Irruputuncu, Chile, Smithsonian

Geology for Engineers

Minerals and Rocks (I)

Minerals and Rocks (I)

Minerals and Rocks (I)

Igneous Rocks

• Key Questions:

• What are igneous rocks?

• From what are they formed?

• How do they crystallise?

• How are they emplaced?

Forms of Igneous Rocks

• Volcanic:– Lavas erupt from volcanoes either as molten

fluids, or are blown out as volcanic ash by violent explosions

– Black volcanoes (effusive, mostly basaltic)– Red volcanoes (explosive, mostly felsic)

This classification is based on composition. Formation of phenocrysts on cooling increases the viscosity

Ash & Lava flow deposits

Main Volcano Types• Shield volcanoes – these may build

volcanic islands, e.g. Hawaii or Iceland– These usually have a low “aspect ratio”

meaning they are much wider than they are high

• Composite volcanoes – these tend to be smaller & are built up of layers of lava and ash– These are much steeper in form

Shield volcanoes

Mauna Loa (left) is the largest volcano on Earth, and is one of the most activeOlympus Mons (right) is the largest volcano, and has an area approximately the size of France.

Composite volcanoes

wapi.isu.edu

Llaima (pictured right), is one of Chile's largest and most active volcanoes•It contains two main historically active craters, one at the summit and the other to the SE.•Frequent moderate explosive eruptions with occasional lava flows have been recorded since the 17th century.

http://www.volcano.si.edu/

Common Volcanic Rocks

Vesicular basalt

Welded tuff

www.npolar.no

Common Volcanic Rocks

Pumice - froth of felsic volcanic glass

Tuff - pyroclastic texture shows a mixture of rock fragments, pumice, and volcanic ash

www.pitt.edu

Common Volcanic Rocks

Pahoehoe lava flow - Ropy lava - formed when a lava flow skins over while flowing very slowly

Aa lava flow - Jagged lava formed by the shattering of a solid skin that forms over a rapidly moving flow

www.npolar.no

Submarine Volcanism

• Basalt lava is erupted onto the sea floor at submarine spreading centres

• In these conditions, eruption styles differ from volcanic eruptions on land

Submarine Volcanism

“Pillow Basalts”

Hypabyssal Rocks

• These are igneous rocks emplaced at a high level into the crust they may occur as:

– Volcanic pipes, dykes or sills– Ring dykes and cone sheets

Hypabyssal Rocks

Feldspar phenocrysts

Intrusion of a dyke

Plutonic Rocks

• Further down in the magma chamber, cooling happens at a slower rate and crystals form larger sizes.

• We can only see plutonic rocks when erosion has stripped away the covering rocks, or faulting has brought them up

Plutonic Rocks

The Brandberg is an intrusion in the Namib desert in western Namibia with a diameter of about 15km and with a height of 2573m is the highest point in Namibia. Southwest of it is the Messum Crater, and behind, the Erongo Mountains

www.earth.imagico.de

Plutonic Rocks

Typical granite