Plate Tectonics and Rock Cycle

what is the lithosphere and what is it made up of

• Rigid outer shell of Earth

  • Bends/breaks when a force acts on it

• Made up of:

  • Earth's Crust

  • Lithospheric mantle

    • Cooler, upper layer of mantle

    • Floats on a softer, warmer mantle layer (aesthenosphere)

    • Aesthenosphere convects

types of crust

• Continental - lithosphere under continents

  • Thicker crust and total thickness

    • Surface lies at a higher elevation than oceanic lithosphere

    • Continental lithosphere sinks

  • Rocks are rich in silica + less dense

  • Same mantle composition

• Oceanic - lithosphere under oceans

  • Thinner crust and total thickness

  • Rocks are less rich in silica and more dense

  • Same mantle composition

plate boundaries and types

where two/more plates meet

• Lithosphere is made up of 12 major plates and some microplates

• Type of plate boundary depends on relative motion between plates

  • Divergent - moving away

  • Convergent - moving towards

  • Transform - slide past each other (e.g. San Andreas Fault line)

Divergent boundaries + importance

• Plates move apart creating a ridge

• New oceanic lithosphere forms

  • Mantle rises up and cools to create new crust

• New sea floor forms along centre line of ridge

  • Sea floor moves laterally away from ridge

  • Further away from the ridge = older the sea floor

describe the mid ocean ridge system

•  divergent ocean-ocean plate boundaries

• Lies at water depth 2-2.5km

• Sea floor slopes away from ridge axis/centreline

  • Sea floor deepens away from ridge axis

• Hot aesthenosphere rises beneath ridge -> melts -> produces magma

  • Magma is less dense than crust

  • It rises and accumulates in magma chambers - place below ridge axis in crust

• When magma solidifies = new oceanic crust

  • Solidifies along side of chamber = make gabbro

  • Solidifies along vertical cracks = makes sheet-like dykes of basalt

  • Rises and solidifies at surface of sea floor = underwater volcanoes

• Lithospheric mantle does not exist at ridges

  • Too hot for aesthenosphere mantle to cool into lithospheric man

Convergent boundaries

• what are they

• which plate sinks

• what natural phenomena mainly occurs here

• Plates move towards each other and subduction occurs

  • Subduction - bending and sinking of one plate under another

• One oceanic plate subducts under another oceanic/continental plate into its aesthenosphere

  • If oceanic lithosphere is >10 million years old, it will be denser than aesthenosphere

  • It sinks in aesthenospheric mantle

• Continental crust is too buoyant to subduct

  • It is less dense due to its rock makeup

  • If it subducts, it will float and eventually subduction will stop

• Earthquakes occur along subducting plate down to 660km

  • Position of subducting plate is defined by its band of earthquakes that occur along it when it subducts

  • Called Wadati-Benoff Zone

describe the Features of convergent boundaries

• Trench = depression where the oceanic plate bends and subducts under overlying plate

• Accretionary prism = the scraped off sediment from the subducting plate onto overlying plate

  • Overriding plate is like a bulldozer

• Volcanic arc = chain of volcanoes created behind accretionary prism on the overriding plate

  • Continental volcanic arc -> overriding plate is continental

  • Volcanic island arc -> overriding plate is oceanic

• Back-arc basin = small oceanic basin between volcanic arc and continent due to the pulling apart of the overriding plate

Transform Plate Boundaries

• Fracture zones = thin belts of broken seafloor intersecting perpendicular to and extending beyond ridge segments

  • Old theory:

    • Fracture zones extended beyond the sea ridge and cause each segment to slide against each other

  • New theory:

    • Fracture zones connect different segments of mid ocean ridges and slide when new sea floor is being made

    • Direction of transform fault is OPPOSITE to what we originally predicted

• Transform boundary = active slipping segment of a fracture zone between two ridge segments

  • Plates slide past one another

  • Transform faults can cut across continental lithosphere (e.g. San Andreas Fault)

what is Continental rifting and its effect

One continent splits into two continents

• Lithosphere is stretch horizontally and gets thinner

  • At the surface, rocks break and faults develop

  • Thinned surface is filled with sediment

• As continental lithosphere thins = hot aesthenosphere rises and magma erupts as volcanoes along rift

• Over time rifting causes a mid-ocean ridge to form + sea floor spreading

  • Sea floor spreading - the creation of new crust as mantle cools along the ocean ridge

• If rifting stops early = fault-bounded trough with volcanoes is made

types of continental rifting

• passive = tectonic forces

  • plate divergence with extensional

    forces transmitted through the lithosphere

  • lithosphere stretches and thins first, which forces the underlying asthenosphere to passively rise up to fill the void

