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

mineralsEach reacts differently to Earth

forces/processes… fall lines and

escarpments… mesas and canyons

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

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 weatheringoxygen+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 historyFrancis 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