Explain the relationship between igneous activity, magmatic

composition, rock types and plate boundaries.

Constructive Plate Margins

What is it that melts?Upper mantle/peridotite.What is it’s composition?Ultrabasic.Why does it melt?

Reduced P as crust above is stretched and thinned.Increase in T due to hot rising convection currents.

Constructive Plate Margins 2

What kind of magma is produced by melting the ultrabasic mantle?

Basic.

What does the magma do?

Rises up through the thin crust to the surface.

It may erupt underwater to produce what?

Basalt pillow lavas.

Constructive Plate Margins 3

Some of this basic magma will cool the the fractures feeding up to the surface.

What rock and structure will form?

Sheeted dolerite dykes.

Some magma will cool in the magma chamber to form what rock?

Gabbro.

Ocean v Ocean CollisionThere are 3 reasons why crust may melt.

Which is occurring in this situation? Increased T as the plate sinks

deeper. Also the plate is saturated in

water.

The melting oceanic crust is basic so what kind of magma will form when it melts?

More SiO2 rich and so will move towards being intermediate.

This magma will move upwards through the overlying mantle and thin crust to the surface.

Then what will happen?

Oceanic v Oceanic 2Explosive volcanic activity.

Basic/intermediate magma will be fairly violent.

What will the products be:

Lavas will be as important as pyroclastics.

Lavas: Mainly andesites.

Pyroclastic deposits:Ash = tuff.

Pyroclastic flow deposit = ignimbrite.

Large blocks close to the vent = (vent) agglomerate.

Oceanic v Oceanic 3Will the basic/intermediate magma undergo fractional crystallisation/magmatic differentiation?

Yes.

So what will happen to the magma over time feeding a particular volcano?

It will become more SiO2 rich.

What will that do to the style of eruptions?

Become more explosive.

Oceanic v Oceanic 3What intrusive rocks may form?Plutonic intrusions that form deep down in batholiths.What will be the crystal size?Coarse.They will tend to be intermediate so what will the rock be?Diorite.

Oceanic v Oceanic 4Hypabyssal intrusions (just below surface and small – dykes and sills).

What will their crystal size be?

Medium.

So what will the rock type be?

Microdiorite.

Ocean v Continental CollisionThe subducting oceanic plate will melt just as in ocean v ocean.

The magma produced will be the same. Which is?

Basic to intermediate.

It will rise up into the continental crust and this is where the differences occur.

Ocean v Continental Collision 2What happens to the melt as it enters the continental crust?Remember that the magma is now intermediate(ish).It travels up through the continental crust that has what composition?Acid.The hot magma melts the continental crust (which has low T minerals) and adds SiO2 rich crust to the intermediate magma to make magma that is very intermediate or acid.

Ocean v Continental Collision 3There will be a greater proportion of pyroclastics compared top lavas.What are the products?Lavas: Mainly andesites, some rhyolites.Pyroclastic deposits:

Ash = tuff.Pyroclastic flow deposit = ignimbrite.Large blocks close to the vent = (vent) agglomerate.

Ocean v Continental Collision 4What intrusive rocks may form?

Plutonic intrusions in batholiths will be coarse and will undergo fractional crystallisation and so a wide range of rock types may occur.

Remember they can form zoned intrusions with basic at the margin acid in the core and intermediate between he two.

Ocean v Continental Collision 5What will the rock types be:

Acid = granite

Intermediate = diorite

Basic = Gabbro

There will also be hypabyssal intrusions forming medium grained equivalents:

Acid = Microgranite

Intermediate = microdiorite

Basic = microgabbro or dolerite.

Continent v Continent CollisionWhat is melting?The base of the continental crust.Why is it melting?Because the crust is so thick that it is hot enough to melt.What is the composition of the rock that melts?Acid/intermediate.So what will the resultant magma be?Acid.

Continent v Continent Collision 2What happens to this magma?It will intrude higher.Will it reach the surface?No.Why not?For 3 reasons:

1. The magma is very viscous.

2. The magma is relatively cool (it is just hot enough to melt).

3. The crust is too thick.So it crystallises deep in the earth/plutonic.

Continent v Continent Collision 3This acid magma cools at depth in batholiths.What rock type will it form?Granite.The magma may undergo some fractional crystallisation and so there can be some zoning with possibly intermediate rocks at the edge and very acid rocks in the centre.Smaller intrusions may produce microgranites.