Earth Science 1121H Igneous Rocks Mantle Melting and Origin

Anhydrous melting

melting temperature increases with increasing pressure
- under dry conditions

Silicate melt is less \____ than solid rock

dense

As melting progresses

liquid will segregate from the solid residuum and ascend to the earth's surface

Melting mechanisms process

1. mantle rises towards the surface from point a will intersect with solidus on point b
2. at point b, the rising mantle will begin to melt
3. therefore melting ensues when mantle material moves to lower pressure or shallower depth

decompression melting

melting that occurs as rock ascends due to a drop in confining pressure
- occurs in passve respones to lithosphereic stretching and thinning as plates are pulled apart
- occurs in actively upwelling, convecting mantle

Decompression melting occurs at

divergent margins
and beneath plate interiors

oceanic lithosphere is created at

divergent margins

Plume

known as hot spots
- active decompression melting

The heat associated with plumes creates

elevated topography

Large volumes of magma are erupted from

the heads of plumes
- preserved in flood basalt provinces on continents

Oceanic plateaus form from

immense short-lived eruptions of basalt

Most recent volcanoes are located at

constructive and destructive plate margins

Melting to produce magmas must occur

in the mantle beneath lies of active volcanism

volcanism at convergent margins

- often explosive
- eruptions release large quantities of water

Water dramatically lowers

the melting temperature of pure minerals
- relative to the melting temperature under anhydrous conditions
- melting temperature decreases ith increasing pressure

Addition of water to the mantle may trigger

melting at subduction zones

Interaction of seawater and basalt

seawater oceanic crust penetrates along fractures
- water is heated and reacts with basalt to form hydrated oceanic crust

Magma genesis

1. Melting occurs beneath a mid-ocean ridge
2. mantle plume and a hot-spot volcano
3. subduction yields a volcanic arc