Introduction to Landforms

Structure of the Earth

Understanding of Earth's structure is limited due to the minute fraction of total depth explored (less than 8 miles).
Much understanding is inferred through geophysical means.
Four main regions: crust, mantle, outer core, and inner core.

Crust

Depth ranges from 5 km below the ocean to about 20 km below land.
Less than 1% of Earth's volume and 0.4% of its mass.
Moho discontinuity: a significant change in mineral composition.

Mantle

Extends to a depth of 2900 km (1800 miles).
Largest of the four shells, comprising 84% of total volume and 67% of total mass.
Three sublayers: Lithosphere, Asthenosphere, and Rigid rocks (lower mantle).

Outer Core

Molten (liquid).
Not the source of lava.
Extends to a depth of 5000 km.
Earth's cores drive the slow movement of hot rock through the mantle toward the surface (convection).
Rising material melts due to lower pressure; otherwise, it remains solid.

Inner Core

Dense mass with a radius of about 1450 km.
Primarily made of iron/nickel or iron/silicate.
The two zones combined make up 15% of Earth's volume and 32% of Earth's mass.

Magnetic Field

Controlled by the outer core.
Turbulent convection of radioactive heating and chemical differentiation in the outer core.
Similar to an electrical generator, Earth's conducting iron creates electric currents that generate their own magnetic field.
Magnetic poles are not the same as the axial poles.

Plate Tectonics

1900s: Theory of Continental Drift - Initially unpopular.
1960s: Revived and expanded into the theory of plate tectonics - Accepted in the science community.

Land-Based Clues to Continental Drift

Geographical: Continents fit together like a puzzle.
Geological: Old mountain zones of matching ages appear as belts across southern continents.
Climatic: Glacial deposits and rocks scratched by stones in moving ice indicate ice cover over huge tracts of southern continents 300 million years ago.
Paleomagnetic: Alignments of magnetized particles in old rock show that southern continents all lay near the South Pole 300 million years ago.
Biological: Identical fossil land plants and land animals crop up in the southern continents now widely separated by sea.

Plate Tectonics (Movement)

Lithosphere is broken into large slabs (plates) that float on the asthenosphere.
Plate boundary movement is driven by convection.
Plates pull apart, collide, and slide past each other.
Responsible for internal processes: faulting, folding, volcanic activity.

Composition of the Earth

Minerals

Naturally formed compounds and elements of Earth.

Classes of Rock

Igneous, Sedimentary, and Metamorphic rocks.
Includes continental and oceanic plates.

Zhangye Danxia

Formed 24 million years ago by deposits of sandstone and other minerals.
Tectonic plates responsible for the Himalayas tilted the layers.
Wind, rain, and time sculpted the shapes.

Characteristics of Minerals

Solid
Found in nature
Inorganic
Specific chemical composition
Atoms arrange in patterns to form crystals

Common Rock-Forming Minerals (Examples)

Olivine: $(Mg, Fe)2SiO4$
Pyroxene group (Augite): $(Mg, Fe)SiO_3$
Amphibole group (Hornblende): $Ca2 (Fe, Mg)5 Si8 O{22} (OH)_2$
Biotite mica: $K(Mg, Fe)3 AlSi3 O{10} (OH)2$
Muscovite mica: $KAl2 (AlSi3 O{10}) (OH)2$
Potassium feldspar (Orthoclase): $KAlSi3 O8$
Plagioclase feldspar: $(Ca, Na) AlSi3 O8$
Quartz: $SiO_2$
Hematite: $Fe2 O3$
Magnetite: $Fe3 O4$
Corundum: $Al2 O3$
Galena: $PbS$
Pyrite: $FeS_2$
Chalcopyrite: $CuFeS_2$
Gypsum: $CaSO4 Imes 2H2O$
Calcite: $CaCO_3$
Dolomite: $CaMg(CO3)2$
Halite: $NaCl$
Fluorite: $CaF_2$
Gold: $Au$
Silver: $Ag$

Rocks

Fewer than 20 minerals make up 95% of the composition of crustal rocks.
Outcrops: exposed bedrock.
Regolith: layer of unconsolidated rocky material covering bedrock.
Petrology: the study of the characteristics of different rocks.

