GEO 200: Physical Geography

Introduction to Landform Study

Big topic

• The Earth’s size is difficult for humans to comprehend.– Diameter: 12,800 km

– Circumference: 40,000 km

• Geographers can, however, largely ignore the interior and focus on the surface.

The unknown interior, part 1

• Humans have not penetrated more than one-thousandth of the Earth’s radius.

• We have inferential knowledge of the interior, through monitoring shock waves transmitted through Earth from earthquakes or from human-made explosions.

– Our knowledge is incomplete.

– Scientist have deduced that the Earth has a heavy inner core surrounded by three concentric layers of various composition and density.

The unknown interior, part 2

• Inferential knowledge (continued)– The four regions are the crust, mantle, outer core,

and inner core.• The crust is the outermost solid layer of Earth, consisting of

a broad mixture of rock types.– On average, the crust is three times as thick under continents

as under the ocean.

– It makes up less than 1 percent of Earth’s volume.

– The Mohorovičić discontinuity (Moho) is the boundary between the crust and mantle.

– It is thought to be a narrow zone with a significant change in mineral composition.

The unknown interior, part 3

• Inferential knowledge (continued)– Four regions (continued)

• The mantle is that portion of the Earth beneath the crust and surrounding the outer core, to a depth of about 2,900 km.– It has the largest volume of all four shells.

– Scientists believe there are three zones within the mantle: lithosphere, asthenosphere, mesosphere.

– The lithosphere is the uppermost zone of mantle and the crust together. (The term also refers to tectonic plates consisting of the crust and upper rigid mantle. It is also sometimes used as a general term for the entire solid Earth.)

The unknown interior, part 4

• Inferential knowledge (continued)– Four regions (continued)

• The mantle (continued)– The asthenosphere is a plastic layer of the upper mantle that

underlies the lithosphere. Its rock is very hot and therefore weak and easily deformed.

– The mesosphere is the rigid part of the deep mantle, below the asthenosphere. (The term also refers to the atmospheric layer above the stratopause).

• The outer core is the (molten) liquid shell beneath the mantle that encloses Earth’s inner core.

• The inner core is the supposedly solid, dense, innermost portion of Earth, believed to consist largely of iron/nickel or iron/silicate.

The unknown interior, part 5

• Inferential knowledge (continued)– Understanding of crust and upper mantle has

fundamentally changed in last three decades.• Continental drift is the theory that proposes that the present

continents were originally connected as one or two large landmasses that have broken up and literally drifted apart over the last several million years.

• Plate tectonics, a descendant of continental drift, is a coherent theory of massive crustal rearrangement based on the movement of continent-sized lithospheric plates.

Focus on the surface

• Geographers are interested in the Earth’s interior only as it influences topography and other surface features, how it affects the nature and characteristics of surface.

Composition of the crust, part 1

• A mineral is a naturally formed inorganic solid substance that has an unvarying chemical composition and characteristic crystal structure.

– About 3,500 minerals are known, with about 50 new types discovered each year.

– Fewer than 20 minerals account for more than 95 percent of composition of all continental and oceanic crust.

Common minerals, part 1

• There are seven principal categories of rock-forming minerals (on basis of chemical properties and internal structure).

– Silicates are a category of minerals composed of silicon and oxygen combined with another element or elements.• They are the largest and most important group; most are hard and

durable.

– Oxides are a category of minerals composed of oxygen combined with another element.• Quartz has chemical composition of oxide, but classified as

silicate because of its internal structure.

Common minerals, part 2

• Seven principal categories (continued)– Sulfides are a category of minerals composed of

sulfur, combined with another element or elements. • They include many of the most important ore minerals.

– Sulfates are a category of minerals composed of sulfur and oxygen, combined with another element or elements. • Calcium is the principal combining element.

– Carbonates are minerals that are a carbonate compound of calcium or magnesium.

Common minerals, part 3

• Seven principal categories (continued)– Halides are a category of minerals that are notably

salty.• They are the least widespread.

– Native elements are those minerals that aren’t combined chemically with others, but appear as discrete elements (e.g., gold and silver).

Composition of the crust, part 2

• Magma is a molten material in Earth’s interior.– The quantity is unknown.

• Rock is a solid material composed of aggregated mineral particles (in lithosphere).

• An outcrop is the surface exposure of bedrock.• Bedrock is a buried layer of residual rock that

has not experienced erosion.

Composition of the crust, part 3

• Igneous rocks– Igneous rock is rock formed by solidification of molten

magma.• There are many kinds, but the principal shared trait is

crystalline structure.

– Extrusive rock is molten rock ejected onto Earth’s surface, solidifying quickly in the open air.

– Intrusive rock is rock that cools and solidifies beneath Earth’s surface (it may be pushed up to surface or exposed through erosion). • Granite is the most common and well known.

Composition of the crust, part 4

• Sedimentary rocks– Sediment is small particles of rock debris or organic

material deposited by water, wind, or ice.

– Sedimentary rock is rock formed of sediment that is consolidated by the combination of pressure and cementation.• During sedimentation, materials are sorted roughly by size

(the finer particles are carried farther than heavier particles).

• Strata (plural; stratum, singular) are distinct layers of sediment which result in parallel structure (stratification), with layers varying in thickness and composition.

