The Rock Cycle, Isostasy, and the Dynamics of the Lithosphere

Siccar Point, Scotland

Rock - aggregates of mineralsRock 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 are basalt. Mars has volcanism.

Mars does not have:• Plate Tectonics• Oceans• Water-rich atmosphere

The Rock Cycle

Igneousrock

Cooled frommolten 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.

Physical Geology - Rock Cycle and Isostasy

Dry mantle(basalt, gabbro)

Wet mantle(andesite)

Continental crust(rhyolite, granite)

Plate Tectonic Setting

The Rock Cycle

Igneousrock

Cooled frommolten rock

Magma / Lava

uplift and weathering

GravelSandMudDissolved ions

Physical Geology - Rock Cycle and Isostasy

Weathering / Erosion • the physical and chemical breakdown

of rock.• occurs at the Earth’s surface.

Physical Geology - Weathering

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

Lysefiord, Norway

Passed geology 1CFailed geology 1C

Physical Geology - Rock Cycle and Isostasy

Sheeting of Granite bedrock

Enchanted Rock, Texas

Physical Geology - Weathering

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 WeatheringThe chemical reaction of minerals with air and water.

• hydrolysis (acid attack)• oxidation• dissolution (leaching)

CO2 + H2O = H+ + HCO3

-

H+ + feldspar = Ca++ clay + silica

Chemical Weathering

HCO3-, Ca+, silica

to the ocean

Physical Geology - Weathering

Minerals prone to chemical

weathering• feldspars• pyroxenes• amphiboles• micas

• quartz • clay minerals (e.g.

kaolinite)• iron oxides (e.g.

hematite)• carbonate (calcite)

Stable mineral products of chemical

weathering

Common in Igneous Rocks Common in Sedimentary Rocks

Sedimentary Rocks (from the L. sedimentum– to settle)

•Grains produced by the weathering of other rock.

•Layers form at the Earth’s surface.•Layers preserve fossils and other information about past environments.

• Composed of layers of mineral grains.

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.

Coal - carbon derived from plant matter

Tar, oil, natural gas - hydrocarbon from marine algae

Fossil Fuels are found in Sedimentary Rock

Plants Peat CoalLignite

coal

Fossil Fuels

oil

Igneousrock

Cooled frommolten rock

The Rock Cycle

Magma / Lava

SedimentaryRock

Deposited inlayers on thesurface.

Uplift and

Erosion

Redeposition

burial and subsidence

uplift and erosion

Metamorphic Rocks (from the G. meta– to change, and morphos– form, shape)

• Changes texture and mineral composition.

• Usually happens when rock is buried beneath mountain ranges during plate collisions.

• Rocks altered by heat and pressure.

Physical Geology - Rock Cycle and Isostasy

Sedimentary Rock

Low Grade Metamorphic Rock

High Grade Metamorphic Rock

Migmatite

Magma

200° C 3000 ATM

600° C

Slate Phyllite Schist

Low Grade High Grade

Progressive Metamorphism

Igneousrock

Cooled frommolten rock

The Rock Cycle

Magma / Lava

Uplift and Erosion

SedimentaryRock

Deposited inlayers on thesurface.

Uplift and

Erosion

Redeposition

burial and subsidence

MetamorphicRock

Changed by heat and pressure.

melting

uplift and erosion

Burial and subsidence

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.

changes in density of the crust.changes in thickness of the crust.

What causes the lithosphere to float higher or lower on the underlying asthenosphere?

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.

Physical Geology - Rock Cycle and Isostasy

Isostasy - equilibrium position of lithosphere “floating” on asthenosphere.

oceanic lithosphereoceanic lithospherecontinental lithosphere

Asthenosphere

Isostacy

oceanic lithosphereoceanic lithospherecontinental lithosphere

Asthenosphere

Rock at earth’s surface.

IsostacyCrust 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

Rebound

Once deeply buried rock has been uplifted back to the surface

Igneousrock

Cooled frommolten rock

The Rock Cycle

Magma / Lava

Uplift and Erosion

SedimentaryRock

Deposited inlayers on thesurface.

Uplift and

Erosion

Redeposition

burial and subsidence

MetamorphicRock

Changed by heat and pressure.

melting

uplift and erosion

Burial and subsidence