• active = mantle plumes pushing up

  • hot plume pushes upward, thermally weakening, swelling, and doming the continental lithosphere before significant horizontal pulling occurs

what is plate collision

When two buoyant pieces of lithosphere converge and squeeze together

• When two continental plates collide

  • Oceanic plate sinks

  • Rock and sediment from both plates squeeze together and ride up to make mountain belts

    • E.g. Himalayas and Alps

• Collision causes crust to thicken and earth's surface to rise

current model of how plate tectonics work

Ridge push force	Force from ridge pushes Lithosphere closer to ridge is more elevated than lithosphere further away from ridge Gravity causes higher lithosphere to push on lower lithosphere Causes two sides of the ridge to move away = seafloor spreading Aesthenosphere rises and cools to fill in the gap Rising of aesthenosphere DOES NOT CAUSE seafloor spreading It is a RESULT
Slab-pull force =	Force applied on the subducting plate on the oceanic lithosphere at the convergent boundary Oceanic lithosphere >10 million years = denser than asthenosphere It sinks into asthenosphere

what is a rock

coherent, naturally occurring solid consisting of minerals or a mass of glass

• Coherent = one connected mass that can be broken up into smaller pieces

• Naturally occurring = only made by environment and not man made

• An aggregate of minerals or a mass of glass = most rocks are made of many mineral grains/crystals stuck together

  • Can be made of one or many kinds of minerals

  • Rocks at volcanoes can have glass inside -> e.g. obsidian is volcanic glass

define

• grain

• clastic

• crystalline

• Grain = a fragment or a piece of mineral, rock or glass

• Clastic = rocks whose grains are stuck together by cement

  • Cement has precipitated from water

  • Filled the space between grains to stick it together

  • E.g. Sandstone

• Crystalline = rocks whose grains interlock with one another

  • Like pieces of a jigsaw puzzle

  • E.g. granite

main rock groups

igneous

sedimentary

metamorphic

igneous rock

• how they form

• grain size

• types

Form by solidification of molten rock (magma/lava)

• E.g. Granite

• Involves cooling of melted products (magma)

 

Grain size is controlled by rate of cooling

• Cools fast = small grains -> generally extrusive

  • Closer to the crust is cooler

• Cools slowly = larger grains -> generally intrusive

 

Types of igneous rock

• Intrusive = cool inside earth's crust

  • Erosion overtime can expose intrusive rock

• Extrusive = cool outside earth's crust

composition of igneous rock

how are sedimentary rock formed

Form by

• Cementing of grains broken off pre-existing rock

• Precipitation and compaction of minerals out of water solutions near earth's surface

what are metamoprhic rock

Form when pre-existing rock changes character due to a change in pressure, temperature conditions, squashing, stretching or shear

• Metamorphic change occurs in solid state

• Does not require melting

• Can come from sedimentary or igneous rock

Physical characteristics to distinguish rocks

• grain

• texture

• composition

• layering

grain size

•  dimension of individual grains

  • Fine vs course

  • Are dimension same in all directions? Equant vs inequant

  • Are the grains the same size vs range of sizes

composition

proportion of different minerals making up rock

texture

• arrangement of grains

  • Way grains connect

  • Are inequant grains aligned parallel to each other

layering

•  defined bands of different composition or textures, or the alignment of inequant grains

  • Bedding - layering in sedimentary rock

  • Metamorphic foliation - layering in metamorphic rock

what are rock forming environments for igneous rocks

Igneous rocks develop when magma rises and cools

• Intrusive igneous rock = magma cools underground

  • Never reach the surface when they form

• Extrusive igneous rock = magma erupts at the surface as lava/ash and cools above ground

what are rock forming environments for sedimentary rocks

• Weathering/erosion break up existing rocks to make sediment

  • Different sedimentary rocks form in different places

  • Reflects composition of the source and setting in which sediment accumulates

  • E.g. desert dunes, river channels, deltas, coastlines, deep sea

what are rock forming environments for metamorphic rocks

• Metamorphism = changing of pre-existing rocks in their solid state

  • Makes metamorphic rock

  • Contact metamorphism

    • Produced around areas of intrusive igneous rock

    • Due to heat transfer

  • Regional metamorphism

    • Tectonic rocks cause rocks to be buried deeply into earth

what is rock cycle + describe it

The transformation of earth materials from one rock type to another

• Mass transfer cycle

  • Involves the transfer or movement of materials to different parts of the earth system

• Many paths can be followed

  • Igneous erode and compact to form sedimentary which can experience pressure to form metamorphic which can melt in the magma and then cool to form igneous

  • Igneous erode and compact to form sedimentary which can experience pressure to form metamorphic which can erode and compact again to form sedimentary

  • Igneous -> metamorphic -> sedimentary