Geology 309 - Lecture 6

Magma sources, Types of basalt

Use lecture notes, read chapter 3

Most mafic type of basalt with high olivine content
Approximately 200 degrees C hotter than most typical basalts
Most common in the Archean, when the Earth was a lot hotter (none erupted today).
Click here to see an example of what this rock looks like in the field

MORB - Mid Ocean Ridge basalts. Rising limb of convection cells in mantle at spreading ridge.
OIB - Ocean island basalts. Hotspots such as Hawaii. Controversy over source. Is it core-mantle boundary or shallower in mantle?
IAB - Island arc basalt. Quite chemically different from others. Dehydration of subducting slab
CAB - Continental arc basalt. From a continental arc rather than an island arc.
Continental Rift basalts - typically very exotic. Very alkaline.
Continental flood basalts - similar to OIBs, but massive proportions usually erupted on continents.

Lunar mare (basalt) extends over huge areas, no conspicuous fissures.
- Similar to flood basalts on Earth.
- Nice flat surfaces, made nice landing pads for lunar missions.
- Click here for some nice pictures
Age - 3.95 to 3.15 billion (book uses British term, thousand million)
(Compare to oldest MORB on Earth - no MORB older than 180 million years. Why?)
Origin of lunar basalts - Formation of the Moon
- Soon after Earth formed from solar nebula, struck by a huge impacting body. Size of Mars.
- Large chunk of the Earth was blasted off
- Debris settled into orbit around Earth, rapidly accumulated to form the Moon
- Entirely molten at first, then had global magma ocean around a denser solid core (4.4 by). Depth was a few hundred kilometers
- Developed chilled skin on top (basalts)
- Deeper, crystals formed, dense ones settled to bottom (olivine and pyroxene), buoyant ones floated to top (feldspars). These buoyant ones created the anorthosite lunar highlands that rise above the lunar mare
- Total solidification