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CHAPTER 8
The Dynamic Planet
The 20th Century brought the greatest
scientific clarification of Earth structure
… discoveries were made that
revolutionized [a term totally over-utilized]
knowledge of Earth structure and
change processes
…conversely, we know very little about
the “whole” of the Earth system
[why an earthquake we know / when an
earthquake is beyond us]
This chapter takes the earth science side
of physical geography and with the help
of some geology gives us
geomorphology (Earth form description)
Aside
I leave geologic time scale discussions to the
geologists, but the section The Pace of Change
author brings up a potentially interesting
discussion – that of a Anthropocene – a human
epoch in geologic time
… I am not sure how I feel about such a
concept – elevating Man to the position of the
most important species and his activities to
the position of planet shaping / planet
directing
catastrophism
vs.
uniformitarianism
Punctuated Equilibrium
The Earth system seeks dynamic equilibrium and
does not always achieve it. Variations in
insolation or surface albedo; variations in
discharge of internal heat; extra terrestrial
visitation; etc, upset the equilibrium
By definition, catastrophes (meteors, flooding,
etc) have and do play(ed) a role in shaping the
Earth surface
The Earth system seeks a return to dynamic
equilibrium
At any point in time the landform /
landscape of the Earth is the result of
conflict between two forces
(1) building – (nuclear) (endogenous)
tectonics
(2) reducing – (solar and gravity)
(exogenous) weathering;
mass wasting; erosion
The Structure of the Earth
“Scientists still know relatively little
about the interior of the Earth”
Additional data on composition /
processes will make process
understanding; hazard mitigation;
mineral exploration; etc. easier
We have been able to assemble
information only about the Earth’s
crust… and most of that by indirect
means… seismic waves
(1) P(primary) waves
(2) S(secondary) waves
(3) L (longitudinal) waves
Actions of seismographic waves, with
magnetic readings, gravity studies, etc.
suggests that the Earth is composed of
a series of roughly concentric “layers”
(1) Core – Inner and Outer
(2) Mantle
(3) Crust – oceanic and continental
[I like the image and explanation of distance in Fig 8.3]
The Composition of Earth’s Crust
Earth’s crust is composed of a
variety of rocks and minerals
Each reacts differently to Earth
forces/processes
… fall lines and escarpments
… mesas and canyons
The Geologic Cycle
The Rock Cycle
Rock – mineral particles that are
physically separate and chemically
distinct
Mineral – generally inorganic compound
having a distinct compound of elements
… each can be assigned a unique
chemical formula
Combinations of elements give minerals
particular characteristics
hardness; luster; color; etc.
Certain elements unite more readily, and
are more widely distributed spatially
and chemically
Oxygen; Silicon
[see Table 8.1]
Silicates – lg.est/most important
group; formed by crystallization of
cooling magna;
silicon+oxygen+ metal/base
Oxides – not formed “in mass”; more
commonly the result of weathering
oxygen+another element
Carbonates – both organic and
inorganic; ease that carbon bonds
(part. with O2) makes them
important in nature
In halides (chlorine replaces O2) and
sulfides (sulfur replaces O2)
Remaining mineral group is single
element (gold; silver; copper, etc)
Classification of Rocks
We classify rocks and minerals into three types [Fig 8.6]:
(1) Igneous – from cooling and solidification of molten
material (magma); dominant crustal rock
…. Speed of cooling dictates grain size from coarse
(slow) to glassy (faster)
… intrusive forms are often the core of mountain
systems
… extrusive form (basalt) constitutes the bulk of
the floor of the oceans… at 71% of the Earth
surface, this basalt would be the most
frequently occurring rock form
(2) Sedimentary – accumulated material
from degradational forces [weathering
and erosion] or organic sediments [bones;
shells; etc]
- constituent material is transported
[wind; water; ice; gravity] as clastic sediments
[moved mechanically] or chemical sediments [moved in
solution]
and deposited [often far from parent material] in
horizontal “beds” of compacted and
cemented materials - lithification
Stratigraphy studies the sequence, spatial
location and distribution, and
characteristics of these beds in order to
ascertain the age and origin of the
sedimentary material
(3) Metamorphic – [“changed form”] “changed”
from heat [ex.s: tectonic subduction or contact
metamorphism from surrounding molten rock] and/or
pressure [ex.s: tectonics or regional metamorphism of
accumulated sedimentary rock material] into something
“new”
… can happen to any igneous or sedimentary
rock
… can be changed physically and chemically
into a generally denser / more weathering
and erosion resistant rock form
Plate Tectonics: A Unifying Theory
The current paradigm for Earth landforms
is plate tectonics (continental drift)
That continents “fit together” has been
noticed in history
Francis Bacon (1620)
Abraham Ortelius (Thesaurus
Geographicus, 1596)
Robert Sirk (Mrs. Allen’s 5th grade,
1966)
Alfred Wegener
German meteorologist proposed drifting
continents in Origin of the Continents
and Oceans (1915)
Hypothesized continents once part of one
or two “proto-continents” circa 225 mill
B.P.
Pangaea or Lauasia + Gondwanaland
Support
- fossil evidence
- climate evidence
Opposition
- location?
- why the split?
- the continent moving force?
Supporting Evidence for Continental
Drift
It was a half-century before evidence was
derived
(1) paleomagnetism [Fig 8-14]
(2) fossil discoveries
(3) climatology
(4) seafloor spreading [Fig 8-13]
… mid-ocean ridges
… mirrored rock age
… mirrored rock temperature
Tectonic Motion
Convection is the mechanism for plate
movement … transferring Earth
heat/molten material to the surface
(1) divergence (constructive)
(2) convergence (destructive)
… source of global subduction
… on continents builds mountains by destroying
crust
(3) transverse or transform
(hard to say)
Future of Plate Tectonics
The Earth is losing tectonic energy
(1) drifting of the continents will
slow/stop in the far distant future
With building force absent, reducing force
will take over
(2) growth of the continents at the
expense of the ocean basins