Download 3A8 Week 01 Lecture 02-Rocks and minerals 01

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Geology for Engineers
Minerals and Rocks (I)
Irruputuncu, Chile, Smithsonian
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
• Composite volcanoes – these tend to be
smaller & are built up of layers of lava and
– These are much steeper in form
Shield volcanoes
Mauna Loa (left) is the largest volcano on Earth, and is one of the most active
Olympus Mons (right) is the largest volcano, and has an area approximately the
size of France.
Composite volcanoes
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.
Common Volcanic Rocks
Vesicular basalt
Welded tuff
Common Volcanic Rocks
Tuff - pyroclastic texture shows a
mixture of rock fragments, pumice,
and volcanic ash
Pumice - froth of felsic volcanic
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
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
Plutonic Rocks
Typical granite