Lecture 5 Tectonic and Structural Geomorphology
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Transcript of Lecture 5 Tectonic and Structural Geomorphology
Tectonic and Structural Geomorphology
1. Examine the structure of the earth’s crust
2. Establish the mechanisms that drive uplift/subsidence of
the earth’s surface
3. Examine the linkages between the uplift of mountains,
the development of topographic relief, and climate
4. Examine a class of landforms that are controlled
primarily by local structural geology and internal processes.
Goals of the Topic
Structure of the Earth’s Crust
Layers Thickness Properties
Core (Barysphere)
1200 km 2300 km
Composition: Iron & nickle (NIFE); Temp is 1927°C Outer core: liquid which controls earth magnetic field Inner core: solid Convective currents
Mantle (Mesosphere)
2,900 km
Composition: Silicate rocks rich in iron, magnesium, & olivine Mantle convection-plate tectonic processes
inner
outer upper
lower
Structure of the Earth’s Crust
Layers Thickness Properties
Crust
5–10 km 35-70 km
Composition: felsic rocks: silica & alumina (SIAL) rocks, e.g. granite; density is 2.7 mafic rocks: silica, iron & magnesium rocks (SIMA), e.g. basalt; density is 3
Lithosphere
The uppermost mantle together with the crust constitutes the lithosphere Floats on a semi-liquid layer known as the asthenosphere. Made of plates
Hydrosphere oceans
oceanic
continental
Theory of Plate Tectonics
Background to the concept of plate tectonics
1.Wegner’s continental drift hypothesis
2. Sea floor spreading (Plate convergence and spreading)
3.Mantle convection cells
•The movement and interaction between crustal plates
Continental Drift
•Concept proposed by Alfred
Wegner (The origins of Continents and Oceans, 1937) to explain glacial
history of the southern continents.
• Supercontinent (200 million yrs) Pangaea. Separated into northern block
(Laurasia) & Gondwanaland (southern hemisphere).
Evidence
•Wegner’s theory was based on the apparent ‘fit’ between S.
America and Africa,
• the similarity of rock formations on either side of the Atlantic,
•Tropical fossils found in Antarctica.
Continental Drift
Evidence of Continental Drift: Mantle convection
Convection is the process
where heated fluid rises and
cooled fluid falls. This
establishes convection cells
in the mantle.
Spreading plates form at
rising limbs of convection
cells and convergent
boundaries are formed by
magma that has cooled after
a trip to the surface.
Evidence of Continental Drift: Sea Floor Spreading
Arthur Holmes 1944,
English geologist who
suggested that, it was
the sea floors that are
moving & dragging the
continents along with
them abt 1 – 10cm/yr
Cause by mantle
convection – subduction
•Causing magnetic a reversal of the earth magnetic field
•Widening the Atlantic Ocean
•Earthquakes/fold mountain formation
Further evidence of Continental Drift:
Earthquake distribution
Concentration of
earthquakes along
plate boundaries.
J.P. Rothé (1954,
Royal Society of
London):
Modern Plate Tectonics
Surface of Earth is composed of moving internally rigid plates
which separate at divergent zones (spreading) and meet at
convergent zones (subduction and mountain building)
Expressions of Tectonics at Earth’s Surface: Mountain Formation
• Tectonics provides the mechanisms for mega-scale mountain
uplift, but at the scale of individual mountains or groups of
mountains, there are distinctive expressions of tectonic activity that
control land form.
• These expressions include faults and folds, which may have been
altered by erosion since tectonic activity.
• Another control on local scale landscape evolution is igneous
activity which includes various types of volcanoes, lava flows, and
intrusion of plutons that may be exposed at earth’s surface by
erosion.
• Together, these expressions of tectonic activity enforce a first
order control on the landscape at the local scale.
Fold Mountains: Types of Folds syncline anticline
nappe overfold
Syncline and Anticlines
Fault Mountains: Types of faults
Selby, Earth’s Changing Surface, 1985.
•Normal Fault - tension
•Reverse/thrust Fault -
compression
•Strike-slipe Fault –
stress
Fault Mountains: Types of faults
Tension Compression
stress Compression
Fault Mountains: Types of faults
Fault Mountains: Types of faults
Reverse fault, Terlingua Creek, TX
Fault Mountains: Types of faults
Overthrust fault, Near Banff, AB
Fault Mountains: Horst and Grabens
Fault Mountains: Horst and Grabens
Fault Mountains: Horst and Grabens
Puu Oo, a type of spatter cone, Kilauea, Hawaii
As geomorphologists, we are primarily interested in the
shapes produced by volcanic activity at the earth’s surface.
Volcanism & Earthquakes
Types of Surface Expression of Igneous Activity
1) Intrusive: igneous landforms exposed by erosion
Intrusive: Magma rose within the crust and cooled is
intrusive.
Examples of intrusive landforms:
• Batholiths, domes (laccoliths), dykes, sills
2) Extrusive: volcanic or depositional landforms
• Extrusive: Magma that is forced to surface and cooled is
extrusive.
Examples of extrusive landforms:
• Lava flows, ejecta, ash, volcanoes
Landforms of Igneous Intrusions
Phacolith is a lens-shaped mass of igneous rocks occupying the
crest of an anticline/syncline being fed by a conduit from beneath
Types of
extrusion
1. Lava flows – lavas
2. Pyroclastics - coarser
volcanic materials
3. Ashflows – ashes/volcanic
dust
Lava flows
Ashflows
Pyroclastics
Types of Lavas
Basic Lava
•Highly fluid, very hot, dark rich in iron & magnesium but low in
silica
•Silent emissions of magma, fast flow (16 – 48 km/hr), forming
gently sloping volcanoes (shield/dome shaped).
•Acidic Lavas
•Highly viscous, light in colour, low density & high silica content
•Violent emissions of magma, slow flow, steep-sided mountains
Types of Lavas
Pyroclastic flows descend the south-eastern flank of Mayon
Volcano, Philippines
Most extrusions are some
combination
of all three types of flow
Types of extrusion
Characteristics of the magma
1. Composition of magma: silica content controls the viscosity of the
magma and, consequently, its ability to plug up vents. Basalt 50%
SiO2 (basic lava); Andesitic 60% SiO2; Rhyolitic 70% SiO2 (acidic
lava)
2. Magma Temperature: also controls viscosity and
likelihood that lava will flow. Melting point of Basalt is 1100°C.
Melting point of Rhyolite is 650-700°C.
3. Gas Content: controls explosiveness and, ultimately, deposition
patterns and flow types
Types of Volcanoes
Active – frequent eruptions
Dormant – occasional eruptions with signs of future
occurrences
Extinct – not experience eruptions in historic times
•Cause by moving plates (Plate Tectonics)
•Plates rub against each other in some places (like the San
Andreas Fault in California) – striking.
•Sink beneath each other in others (like the Peru-Chile Trench
along the western border of South America) – subduction
•spread apart from each other (like the Mid-Atlantic Ridge).
•At such places the motion isn't smooth- the plates are stuck
together at the edges but the rest of each plate is continuing to
move, so the rocks along the edges are distorted (what we call
"strain"). As the motion continues, the strain builds up to the
point where the rock cannot withstand any more bending. With a
lurch, the rock breaks and the two sides move.
•The energy that is transmitted in the form of vibrations when the
rocks break - earthquake.
Earthquakes
•The point at which the shock emanates - origin/focus
•The point on the earth’s surface directly above the focus –
epicentre
•Seimograph – Richter scale
Earthquakes