| Plate Tectonic Setting |
Process and location of magma formation |
Lavas erupted (and why!) |
| Divergent Boundaries |
|
|
Mid-ocean ridges
|
Decompression partial melting of
asthenosphere produces mafic magma. Magma formed at depths of as little as 10 km, rises
and pools in magma chamber in the lower oceanic crust |
Mafic lavas erupt to form pillow basalts.
Magma also cools at depth to form sheeted basalt dikes and the gabbros of the lower
oceanic crust |
Continental rifts
|
same as mid-ocean ridge |
Bimodal eruptions. Mafic lavas erupt to
form basalts, but rising mafic magmas also melt continental crust causing felsic lavas to
erupt forming rhyolites. |
| Convergent Boundaries |
|
|
Oceanic-oceanic subduction zones
|
Mafic to intermediate magmas formed at
depths of about 150 km by partial melting of the water-rich ocean crust.
This dewatering of the subducting plate causes hydration of the surrounding
asthenosphere which, in turn, partially melts forming mafic magmas. |
Eruptions range from mafic to intermediate.
Often early eruptions are mostly mafic becoming more intermediate as the island arc grows. |
Oceanic-continental subduction zones
|
same as oceanic-oceanic |
Eruptions are predominantly intermediate.
Rising magmas are altered by fractional crystallization and assimilation of
continental crust. Some mafic magmas erupted. Heating and hydration of the continental
crust also causes felsic magmas to form which usually solidify at depth forming granitic
plutons. |
Continental collisions
|
Felsic magmas formed as wet felsic material is
compressed due to crustal thickening |
None. These felsic magmas solidify before
reaching the surface forming granitic plutons. |
| Hot Spots |
|
|
Under oceanic crust
|
Partial melting in the asthenosphere produces
mafic magmas |
Mafic magmas erupted |
Under continental crust
|
same as under oceanic crust |
Bimodal eruptions. Mafic lavas erupt to
form basalts, but rising mafic magmas also melt continental crust causing felsic lavas to
erupt forming rhyolites. |