The Rock Cycle, Isostasy,
and the Dynamics of the
Lithosphere
Siccar Point, Scotland
Rock - aggregates of minerals
Rock cycle - movement and
transformations of rock in the lithosphere
What is the ultimate origin of
rock in the lithosphere?
Molten magma from the
Earth’s mantle.
Basic geology on Mars would be easy to learn - almost all of the rocks
Mars has volcanism.
are basalt.
Mars does not have:
• Plate Tectonics
• Oceans
• Water-rich atmosphere
The Rock Cycle
Cooled from
molten rock
Igneous
rock
Magma / Lava
Igneous Rocks
(from the L. ingnis – fire)
• rocks that have crystallized
(solidified) directly from a molten
state.
• Molten rock - magma / lava
• Composed of randomly interlocked
crystals of silicate minerals.
Plate Tectonic
Setting
Dry mantle
(basalt, gabbro)
Physical Geology - Rock Cycle and Isostasy
Wet mantle
(andesite)
Continental crust
(rhyolite, granite)
The Rock Cycle
Gravel
Sand
Mud
Dissolved ions
Cooled from
molten rock
Igneous
rock
uplift
and weathering
Magma / Lava
Weathering / Erosion
• the physical and chemical breakdown
of rock.
• occurs at the Earth’s surface.
Physical Geology - Rock Cycle and Isostasy
Weathering
does several very important things:
• Breaks down rock to make
sediments.
• Forms soils.
• Forms economic mineral
deposits.
• Removes CO2 from the
Earth’s atmosphere.
Physical Geology - Weathering
Mechanical Weathering
physical disintegration of rock
• jointing
• sheeting
• wedging
Physical Geology - Weathering
Vertical jointing - tectonic stress + gravity
Failed geology 1C
Passed geology 1C
Physical Geology - Weathering
Lysefiord, Norway
Sheeting of Granite bedrock
Enchanted Rock, Texas
Physical Geology - Rock Cycle and Isostasy
Ice and Frost Wedging
• caused by freeze - thaw cycles.
• water expands when it freezes.
• ice has 9% more volume than liquid
water.
Physical Geology - Weathering
Chemical Weathering
The chemical reaction of minerals with air and water.
• hydrolysis (acid attack)
• oxidation
• dissolution (leaching)
Physical Geology - Weathering
CO2 + H2O =
H+ + HCO3-
Chemical
Weathering
H+ + feldspar =
Ca++ clay + silica
to the ocean
HCO3-, Ca+, silica
Minerals prone to
chemical
weathering
• feldspars
• pyroxenes
• amphiboles
• micas
Common in Igneous Rocks
Stable mineral
products of chemical
weathering
• quartz
• clay minerals (e.g.
kaolinite)
• iron oxides (e.g.
hematite)
• carbonate (calcite)
Common in Sedimentary Rocks
Physical Geology - Weathering
Sedimentary Rocks
(from the L. sedimentum– to settle)
• Composed of layers of mineral grains.
• Grains produced by the weathering
of other rock.
• Layers form at the Earth’s surface.
• Layers preserve fossils and other
information about past
environments.
Alluvial fan depositional environment, Death Valley, California
River System depositional environment, New Zealand
Fossils are organic
remains buried in
sediments and preserved
in sedimentary rock.
These Cape Buffalo
carcasses are on their
way to becomming
fossils.
Ankylosaur skeleton
being excavated from
sandstone, Mongolia.
Ankylosaur skeleton
being excavated from
sandstone, Mongolia.
Fossil Fuels are found in Sedimentary Rock
Coal - carbon derived from plant matter
Plants
Peat
Lignite
Tar, oil, natural gas hydrocarbon from marine algae
Coal
Fossil Fuels
oil
coal
The Rock Cycle
Uplift
and
Erosion
burial and subsidence
Redeposition
Deposited in
layers on the
surface.
Sedimentary
Rock
Cooled from
molten rock
uplift
and erosion
Igneous
rock
Magma / Lava
Metamorphic Rocks
(from the G. meta– to change, and
morphos– form, shape)
• Rocks altered by heat and pressure.
• Changes texture and mineral
composition.
• Usually happens when rock is buried
beneath mountain ranges during plate
collisions.
Sedimentary Rock
3000 ATM
200° C
Low Grade Metamorphic Rock
High Grade Metamorphic Rock
Migmatite
600° C
Magma
Physical Geology - Rock Cycle and Isostasy
Progressive Metamorphism
Phyllite
Slate
Schist
Low Grade
High Grade
The Rock Cycle
Uplift
and
Erosion
burial and subsidence
Redeposition
Deposited in
layers on the
surface.
Changed by
heat and
pressure.
Sedimentary
Rock
Uplift and
Erosion
Cooled from
molten rock
uplift
and erosion
Igneous
rock
Magma / Lava
Metamorphic
Rock
Burial and
subsidence
melting
Uplift - the movement of the crust
upward.
Subsidence - the movement of the crust
downward.
The crust can move
because it is floating on the
asthenosphere.
What causes the lithosphere to float higher or lower
on the underlying asthenosphere?
changes in density of the crust.
changes in thickness of the crust.
What changes the density of rock?
• Heating - rock expands, becomes less
dense.
• Cooling - rock contracts, becomes
more dense.
• Changes in density cause the
lithosphere to float higher or lower
on the asthenosphere.
What changes the thickness of the
crust?
• Plate collisions
• Shortening and thickening of the
crust in the collision zone.
• Thrusting of one plate on top of
another.
• Erosion
• Thinning of the crust exposed
above sea level.
Isostasy - equilibrium position of lithosphere
“floating” on asthenosphere.
continental lithosphere
oceanic lithosphere
oceanic lithosphere
Asthenosphere
Physical Geology - Rock Cycle and Isostasy
Isostacy
continental lithosphere
oceanic lithosphere
Rock at earth’s surface.
Asthenosphere
oceanic lithosphere
Isostacy
Crust is “loaded” with rock, increasing its
thickness.
Surface rock is buried.
Loaded crust subsides.
Loaded crust subsides.
Loaded crust subsides.
Loaded crust subsides.
Loaded crust subsides.
New isostatic equilibrium is achieved.
Surface rock is now deep in the Earth’s interior.
Surface topography erodes.
Crust rebounds to new isostatic equilibrium.
Erosion
Rebound
Erosion
Rebound
Erosion
Rebound
Erosion
Rebound
Erosion
Rebound
Erosion
Rebound
Erosion
Rebound
Erosion
Once deeply buried rock has been
uplifted back to the surface
Rebound
The Rock Cycle
Uplift
and
Erosion
burial and subsidence
Redeposition
Deposited in
layers on the
surface.
Changed by
heat and
pressure.
Sedimentary
Rock
Uplift and
Erosion
Cooled from
molten rock
uplift
and erosion
Igneous
rock
Magma / Lava
Metamorphic
Rock
Burial and
subsidence
melting