Three Classes of Rocks

Igneous: formed from magma or lava.
Sedimentary: formed from sediments.
Metamorphic: formed when other rocks are changed by heat and pressure.

Igneous Rock

“fiery inception”
Magma: molten rock beneath Earth's surface.
Lava: molten rock when it flows onto Earth's surface.
Pyroclastics: Fragments of rock erupted by volcanoes.

Classification of Igneous Rocks

Based on mineral composition and texture.
Texture based on how rocks cool.
Interlocking crystals formed as the molten rock cooled down.
The smaller the crystals - the faster the cooling.

Plutonic (intrusive) Rocks

Cool beneath Earth's surface.
Surrounding rocks insulate the magma intrusion, slowing cooling.
Individual minerals can grow to large size.
Example: Granite.

Volcanic (extrusive) Rocks

Form on Earth's surface
Cool rapidly
Generally do not show individual mineral crystals, but can if the crystals are formed from shattered rock that was explosively ejected
Example: Basalt

Common Igneous Rocks

Felsic: Contain large portions of light-colored silicate minerals such as quartz and feldspar; Higher silica content of magma.
- Plutonic: Granite
- Volcanic: Rhyolite
Intermediate: Mineral composition between Rhyolite/Granite and Basalt/Gabbro.
- Plutonic: Diorite
- Volcanic: Andesite
Mafic: Contain large portions of dark-colored silicate minerals such as olivine and pyroxene; Lower silica content of magma.
- Plutonic: Gabbro
- Volcanic: Basalt

Sedimentary Rocks

External processes cause rock disintegration.
Material transported by water as sediment.
Over long periods, large amounts of sediment build to large thicknesses.
Exert enormous pressure that causes particles in sediment to interlock.
Chemical cementation takes place, forming sedimentary rock.
Strata: horizontal layers of sedimentary rock; sometimes tilted into vertical by Earth processes.
Most derive from the deposited remains of older rocks.
Rounded mineral grains joined by natural cements.

Clastic Sedimentary Rocks

Composed of fragments of preexisting rocks.
Also known as detrital rocks.
Shale is an example.
Conglomerate: composed of pebble-sized fragments.

Chemical and Organic Sedimentary Rocks

Formed by precipitation of soluble materials or complicated chemical reactions.
Limestone and coal are examples.
Organic sedimentary rocks such as coal form from remains of dead plants and animals.

Metamorphic Rocks

Rocks that were originally igneous or sedimentary and have been changed by heat and pressure.
Causes a “cooking” of rocks.
Rearranges the crystal structure of the original rock.

Contact Metamorphism

Rock contacts magma and is rearranged.

Regional Metamorphism

Large volumes of rock are subjected to heat and pressure over long time scales.
Limestone becomes marble.
Sandstone becomes quartzite.
Shale becomes slate.

Metamorphic Rocks - Schist

Metamorphic rocks with narrow foliations.

Metamorphic Rocks - Gneiss

Broad, banded foliations.

Rock Cycle

Processes where rocks can transition between the three rock types, driven by solar energy, melting, metamorphism, weathering, erosion, deposition, compaction, and cementation.

Continental and Ocean Floor Rocks

Sedimentary rocks make up 75% of the continents.
Sedimentary cover is not thick.
Continental crust – sial (silicon and aluminum).
Ocean floor crust – sima (silicon and magnesium).
Ocean lithosphere is more dense than continental lithosphere.
Ocean crust can be subducted into the asthenosphere.

Isostasy

Recognition of differences between oceanic crust, continental crust, and mantle.
Crustal subsidence and rebound due to glacial ice.

