Magma & Volcanoes, pt. 1, mafic rocks

A. Magma forms when rock is heated up to its melting point to produce molten liquid.

1. Basalts melt at temperatures above 1000° C.

2. Granites melt at temperatures above 750° C.

3. Rocks that are heated to temperatures between these two numbers usually experience partial melting.

a. Silica-rich minerals, like quartz and feldspar, are melted in these circumstances.

b. The mafic minerals still remain crystalline.

B. Basaltic magmas are derived from the partial melting of mantle rock.

1. The magma rises toward the surface from the mantle and is usually within 5 miles of the surface.

2. Because the silica is diluted with other mineral components at a high temperature, the magma is very runny and thus not very viscous.

a. Viscosity is a measure of a fluid's resistance to flow.

b. Honey left in the refrigerator will be more viscous than tap water.

c. Basaltic magmas contain about 50% silica.

C. Andesitic and felsic magmas are formed by partial melting of pre-existing rock.

1. These magmas contain about 60% and 70% silica, respectively.

2. As a result of the additional silica, these magmas are more viscous than basaltic magmas.

II. How do basaltic eruptions occur?

A. Basaltic magmas most commonly erupt on the ocean floor at places where two tectonic plates move away from each other.

1. Such places are called spreading centers or mid-ocean ridges.

2. Iceland is one place on a spreading center where basaltic eruptions occur.

a. The island of Surtsey is located off the coast of Iceland and was formed out of a volcanic eruption in 1963.

b. Volcanic eruptions are a common occurance in Iceland, and they provide geothermal energy to much of the country.

B. Sometimes basaltic eruptions originate at the ocean floor where there is no spreading center.

1. These eruptions are caused by hotspots; this is a topic that will be discussed further in a later lecture.

2. The volcanic eruptions on the Hawaiian Islands are an example of such a phenomenon.

3. Hot spot eruptions often can produce great floods of basalt, which are sometimes called flood basalts or plateau basalts.

C. Volcanoes that extrude basaltic lavas are usually called shield volcanoes.

III. What features are associated with eruptions?

A. Cooling basaltic lavas (Hawaiian-type lavas) will form characteristic types of lava flows.

1. Pahoehoe flows are formed when the surface of the lava flow congeals while the still-molten subsurface continues to advance.

2. Aa flows form when the outer surface of the lava flow cools, expands in response to pressure from molten material below, fractures and breaks into rubbly rock. This texture is so named because ethnic Hawaiians used to make the sound "ah, ah" when they walked over this material in bare feet.

3. Pillow lavas form when basaltic magma erupts at the ocean floor.

4. Columnar joints form pillar-like columns in lava flows; Devil's Post Pile National Monument in California has a classic outcrop of this feature.

B. Eruptions involve the extrusion of gases, water, and rock fragments as well as the fluid lava.

1. Fissures that extrude mainly gas and vapor are called fumaroles.

2. Gaseous extrusions are often a precursor to magmatic eruptions.

3. Also preceding an actual eruption of lava, ash and pyroclastics may be extruded.

a. Pyroclastics are rock fragments and chunks of lava that are thrown out of a volcano.

b. These fragments may range in size from dust to large volcanic blocks and bombs (check out page 89 of your text for a real cool picture of bombs).

c. Ash that is extruded in an eruption, settles to the ground and solidifies, forms a kind of rock called a tuff.

C. Lava extruded in eruptions can produce different textures.

1. If the lava contains a lot of gas, this gas will try to escape once the molten liquid reaches the surface.

a. If the lava cools before all of the gas escapes, little holes or vessicles may be left in the rock.

b. This rock is called scoria.

2. Shield volcanoes are composed mostly of lava flows with very little pyroclastic material.

3. The slopes are usually very gentle, and the actual volcano is often quite large.

D. Eruptions of basaltic magma often form spectacular fountain-like jets of lava; these eruptions are called Strombolian eruptions.

Last update 2/23/2005
Webpage designed by Hiram Jackson.
Contact Geology webmanager, Hiram Jackson, at jacksoh@crc.losrios.edu