chapter 8 the dynamic planet. the 20 th century brought the greatest scientific clarification of...
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CHAPTER 8
The Dynamic Planet
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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]
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This chapter takes the earth science side of physical geography and with the help of some geology gives us geomorphology (Earth form description)
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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
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catastrophism vs.
uniformitarianism
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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
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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
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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
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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
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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]
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The Composition of Earth’s Crust
Earth’s crust is composed of a variety of rocks and
mineralsEach reacts differently to Earth
forces/processes… fall lines and
escarpments… mesas and canyons
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The Geologic Cycle
The Rock CycleRock – 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
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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]
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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 weatheringoxygen+another element
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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)
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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
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(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
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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
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(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
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Plate Tectonics: A Unifying Theory
The current paradigm for Earth landforms is plate tectonics (continental drift)
That continents “fit together” has been noticed in historyFrancis Bacon (1620)Abraham Ortelius (Thesaurus Geographicus, 1596)Robert Sirk (Mrs. Allen’s 5th grade,
1966)
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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
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Support- fossil evidence- climate evidence
Opposition- location?- why the split?- the continent moving force?
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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
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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)
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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