A is for Apples,B is for Basalts…

*Picture from USGS http://volcanoes.usgs.gov/Products/Pglossary/basalt.html

3 Basic Types of Rocks

• Igneous - formed from melted rock that has cooled and solidified. basalts

• Sedimentary - are layered accumulations of sediments-fragments of rocks,

minerals, or animal or plant material.• Metamorphic - sedimentary and igneous rocks

are subjected to pressures so intense or heat so high that they are completely changed, but not melted.

Igneous Rocks

• Basalt is the most common rock type in the Earth's crust. Makes up most of the ocean floor and many islands.

• They are dark in color and usually relatively dense.

• Basalt compositions are rich in MgO and CaO and low in SiO2 and Na2O plus K2O relative to most common igneous rocks. Basaltic magma is commonly produced by direct melting of the Earth's mantle.

• Basalt is erupted at temperatures between 1100 to 1250° C.

Continental Basalts

• Vary more in composition than those that erupt in the oceans

• Eruption at surface is preceded by ascent through tens of kilometers of continental crust.

Subaerial Basaltic Lava

• Pahoehoe flows – “ropy,” highly fluid lava moves swiftly down a steep slope, spreading out rapidly into sheets (~1 m thick).– Smooth, ropy…

• ‘A’a flows – as basaltic lava flows farther from the vent it cools & flows more slowly, forming a thick brittle crust which breaks and reforms as the flow moves.

• Jagged, rough, very sharp * ‘a’a was the Native Hawaiian term for “ahhh,” as in a painful scream…

• Basalt flows may also contain lava tubes, where lava moves under the surface of solidified lava & drains out as the eruption wanes, leaving them empty.

• Vesicles – gas holes that can be preserved at the top of the basalt when it cools. (scoria)

• Columnar jointing – as basaltic lava cools, it shrinks in volume, often producing a pattern of cracks, which as cooling proceeds, extends inward from the top & bottom, forming hexagonal columns of rock.

Subaqueous Basalt Eruptions

• Much more common than subaerial.• Basalt is the principal rock of the ocean floor.• Pillow basalts – resembles pillows… Formed

when lava is instantly chilled by cold water, forming a “skin” that expands, sort of balloon-like, as lava continues to flow into it, until it cracks and the process starts again…

Basalts in Sicily