flight: Insects
reached the largest sizes ever reached in geologic history.
Dragonflies (first in flight) such as Meganeura had wingspans of 75 cm
(about 2.5 feet). Millipedes
reached lengths of eight feet.
b.
Amphibians also reached exceptionally large sizes
8.
Putting
on the breaks…How to stop this?
a.
Cooling
temperatures led to glaciation in Gondwanaland.
Sea levels dropped, coastal, wetland areas started to dry out with sea
level drop.
b.
Fungi and bacteria began breaking down lignin.
c.
Peak of oxygen rise for the Pennsylvanian is
estimated to be about 35%, compared with modern 21%.
Estimates are drawn from several sources, including stable isotopes.
9.
Legacy
effect-the huge coal deposits of the modern world were produced almost
exclusively in Pennsylvanian time under unusual circumstances, but those
deposits fueled the Industrial Revolution and are still a major source of global
energy today.
10.
The
Permian Period is the final geologic period in the Paleozoic.
Several major events occurred during this time:
a.
The high
atmospheric oxygen level seen in the Pennsylvanian period dropped to levels
below modern.
b.
The landmasses converged to build the
supercontinent Pangaea
c.
The global temperatures started rising, so that
the late Permian was much warmer than the Pennsylvanian period
d.
The Permian ended with the Earth’s greatest
extinction event in history
11.
Ecological
changes that should be considered with the development of a supercontinent
include
a.
Continental
landmasses continuously arrayed from pole to pole-lots of possibilities for
migration, gene pool expansion
b.
Large expanses of midcontinental interiors-“continentality”
prevails-higher highs and lower lows in temperature and precipitation swings
c.
Loss of coastal embayments and estuaries with the
convergence of landmasses
12.
The drop
in atmospheric oxygen coincides with
a.
The
extinction of gigantic insects, amphibians
b. The development of Permian and Triassic “redbeds”
c.
The warming of the planet
13.
During
Pennsylvanian time, two important vertebrate groups arose from the stem group of
early tetrapods (like Tiktaalik).
These were the Synapsids and the Diapsids.
Both groups were Amniotes
(vertebrates that produced eggs).
a.
Diapsids-ancestral
line of reptiles, birds
b.
Synapsids-ancestral
line of mammals
14.
By Permian time, these groups included the top
carnivores of the continents, as well as numerous herbivores.
a.
The Permian Gorgonopsians, a group of synapsids,
were common carnivores and herbivores, but became extinct at the close of the
Permian.
