Physical

Question 1

What are Bathymetric profiles

Answer 1

A map showing sea floor depth

Question 2

What is the Hypsographic curver

Answer 2

A cumulative frequency curve based on the histogram showing actual frequency depth. It is a curve showing the percentage of the earth showing that lie above or below any giving levels. (max height- 8.8km aboveand max depth (11.04 km)

Question 3

What is the Abyssal plane

Answer 3

It is 4.5km deep usually lying between the continental rise and the MOR.

Question 4

What is Isostasy

Answer 4

Isostasy is the proposition that, at some depth under the ocean, the combined weight of the overlying mantle, ocean crust and water is same as the combined weight of the depth under the continents ( similar to the iceberg). Isostatic balance between oceanic and continental crust produces a 5km step at the ocean basin edge.

Question 5

What is sea floor spreading

Answer 5

If the head generated by natural radioactivity in the Earth’s deep interior was not brought to the surface and lost into space, the mantle would melt. This head is transported by convection, which involved the flow of the underlying mobile asthenosphere as well as transport of the overlying lithosphere. The lithospheric plate forms at the axial ridge of a mid-ocean spreading centre when the asthenosphere cools. It is continuously replaced by new asthenosphere rising and cooling at the ridge axis; it then thickens as it moves away and cools.

Question 6

At the mid ocean ridge axis

Answer 6

the lithosphere is progressively younger, warmer and less dense. Its lower density makes it more elevated

Question 7

What is Airy Isostatic Balance

Answer 7

The basis of the model is the Pascal’s law, and particularly its consequence that, within a fluid in static equilibrium, the hydrostatic pressure is the same on every point at the same elevation.

Question 8

Convection, conduction and liithospheric plates

Answer 8

The earth is 4.6 million years old but, if conduction was the only way of getting heat out of the interior, much of the mantle would have melted after only 2.7million years.

Question 9

What is internal mobility

Answer 9

(the asthenosphere). is necessary to allow convection to carry away heat generated by the decay of natural radioactivity within the earth. A breach in the lithosphere is necessary in order to allow this heat to escape because the lithosphere acts as a insulating blanket., the lithospheric plate cools by conduction of heat to sea water. As it cools, the lithosphere contracts and so becomes of higher density. Also as it cools the depth of the critical- 800c isotherm increases and so the lithosphere thickens

Question 10

Schematic Profile through the oceanic crust- mantle

Answer 10

constant 7 km thick ocean crust and low density. Mantle lithosphere increases in thickness with time as it cools. as it cool, it contracts and becomes more dense with asthenosphere. Asthenospheric mantle and so is more expanded and less dense.

Question 11

Age of oceanic crust

Answer 11

Near ridges, the percolation of sea-water through cracks- hydro-thermal circulation (important mechanism of heat loss. As the underlying crust is carried further away from the spreading centre, the depth of the sea increases: young (warmer less dense) floats higher than older (cooler more dense) lithosphere in isostatic balance. When a plate cools by loosing heat from its top surface only, the depth to the critical isotherm increases like the square root of time.

Question 12

The magnetic tape recorder

Answer 12

sea floor basalts are rich in iron and can be magnetised. when heated above a characteristic temp.they, like all magnetic materials, lose their ability to be magnetised but gain it again will cooling. volcanic rocks cooling through their curie temp acquire a remnant magnetization in the direction of the earths magnetic field at that time.

Question 13

The earths magnetic field reverses.

Answer 13

spreading sea floor, formed of cooling basaltic lavas. act as a magnetic tape recorder of the reversal sequence of the earths magnetic field. measuring the pattern of stronger magnetisation and weaker magnetisation confirms the sea floor spreading:the pattern is symmetrical

Question 14

How do we know the age of the earths crust

Answer 14

Hot spot plumes.

Question 15

How did the oceanic crust form

Answer 15

at shallower depths, the pressure is less. A decrease in pressure lowers the melting point of mantle rocks. The thinner lithosphere at a mid-ocean ridge causes the asthenospheric isotherms to be shallower and domed up there. Under normal temps conditions about 13% of partial melting occurs because of the uplifted isotherms. the mineral components of the mantle which melt first rise to the surface where they cool to form basaltic ocean crust. 13% of melt results in an ocean crust which is 7km thick

Question 16

Ocean crust notews

Answer 16

If the temperature of the undisturbed mantle happens to
be higher, there is a greater proportion of partial melting,
producing a thicker oceanic crust. Iceland is a place where the
spreading centre straddles a ‘hot spot’ (the top of a ‘jet’ of
rapid mantle convection) and crust up to 25 km thick has been
produced there.
(ii) Using equation 1, which ignores the density reduction due
to thermal expansion of young lithosphere, implies a water
depth of ~1.6 km for a crustal thickness of 25 km; equation 2
gives the water depth reduction due warmed lithosphere above
the ridge axis (t = 0-1) as 2.0-2.4 km instead of 5 km;
together the two equations suggest that the ‘sea floor’ should
be 400-800 m above sea level - hence Iceland!

Question 17

old plates sink

Answer 17

The mantle component of oceanic lithosphere is cooler (and hence contracted
and more dense) than the underlying asthenospheric mantle. (Thermal expansion
of basalt is about 20 ppm/°C, giving 1.6% contraction at the top surface of the
lithosphere compared with at its base)
The mantle component of the oceanic lithosphere thickens with age: after about
100 million years, it is about 80 km thick. But there is always a fixed thickness
of ocean crust welded on to it.
A more dense material does not float on a less dense one - it sinks! The average
density of the oceanic lithosphere combines the mantle component (fractionally
more dense than the underlying asthenosphere) with the much less dense
oceanic crust.
The higher density mantle component increases in thickness with age and
eventually offsets the buoyancy of the crust welded to it after about 150 - 200
million years of cooling: it is then unstable and sinks.

Question 18

Oceanic lithosphere

Answer 18

Oceanic lithosphere sinks (subducts) at ocean trenches, causing a characteristic dipping zone
of Earthquakes (a Benioff Zone). (Earthquakes can ONLY occur in lithosphere and are
normally only found in the top 20 km where the lithosphere is particularly brittle. Near
trenches Earthquakes can be found to depths up to 700 km.
Any water combined with basalt during hydrothermal circulation near ridges, together with
any sediment accumulated on the volcanic floor of the oceans which is not physically scraped
off as the plate descends, boils off or melts at the higher temperatures found at depth: - a
subduction zone generates a characteristic type of island arc volcano.
Once the lithosphere has begun to sink, the process tends to run away and the down-going
slab may suck lithosphere down the hole which is itself a little to young to be unstable. ‘Slab
pull’ is the main driving force of plate motions.