Physical Geography Tidbits: Oceania

Formation of Oceania Islands

Reason for Movement

Excess heat from radioactive decay creates liquid outer core (and magnetic field), and forces mantle to have convection

Exactly how

mantle moves?

ASU view

3 TYPES OF PLATE INTERACTION

Stream Systems on

Dynamic Earth

New Zealand - 2 of the 3

East

Pacific

Rise

Mid

Atlantic

Ridge

Mid Indian

Ridge

Millions of years before present

Island Arcs from Ocean-Ocean Convergence

Trench

Island arc

Older colder ocean plate

subducts

Convergence & Tsunami

Effects Of Plate Locking

EQ (or landslide or volcanic eruption ) Movement Generates the Wave that travels about 500 mph

Animated gif

Mega-Quake Set off 2004 Tsunami & will happen

again

Energy ripple

Sri Lanka Sumatra Uplift

Indonesia

Indonesia

Has Happened, Will Happen Again

Hilo, 1960

Chile EQ generated Tsunami that destroyed Hilo in 1960

Hot Spots

Often in the middle of a plate

Focus here on Hawaii & Oceania

Hawaiian Hotspot

Hawaii - also example of isostacy

Trail of the Hot Spot

Note change in direction 43 myr agoChange in direction 43 myr ago

Are plumes real?

Alternative Hypothesis

After Formed … Sink

This NOAA animation shows the dynamic process of how a coral atoll forms. Corals (represented in tan and purple) begin to settle and grow around an oceanic island forming a fringing reef. It can take as long as 10,000 years for a fringing reef to form. Over the next 100,000 years, if conditions are favorable, the reef will continue to expand. As the reef expands, the interior island usually begins to subside and the fringing reef turns into a barrier reef. When the island completely subsides beneath the water leaving a ring of growing coral with an open lagoon in its center, it is called an atoll. The process of atoll formation may take as long as 30,000,000 years to occur.

http://www.oceanservice.noaa.gov/education/kits/corals/media/supp_coral04a.html

Fringing Reef

Holand Island

Grand Cayman Is.

Tureia, Coral Atoll Oeno, Fringing Reef

Bora Bora Atoll

Climate: What would you expect?

Realm of the Intertropical Convergence Zone

Hadley Cell

Ascending

moist air

condenses

& rains

ITCZ

Equatorial & Tropical

Latitudes influenced by

ITCZ

0º

Descendingwarms & driesIn subtropics

Desertzone

Descendingair warms& dries insubtropics

Desert Zone

Trade winds Trade winds

Global CirculationPatterns

Hadley Cell

Little Seasonal Change in Temp

Soils of the Tropics

Soils form over tens of thousands of years

Gives you a “map” of the “average” location of rainforests & savanna

Synonym: Oxisol Soils

Process ofLatosol or Oxisol Soils

Laterization soil development

Oberlander & Muller, 1984

Soil Name

OxisolLatosol(synonyms indifferentclassifications)

Clay type(kaolinite)Not holdnutrients

Oxisol Profiles

• Little organic matter

• Red from iron oxides

• Loose & friable texture

Paradox: Poor soils because few nutrients, yet great rainforests

Why? Natural nutrient recycling

With Deforestation

• organic matter leached• nutrients lost• Gradual loss of fertility

With deforestation• soil structure hardened by desiccation and

compaction into laterite

• Can make “laterite” by drying out the soil

Deforestation has always occurred: traditional swidden (slash and burn)

agriculture – burn to release nutrients and move to another plot

Feral relief

Steep landscapes

made by landsliding

from intense rains

in places like Hawaii

Andes where a lot

of relief exists

Steep slopes made possible by vegetation holding soil to steep faces

Intense rains saturates slopes and landslides rush down slopes

Flat valleys eroded by torrential floods

Landslides strip

trees

Net effect is relief gone wild (feral)

Kauai(GoogleEarth)

Mt Rotui, Moorea

Rarotonga, Cook Islands