Study of Landforms

Geomorphology

The study of characteristics and development of landforms
Basic elements: Structure, Process, Slope, Drainage.
Fundamental questions: What, where, why, so what?

Structure

Nature, arrangement, and orientation of the materials making up a landform.
Is it composed of bedrock? If so, what kind?
If not, what are the nature and orientation of the sediments or other depositions?

Process

Combined actions that produce the landform.
Combination of geologic, hydrologic, atmospheric, and biotic processes.
Or from one process - glaciation.

Landform Evolution

Landforms evolve as weaker rock is eroded, leaving the more resistant rock standing as ridges or mountains.
Shale is weak rock easily eroded, forming low valley floors.
Igneous rocks are resistant to erosion, forming uplands or mountains.
Limestone is dissolved by carbonic acid in rain and surface water, forming valleys in humid climates; in arid climates, it forms ridges and cliffs.
Sandstone and conglomerate are typically resistant, forming ridges or uplands.

Geomorphic Processes

Endogenic (internal): Increase relief of land surface; examples include diastrophism (earthquakes, volcanoes, epeirogenic/orogenic movements).
Exogenic (external): Decrease relief of land surface; examples include weathering, erosion by ice, river water, sea, wind, underground water.

Slope

A reflection of the balance among various components of structure and process.
Provides details that are important in analyzing/describing the feature.
Shape of any landform.

Drainage

Fundamental aspect.
Movement of water over Earth's surface or down into soil and bedrock.
Deposition and Erosion.
Stream flow, stream patterns, etc.

Hillslopes

Bedrock hillslopes: Weathering limited, arid.
Soil-mantled hillslopes: Transport limited, humid.
Talus slope created by rockfall from cliffs above.
Low gradient interfluve.
Convex slope dominated by diffusive processes.
Concave lower toe slope dominated by advective processes.

Slope-Water Interaction

Precipitation.
Interception.
Stemflow & Throughfall.
Overland flow.
Infiltration.
Throughflow.
Percolation.
Groundwater flow.

Fundamental Questions in Landform Study

The form of the feature(s).
Distribution and pattern.
Origin and development.
Significance in relationship to other elements of the environment and human life/activities.

Critical Concepts

Geomorphic Processes

Internal and External Processes.

Uniformitarianism

The present is the key to the past.

Geologic Time

Age of Earth and Geologic Time Scale.

Internal Processes

Originate from within earth, increase relief of land surface.
Lithospheric Rearrangement (Plate Tectonics).
Volcanism (Extrusive and Intrusive).
Tectonicism (Diastrophisim) (Folding and Faulting).

External Processes

Originate from sources above the lithosphere, decrease relief of land surface.
Weathering.
Mass Wasting.
Erosion/Deposition.

Agents of Erosion

Fluvial (running water).
Aeolian (wind).
Glacial (moving ice).
Solution (ground water).
Waves and currents (oceans/lakes).

Uniformitarianism

Past thinking believed catastrophism, catastrophes shaped the land surface.
Processes that shaped the landscape of the past are the same that will shape the future.

Geologic Time

Vast periods of time over which geologic processes operate.
Geologic Time and Uniformitarianism?
In order for the slow internal/external processes to operate and make a significant difference in the landscape - very long periods of time are required.
Anthropocene - human activity as the dominant influence on the climate and environment.

Scale and Pattern

Scale

An example of scale – five perspectives of Horseshoe Park.
1. Largest scale - ordinary human experience (walking).
  - Note topographic features as you walk through space.
2. Driving
  - Observe relationships among rock type, vegetation, and slope.
3. Airplane
  - Mountain range
4. Satellite
  - Northern colorado (range almost indistinguishable) within Rocky Mountains
5. Smallest scale - (space craft)
  - North American Continent

Pattern

Major landform assemblages of the world.
Predictability vs Irregularity.
Concentration on processes.