Creation of Magma Unlike snow, rock doesn’t all melt at once, because rocks are made up of several minerals, each with its own melting point. This reflects the strength of its bonds. Partial Melting: as rock is heated, not all minerals melt at once. Some minerals melt but some (with higher melting points) remain solid (e.g., candy bar) Magma! Partially melt peridotite  basalt magma Partially melt basalt  andesite magma Partially melt andesite  rhyolite magma

I. Factors controlling melting A. Heat Temperature within Earth increases with depth: geothermal gradient So rocks at depth will melt. The top of Asthenosphere is ~1% liquid, lithospheric plates “float” on it. Geothermal gradient

B. Pressure: High pressure holds the ions and atoms in a crystalline solid closer together At higher pressures it harder to vibrate and break bonds. Increase pressure (i.e. down in the Earth) the temperature needed to melt the rocks also goes up. Decompression Melting: If the pressure on rock is somehow reduced or removed, as happens when tectonic plates rift and diverge It’s melting point drops below it’s current temperature and it melts. I. Factors controlling melting

I. Factors controlling melting

Decompression melting

C. Water Content : Water (even small amounts <1%) lowers the melting point of rocks Water tugs at ions and weakens (loosens) bonds, so lower temperatures will vibrate the bonds more and break them How this works at subduction zones to create melt - dewatering of slab (due to increase in temp and pressure) Experiments show: melting temp of basalt lowered 100  C with just 0.1 % water. Melting oceanic crust (basalt) forms rocks for continental crust (andesite) I. Factors controlling melting

A. The Basics 1.Eventually every magma cools and solidifies—over range of ~200  C. 2. Lava will crystallize into individual minerals (i.e. freeze salt water, you don’t get frozen saltwater, you get ice and salt (e.g., form separate crystals because separate minerals) 3. Melting Temp = Crystallization Temp a. Minerals that melted first, solidify last (i.e. quartz and orthoclase/K-spar). b. Minerals that melted last, will be the first to crystallize during cooling (olivine, pyx) II. Crystallization of magma solidification

A. The Basics 3. Melting Temp = Crystallization Temp (con’t) a. Minerals that melted first, solidify last (e.g., quartz and orthoclase/K- spar). b. Minerals that melted last, will be the first to crystallize during cooling (olivine, pyx) 4. As magma cools, crystals progressively form. So a partially cooled body of magma contains solid crystals of minerals that crystallize at higher temperatures, and liquid containing the atoms and ions of minerals that will not crystallize until the temperature is lowered further. II. Crystallization of Magma solidification

B. Bowen’s Reaction Series: During cooling, minerals crystallize in a systematic fashion based on their melting points

Bowen’s reaction series: Highest temperature minerals first to lowest temperature minerals.

Discontinuous Series: Each successive type differs in both composition and internal structure Continuous Series: As it cools, the Ca is in the solid plagioclase crystals and Na is in the melt. Na starts going into the crystal more— plagioclase becomes zoned. As minerals crystallize, the composition of the remaining melt changes - Fe, Mg, Ca taken out, and it becomes more Na, K, Si rich Bowen’s reaction series Highest temperature minerals first to lowest temperature minerals.

Rock NameColorCompositionMineralsTemp Occurrence RhyoliteLight~70%wt SiO 2 quartz~800  Ccontinents low in Mg, Fe feldspar amphibole mica AndesiteSalt-pepper~60%wt SiO 2 quartz~1000  Ccontinent- feldspar ocean amphibole boundary pyroxene BasaltDark~52%wt SiO 2 feldspar ~1200  Cocean- hi Mg,Fe olivine ocean pyroxene boundary

A. Fractional Crystallization & crystal settling Crystals form (more dense) sink to bottom of magma chamber and can no longer react with the remaining melt so the minerals that form will be different (no Fe, Mg, or Ca). Rocks that form differ from each other and from the original magma composition. This creates several chemically diverse igneous rocks III. Processes that change the composition of the magma

III. Processes that change the composition of the magma. A. Fractional Crystallization & crystal settling

III. Processes that change the composition of the magma. Magma and early forming crystals (minerals)

B. Assimilation Incorporation of foreign material into the magma Pieces of wall rock may break off the wall and fall into the magma and wholly or partially melt—changes composition of the magma Xenolith: piece of foreign rock that doesn’t melt (temperature not great enough) and just gets carried along for the ride It stays a solid the whole time III. Processes that change the composition of the magma

B. Assimilation Incorporation of foreign material into the magma Pieces of wall rock may break off the wall and fall into the magma and wholly or partially melt—changes composition of the magma Xenolith: piece of foreign rock that doesn’t melt (temperature not great enough) and just gets carried along for the ride It stays a solid the whole time III. Processes that change the composition of the magma

C. Magma Mixing Two different magma bodies may mix and create a new magma—mix by convection Sometimes you can see two distinct magmas in the erupted rocks III. Processes that change the composition of the magma

Processes that change the composition of the magma A. Fractional crystallization and crystal settling B. Assimilation C. Magma Mixing Summary Processes that change the composition of the magma Factors controlling melting of rocks A.Temperature B.Pressure C.Water content Partially melt peridotite  basalt magma Partially melt basalt  andesite magma Partially melt andesite  rhyolite magma