Transcript Mountain Building

Mountain Building
Vocabulary
• Orogenesis
– Oros—mountain
– Genesis—to come into being
• Lithosphere
– The rigid outer layer of Earth, including the crust and
upper mantle
• Accretion
– the increase in size of a tectonic plate by addition of
material along a convergent boundary
Crustal Uplift
• Evidence
– Marine fossils often found in high elevations in
mountains
– Terraces hundreds of meters above sea level
• Isostasy
– A floating crust in gravitational balance
– Example: blocks of wood floating in water
– Mountain belts stand higher above the surface of the
Earth and have roots that extend deeper into the
supporting material below.
– Crustal thicknesses for some mountain chains are
twice as much as the average for the continental crust
Crustal Uplift
– Crust beneath the oceans is thinner than that
beneath the continents
– Oceanic rocks are denser than continental
rocks
**adding weight to the crust makes it subside
**when weight is removed, crustal uplifting
occurs (ex.: cargo ship)
Crustal Uplift
• Isostatic Adjustment
– Ice Age glaciers added weight to the
continents, making them downwarp by
hundreds of meters
– When glaciers melted, uplift occurred
– Erosion of mountains causes uplift, also
Rock Deformation
• Elastic deformation
– When stress is applied, rocks bend, but will
snap back if the stress is relieved
• Plastic deformation
– When the elastic limit is surpassed, rocks
deform plastically or break (earthquakes)
– They are permanently altered through folding
and flowing
Rock Deformation
• Folds
– When flat-lying sedimentary and volcanic
rocks are bent into a series of wavelike
undulations
• Example: pushing on one edge of a carpet until it
folds
– Anticline
• Upfolding or arching of rock layers
– Syncline
• Downfolds, or troughs
Rock Deformation
– Dome
• When upwarping produces a circular or somewhat
elongated structure
– Basin
• When downwarping produces a circular or
somewhat elongated structure
Rock Deformation
• Faults and Joints
– Fractures in the Earth’s crust
– Dip-slip faults
• Vertical movement
• Hanging wall—rock that is higher than the fault
surface
• Footwall—rock that is lower than the fault surface
• Normal—hanging wall moves downward relative to
the footwall
• Reverse—hanging wall moves upward relative to
the footwall
– Thrust faults—have a very low angle
Rock Deformation
– Strike-slip faults
• The dominant displacement is along the strike or
trend, of the fault (horizontal)
• Transform faults—associated with plate boundaries
– Oblique-slip faults—both vertical and
horizontal movement
– Tensional forces—pull the crust apart
• Graben—central block bounded by normal faults;
drop as the plates separate
• Horsts—upfaulted structures that are adjacent to
graben
– Compressional forces—sections of crust are
displaced toward one another
Rock Deformation
– Joints
• Fractures along which no appreciable displacement
has occurred
– Columnar joints form when igneous rocks cool and
develop shrinkage fractures, producing elongated,
pillarlike columns
– Sheeting produces a pattern of gently curved joints that
develop more or less parallel to the surface of large
exposed igneous bodies.
Mountain Types
• Fault-block mountains
– Tensional stresses elongate and fracture the
crust into numerous blocks. Movement along
the fractures tilt the blocks producing parallel
mountain ranges.
Mountain Types
• Folded mountains (complex mountains)
• Upwarped mountains
– Caused by a broad arching of the crust or
because of great vertical displacement along
a high-angle fault
• Volcanic mountains
Mountain Building
• Convergent boundaries
– Volcanic arcs are forming in most modern-day
subduction zones
– Aleutian-type subduction zones occur where
two oceanic plates converge
Mountain Building
– Andean type subduction zones
• Passive continental margin—part of the same plate
as the adjoining oceanic crust
• Becomes active—subduction zone forms and the
deformation process begins
• The oceanic plate descends and becomes magma
while there is an accumulation of sedimentary and
metamorphic rocks along the subduction zone
(accretionary wedge)
Mountain Building
– Continents converge
• Continental lithosphere is too buoyant to undergo
subduction, a collision eventually results
• Example: India colliding with the Eurasian plate
Mountain Building
• Mountain Building and Continental Accretion
– Smaller crustal fragments collide and accrete to
continental margins
– Example: mountainous regions rimming the Pacific
• As oceanic plates move, they carry with them embedded
oceanic plateaus or microcontinents
• The upper portions of these thickened zones are peeled from
the descending plate and thrust in relatively thin sheets onto
the adjacent continental block.
• This increases the width of the continent
• Terrane—accreted crustal blocks