GeologyRocks

I'd guess that it's not only where the crust is thickest, but where the rocks below are more dense. That would produce a positive gravitational anomaly too

~Jenny~

Hi 98kellrs,

As Jenny said, it's not just the thickness of the crust that affects the gravity anomaly. The density of rocks is a major factor. However, the BGS always publishes gravity anomaly maps that have been corrected for topography (the topographic and Bouger corrections - see our tutorial on gravity surveying in the geophysics section). This means that some of the effect of thicker crust has been accounted for.

Compare the geology and gravity maps - you should see high gravity over gabbros, basalts and other basic igneous rocks. Low gravity will occur over drift sediments, granites and sedimentary basins. As topography has been accounted for (over land at least, over the sea it will be a free-air anomaly) most of the differences you see will be due to rock density changes.

The changes in density of rocks can be quite large, from 2.1gm/cm3 for halite to 3g/cm3 for basic igneous rocks. Mineral veins can be even denser. As a gravity metre measures gravity to 0.1 gu - where 1 gu=1micrometre/second.

Hope this helps!

Jon

thats what i thought, but the areas with highest gravitational anomalies are above drift, with slate underneath it and carb limestone beneath that.

however the mid-high anomalies do correlate with lots of dolerite intrusions...its all so confusing!

Gravity studies are confusing! The problem stems from the non-unique solution. A body causing the anomaly could be small and shallow, or large and deep.

However, I think in this case the anomalies are due to deep bodies. As you said the highest anomalies are over the drift sediments (to the north?). These cannot produce a gravity high and hence it must be a deeper body, probably (although I am guessing) a basic igneous intrusion of Tertiary age as there are a lot up the west coast. I don't know if the intrusions go as far south as the Isle of Man, but it is a possibility. A lot of the western isles of Scotland are igneous bodies: Arran, Rum, Eigg, Muck, etc.

Have a look at the gravity map. Do the highest anomalies have a long or short wavelength? By that I mean, do they change slowly (long wavelength) or they noisey, messy looking (short wavelength)? A slow, gradual change indicates a deep body. Rapid fluctuations, with lots of noise indicate a shallow body. I'm betting (even though I have not seen this map!!) it's a slow change.

Let us kow how you get on

Jon

Unfortunately it doesn't provide a graph, only a map.

beneath the highest levels of gravity below the drift sediment is the manx group? Not sure if you've heard of them but they're a Slate based group, classed as a metasediment, as metamorphism has obviously turned them to slate, but fossils have been found. Do these kind of rocks produce a positive, negative, or neutral anomlay?

if not i guess the basalt must be much deeper!

The Tertiary Igneous Province extends down to Lundy Island in the Bristol Channel so the Isle of Man gravity anomalies could be related to a deep seated igneous body... or not! Igneous bodies do not always give rise to gravity highs - the Troodos ophiolite is a case where the mantle sequence - obviously igneous in origin - provides a gravity low over the highest areas of the island as the mantle harzburgite has been altered and hydrated to produce a low density serpentinite rock...

Cheers - sorry for the lack of knowledge regarding Manx Geology!

Simon

You are free to laugh at me but graivty isn't attraction between mass and another mass but mass and energy.

 Yes we have attraction between two bodies of mass even at 0 Deg K but that is because to make that mass you need some amount of energy. If you look at the map of the world you will clearly see that gravity is higher at locations where you have active volcanos or hot spots.

 I'm neither physicist nor geologist but I'm looking for a simple explaination for this issue and seriously people have non related answers. There are many other reasons why I say graivty is attraction between matter and enegry because we would have clumping of dark matter if two dark matter particles could attract each other but that is beside the point.

If you look at the world map or map of your island you'll see gravity is highest near the hot spots.

 Thanks