1. Definition of magma
“Naturally occurring mobile rock material, generated within the earth and capable of intrusion and extrusion, through which igneous rocks are thought to have been derived through solidification and related processes. It may or may not contain suspended solids (such as crystals and rock fragments) and/or gas phases.” Glossary of Geology, 3rd edition.
Magmas are highly diverse in their physical properties, as manifested by their eruptive styles. What factors lead to this diversity?
The Formation of Magma
Mantle rock melts when the temperature increases or the pressure decreases.
Presence of water in these rocks cause water to form crystiline structure with it and lower the melting point.
definition from the Glossary of Geology, 3rd edition. Bates and Jackson.

2. Factors that affect Melting Temperature
Pressure
The melting point of a mineral increases with increasing pressure.
Both temperature and pressure increase with depth in the Earth’s crust.
For this reason, a rock that melts at a given temperature at the Earth’s surface, will require a higher temperature to melt deep in the crust.
Water
The present of gas, especially water vapor, significantly lowers a mineral’s melting temperature.
This occurs because water under high pressure helps break the silica bonds in the mineral.

3. Three types of Magma
The composition of magmas and lavas is controlled by the most abundant elements in the Earth Si, Al, Fe, Ca, Mg, Na, K, H, and O
Three distinct types of magma are more common than others:
Basaltic, containing about 50 percent SiO2
Andesitic, about 60 percent SiO2
Rhyolitic, about 70 percent SiO2

4. Andesitic Magma: this mgma is found along continental margins where oceanic crust is subducted into earth’s mantel. 60% high silica content has intermediate viscosity and have intermediate eruptions.
Basaltic Magma: formed when rocks in the upper mantle melt , this then rises rapidly to the surface and reacts very little with crustal rocks because it is low in viscosity. Because there are small amounts of gaes and silica the volcanoes it fuels erupt quietly.
Rhyolitic Magma forms when molten material rises and mixes with overlying silica and water-risch continetal crust. Since it is highly viscous it also produces very explosive volcanoes.

5. Magma Viscosity
The internal property of a substance that offers resistance to flow is called viscosity
The more viscous a magma, the less easily it flows
Viscosity of a magma depends on temperature and composition (especially the silica and dissolved-gas contents)
The higher the temperature, the lower the viscosity, and the more readily magma flows
The greater the silica content, the larger is the polymerized group
For this reason, rhyolitic magma (70% silica) is always more viscous than basaltic magma (50% silica)
Andesitic magma has a viscosity that is intermediate between the two (60% silica)

6. Factors affecting viscosity continued
Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt)
Dissolved Gases
Gas content affects magma mobility
Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure
The violence of an eruption is related to how easily gases escape from magma

7. Extrusive vs. Intrusive Igneous Rocks
There are two main categories of igneous rocks:
Extrusive (Volcanic rocks)
rocks that form from materials erupted at the Earth’s surface
result from he cooling of lava flows and the consolidation of pyroclastic materials
Intrusive (Plutonic rocks)
rocks that form from magma that cools below the Earth’s surface

8. Deep Igneous Structures
Pluton – a body of magma or igneous rock that crystallizes deep within the Earth’s crust.
They are arbitrarily distinguished by size when they are exposed at the Earth’s surface.
Stock – When a small pluton becomes exposed at the Earth’s surface with an outcrop area of less than 100Km2, it’s called a stock.
Batholith – when the outcrop area of an exposed pluton is greater than 100km2, it’s called a batholith.

9. Shallow Igneous Structures
Dike
A tabular intrusive structure that is discordant relative to any layering of the surrounding country rock; it cross cuts surrounding layers.
Shallow and fine-grained or deeper and course-grained.
Sill
A tabular intrusive structure that is concordant relative to any layering of the surrounding country rock.
They are parallel to existing rock layers (usually sedimentary rock layers)
Victoria Land, Antarctica
Click to play animation
Glacier National Park, MT

10. Anatomy of a Volcano
• A volcano is a mountain formed of lava and/or pyroclastic material.
• A crater is the depression at the summit of a volcano or that which is produced by a meteorite impact.
Volcanic craters with depressions up to 50 Km in diameter are called calderas.
• A conduit, or pipe, carries gas-rich magma to the surface.
Lava erupts through an opening in the crust called a vent.

11. Types of Volcanoes
The three main volcanic types are shield volcanoes, cinder cones, and composite cones.
• Shield volcanoes are broad, gently sloping volcanoes built from fluid basaltic lavas.
Formed when layer upon layer of basaltic lava accumulates during nonexplosive eruptions.
Example: Hawaiian islands (Mauna Loa)
• Cinder cones are small volcanoes built primarily of pyroclastic material ejected from a single vent.
- Less than 500 m high
Steep slope angle
Rather small in size
Frequently occur in groups

12. Composite Cones
• Composite cones are volcanoes composed of both lava flows and pyroclastic material. ( layers of volcanic fragments alternate with lava)
- most are adjacent to the Pacific Ocean ( Mt. Rainier)
- Large size
- interbedded lavas and pyroclastics
- Most violent type of activity

13. Volcanic Material: Rocks thrown into the air during a volcanic eruption are called tephra
• Pyroclastic materials is the name given to particles produced in volcanic eruptions.
• The fragments ejected during eruptions range in size from very fine duct and volcanic ash (less than 2 millimeters) to pieces that weigh several tons.
• Types of pyroclastic material
- Ash and dust – fine , glassy fragments
- Pumice - frothy , air filled lava
- Lapilli – walnut-sized particle
= Cinders – pea – sized particles
Particles larger than lapilli
* Blocks – hardened lava
Bombs – ejected as hot lava