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