Transcript Definition of magma

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.
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.
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
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.
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.
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.
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)
• 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
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
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.
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 coursegrained.
– 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)
Click to play animation
Victoria Land, Antarctica
Glacier National Park, MT
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.
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
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
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