STRUCTURE OF THE EARTH

Structure

Seismic waves are passed thru earth to tell the difference in densities

The Core

Energy is released from decomposing radioactive materials

Uranium decays into Thorium + releases heat

Outer core

Contains liquid Fe, Ni, S

Inner core

Solid Fe, Ni, S

Mantle

  1. Upper mantle

Fluid

Convection currents

  1. Athenospehre

Below lithosphere

Semi-molten

Made of Periodite

  1. Lithosphere

Below rigid crust

Deforms + flows slowly

Solid

Crust + lithosphere = plates

Crust

1-100km thick

Different plates have different minerals

bc of heat from the Earth's formation

Flow + convection currents = plate movement

Tectonic Plates

7 large plates

Continental + oceanic crust

3 main plate boundaries

Constructive

Destructive

Conservative

Make new crust

Destroy (oceanic) crust

No crust destroyed/made

Oceanic Crust

Subduction = younger (180 ma)

Thin (5-10km)

Basalt, magma, silicon, Mg: (SIMA)

High density

Continental Crust

Not subducted = older

Uranium atoms in a crystal used to determine age

Thick (30-70km)

Granite, Al, Si (silica): SIAL

Low density

Continental Drift

Background

Large scale horizontal movement of continents during geological time

Only applies to continental land masses

Uses evidence from the continents; sea floor spreading uses evidence from ocean

Together they provide the evidence for plate tectonics

Alfred Wegner

Set out evidence of CD in 1915

Continental fit, rock distribution + ancient glaciation = evidence

Ideas unaccepted coz there was no mechanism by which the continents would move

50s - palaeomagnetism + seafloor spreading = widespread acceptance

Super continent 225 m/a - Pangea

Pangea

Is only one of several super-continents

Formed by collisions from Laurentia + Gondowanaland

Broke-up from rifting in MA Ridge

Evidence

Gondawanaland

250Ma

S America, Africa, Antarctica, India, Australia

Single continent = same rocks and fossils + was glaciated at same time

Broke up in Jurassic period, 167 Ma

Evidence - fossils in S Am + Africa

Fit

Not exact fit of present coastline of S Am + Africa bc:

S.l. constantly changes - coastline is temporary feature

Deposition + erosion

Is less at continental shelf

Today's continents = Pangea

Matching sequences of glacial, marine + non-marine rocks found across several continents

Thick rock sequences (inc. coal) can only have been formed in tropical conds.

BRITAIN

Coal + evaporite deposits all suggest tropical climates during Carboniferous + Permian times

Assumes that climate belts have followed same pattern always as they are created by relation of Earth to Sun

So GB must have moved from tropical latitudes 250 Ma to current temperate climate belt

Rock Types

Proof that 2 rock types on either side of ocean are from same outcrop, they must have the same:

Distinctive characteristics of mineral composition + physical features

Age - determined by radiometric dating

Precambrian cratons, carboniferous coals

Mountain Chains

Fold mt. chains are linear features

Gondawanaland had a Precambrian fold mt. chain across Africa to S Am

Trend = way to match geology across continents

Fossils

If Africa + S Am were always separated, they should have diff. fossil records - esp. for orgs. that would not have been able to spread across a wide ocean

During the Carboniferous, land-based orgs. are found on both continents

Glaciation

Cannot form over ocean and move over land

Leaves movement of glaciers e.g. striations in bedrock

Orientation of striations in bedrock in Aus.. S Am + India suggest glaciers formed in oceans and moved on land

Glacial evidence is present in rocks on continents that are now near equator

Rock sequences containing glacial deposits cannot have formed near equator

S Am + Africa have sedimentary deposits of angular clasts

This is a fossil boulder clay or tillite deposited by an ice sheet during Carboniferous

Gondawanaland was probs. near the S pole bc ice sheets cannot extend to the equator

Africa + S Am are now much further N - evidence of movement

Paleomagnetism

Iron-rich minerals in magnetic field hold a record of the E's mg. field at the time of their formation

Large no. of rocks collected, dated and direction of the PM measured

Data plotted as a polar wandering curve - the curve joins up the apparent positions of the mg. N over time

S Am + Afria curves suggest that the N pole was once in 2 positions at same time

Mg. pole cannot sig. change position

Assuming mg. pole remained fixed, it must have been the continents that moved

If the 2 continents are repositioned next to each other, the curves match = one position for the pole

The curves diverge only after the continents started to drift apart

CD + Plate Tectonics

Sea Floor Spreading

Rocks can be dated isotopically

Changes in polarity across fold mts.: reversal + normal events

  1. new rock coming thru ridge pushes old rock on either side along
  1. the rocks pushed away is the same on either side
  1. bc of changes in polarity, there is a pattern along the fold mts; there is a ridge in the middle e.g. MA Ridge
  1. All igneous rocks formed at MA ridges contain iron
  1. Fe minerals cool as lava erupts
  1. Fe minerals become aligned with E's m.f.
  1. Fe minerals is rocks becomes a record of magnetic polarity at the time the rock was formed
  1. When polarity of rocks formed on each side of ridge was compared, they were found to match

Paleomagnetic Record

M.f. appears as a striped pattern on ocean floor

Igneous rocks form oceanic crust + floor

Lava erupts + cools and mg. orientation of Fe particles is locked in the rock, depending on polarity at that time

400-500k years polarity changes orientation

Wegner's CD theory in '12 was opposed bc of lack of driving mechanism

In 60s-70s military surveyed oceans and found magnetic field change + ridges = evidence for CD

General

Rapid rotation + liquid conducting interior = strong m.f.

Mantle is convecting but may change dir. or slow down periodically = m.f. reversal

Changes becoming mroe frequent - 200,000 years avg.

Normal polarity - mg. N + geographic N match

Reverse polarity - mg. N + geographic S match

Plate Boundaries

Constructive: Mid-Ocean Ridges

Are submarine landforms

Key process: ridge push

Occurs at Constructive boundaries

Magma rises to surface + heats surrounding rocks

Crust is elevated above the sea flood at ridge

As new rock forms, it cools + becomes denser

Gravity causes it to slide down each side of ridge

Processes + Features

Ridge push = submarine ridge 3.5 km high

Rising magma/convection = rift in centre on slow spreading ridges

Magma melting = fracture zone

Faulting = transform faults

Underwater cooling = pillow lavas

Chemosynthesis = black smokers/hydrothermal vents

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Exposure to Risk + Ability to Cope

Risk

Geophysical events interact with human population = risk + hazard

Increased overlap between events + populations

Increased pop. densities + growth in EDCs e.g. Haiti

Risk + (Hazard * Vulnerability) / Capacity to Cope

Factors of Exposure

Freq. of hazard

Magnitude

Type e.g. effusive v explosive

Type e.g. EQ, liqeufaction, tsunami

Pop. density

Deforestation = floods + soil erosion e.g. Haiti

Frequency - EQs/Year

Mag. + freq. are inversely proportional

Logrithmic scale - needed as crust is moving

Larger EQs occur less frequently

Magnitude + Recurrence Interval

Mag. = size of EQ

Return period/recurrence interval = likelihood of event occurring

Occurrence of Event

No. of EQs is not increasing, but preception is that they are

EQs in populated areas are more apparent - televised more

EQc clustering - people tend to notice clusters but not necessarily the breaks - makes it seem as if more events occur

Global comms. - more people are now aware of EQs + their impacts