Composition of the crust, part 5

• Sedimentary rocks (continued)– Sedimentary rock formation and consolidation

(continued)• Sedimentary rocks are categorized by how they are formed:

mechanically, chemically, or organically. – Mechanically accumulated: fragments of preexisting rocks (for

example, shale and sandstone).

– Chemically accumulated: precipitation of soluble materials or chemical reactions (for example, calcium carbonate and limestone).

– Organically accumulated: remains of dead plants or animals (for example, coal and limestone).

Composition of the crust, part 6

• Metamorphic rocks– Metamorphic rock is rock that was originally

something else (igneous or sedimentary) but has been drastically changed by massive forces of heat and/or pressure working on it from within Earth.• Process recrystallizes and rearranges mineral components.

• The processes are somewhat predictable, such as the fact that limestone tends to metamorphose into marble.

• Sometimes the metamorphosis is so great, the nature of the original rock cannot be determined.

• The most common metamorphic rocks are schist and gneiss.

Metamorphic Rock Foliation

Composition of the crust, part 7

• Relative frequency of rock classes– The lithosphere has a very uneven distribution of

sedimentary, igneous, and metamorphic rocks.• Sedimentary rocks are dominant on the surface of the Earth.

– This dominance, however, is only on the surface, as sedimentary cover is not thick.

– The average thickness of sedimentary rock is less than 2.4 km.

• Assume that igneous rocks make up the bulk of the crust, but metamorphic rocks might because of the enormous pressures at play beneath the crustal surface.

Critical concepts, part 1

• Basic terms– Topography is the surface configuration of the Earth.

– A landform is an individual topographic feature, of whatever size.

– Geomorphology is the study of the characteristics, origin, and development of landforms.

– Relief is the difference in elevation between the highest and lowest points in an area (e.g., the vertical variation from mountaintop to valley bottom).

Critical concepts, part 2

• Uniformitarianism– Uniformitarianism is the concept that “the present is

the key to the past” in geomorphic processes. The processes now in operation have also operated in the same way in the past and should also operate in the future.

Critical concepts, part 3

• Geologic time– Geologic time enumerates temporal expanses of

almost unfathomable scope.• Geologic time encompasses millions and hundreds of

millions of years.– Four eras, three most recent being subdivided into 12 periods.

Two most recent periods divided into seven epochs.

Critical concepts, part 4

• Geologic time (continued)– Unfathomable scope (continued)

• The following table offers a chart of geologic time expressed in an equivalent one-year scale.– For the first four months, the planet was lifeless.

– Single-celled life appeared in early May.

– Multicellular organisms began evolving in early November.

– Antediluvian fishes, the first vertebrates, appeared about November 21.

– Amphibians appeared late November.

– Vascular plants appeared about November 27.

– Reptiles began era of dominance about December 7.

Critical concepts, part 5

• Geologic time (continued)– Unfathomable scope (continued)

• One-year scale (continued)– Mammals arrived about December 14; birds arrived December 15.

– Flowering plants arrived December 21.

– On December 24, the first grasses and first primates appeared.

– The first hominids came New Year’s Eve.

– Homo Sapiens arrived one hour before midnight.

– The age of written history equals the last minute of the year.

• If geologic time was examined in the context of cliff one km high, an individual’s existence would equal less than the thickness of the finest hair.

Study of landforms, part 1

• Land surfaces alone encompass 150 million square km scattered over seven continents and innumerable islands.

– Such study is a complex endeavor that requires an organized approach, including examination of the following four elements.• Structure includes the nature, arrangement, and orientation of

the materials in feature being studied. – It is the geological underpinning of landforms.

Study of landforms, part 2

• 150 million square km (continued)– Four elements (continued)

• Process includes the actions that have combined to produce the landform.– Process encompasses the interaction of forces such as geologic,

hydrologic, atmospheric and biotic.

• Slope is a fundamental aspect of shape for any landform.

• Drainage refers to movement of water (from rainfall and snowmelt).

• After identifying the above four basic elements, geographer can analyze topography.

Study of landforms, part 3

• Fundamental questions of geographic inquiry:– What? (the form of features)

– Where? (the distribution and pattern of landform assemblage)

– Why? (an explanation of origin and development)

– So what? (the significance of the topography in relationship to other elements of environment and to human life and activities)

Geomorphic processes, part 1

• A variety of topography reflects the complexity of interactions between process and structure.

• Internal and external processes– Internal processes operate within the Earth, drawing

energy from heat.• In general, they are building forces, increasing relief of land

surface.

Geomorphic processes, part 2

• Internal and external processes (continued)– External processes operate at the base of atmosphere

(subaerial), drawing energy from sources above t/he lithosphere (atmosphere or oceans).• External processes are better understood than internal

processes; their behavior is predictable.

• In general, they are wearing-down or destructive forces, diminishing relief of land surface.

Question of scale

• Scale of observation affects how one perceives and understands a landscape.

• The following images offer an example of different perspectives of scale, from the largest scale of ordinary human experience, walking though a landscape; to driving thorough it; to flying over it; to a satellite’s viewpoint; then to the smallest scale, from perspective of a spacecraft.

Pursuit of pattern

• Orderly patterns of distribution are much more difficult to discern in geomorphology than in other geographic elements, such as climate.

• Overall, global distribution of topography is very disordered and irregular.

• Comprehending the process (dynamics of topographic development) is more important than the study of landform distribution.