Docsity
Docsity
Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity

Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan

Guidelines and tips
Guidelines and tips
Sell on Docsity
Sell on Docsity
Docsity AI
Docsity AI
Log inSign up

Prepare for your exams

Study with the several resources on Docsity

Find documents

Prepare for your exams with the study notes shared by other students like you on Docsity

Search for your university

Find the specific documents for your university's exams

Docsity AINEW

Summarize your documents, ask them questions, convert them into quizzes and concept maps

Explore questions

Clear up your doubts by reading the answers to questions asked by your fellow students

Earn points to download

Earn points by helping other students or get them with a premium plan

Share documents
download point icon20 Points

For each uploaded document

Answer questions
download point icon5 Points

For each given answer (max 1 per day)

All the ways to get free points
Get points immediately

Choose a premium plan with all the points you need

Study Opportunities

Choose your next study program

Get in touch with the best universities in the world. Search through thousands of universities and official partners

Community

Ask the community

Ask the community for help and clear up your study doubts

Free resources

Our save-the-student-ebooks!

Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors

Igneous Rocks: Geometry, Origin, Texture, and Composition, Lab Reports of Geology

Virginia Polytechnic Institute and State University (Virginia Tech)Geology

An overview of igneous rocks, their formation, and classification. It covers the processes of magma formation, intrusive and extrusive rocks, textures, and mineral composition. Students will learn about the different types of igneous rocks, such as batholiths, sills, laccoliths, and their textures, including aphanitic, phaneritic, pegmatitic, porphyritic, vesicular, and pyroclastic. The document also introduces the concept of bowen's reaction series and its significance in understanding the mineralogical and textural evolution of magmas.

Typology: Lab Reports

Pre 2010

Uploaded on 09/26/2008

wellhan
wellhan 🇺🇸

2 documents

1 / 5

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
Lab 5: Igneous Rocks and Volcanic Hazards
Objectives
1. Explore the geometry and origin of some intrusive and extrusive bodies
of igneous rock
2. Be able to describe and interpret textural features of igneous rocks
3. Be able to describe compositional features of igneous rocks
(mineralogy, color index).
4. Be able to classify common igneous rocks based on their texture,
mineralogy, and color index.
5. Infer the origin of common igneous rocks
First, lets review rock-forming processes for igneous rocks. Rocks are the solid
materials that comprise most of Earth, our Moon, and other rocky planets of our
solar system. Nearly all rocks are solid aggregates of mineral grains (particles),
either mineral crystals or clasts (broken pieces) of mineral crystals and rocks (e.g.,
pebbles, gravel, sand, and silt), with a few notable examples like obsidian, which is
made of volcanic glass.
Rock forming materials come from three main processes:
1. The Earth’s mantle as molten rock called magma while underground and lava
when it erupts to the surface.
2. Space in the form of meteorites.
3. The fragmentation and chemical decay of mineral crystals and other rocks.
Environmental changes and processes affect these materials and existing rocks in
ways that produce three main rock groups: Igneous rocks, Sedimentary rock,
and Metamorphic rocks.
Today we will talk about igneous rocks………
Igneous rocks form when molten rock (magma or lava) cool to a solid form, either
as glass or masses of tightly intergrown mineral crystals. If the rock had a log time
to grow in a slowly cooling magma the crystals would be large, however, if the rock
cooled rapidly the crystals would be very small. Because when molten rocks cool
they form a mass of intergrown crystals and/or glass, all igneous rocks and
fragments of igneous rocks have crystalline or glassy texture.
Igneous processes and rocks
Under the Earth lies magma, which consists of molten rock, dissolved gases (H20,
C02, etc.), and tiny crystals that may grow in size or abundance as the magma cools.
Magma is under great pressure and is less dense than the rocks that confine it,
which causes it to rise through Earth’s cooler crust through any fractures or zone of
weakness in the form of an intrusion.
pf3
pf4
pf5

Partial preview of the text

Download Igneous Rocks: Geometry, Origin, Texture, and Composition and more Lab Reports Geology in PDF only on Docsity!

Lab 5: Igneous Rocks and Volcanic Hazards

Objectives

1. Explore the geometry and origin of some intrusive and extrusive bodies **of igneous rock

  1. Be able to describe and interpret textural features of igneous rocks
  2. Be able to describe compositional features of igneous rocks** **(mineralogy, color index).
  3. Be able to classify common igneous rocks based on their texture,** **mineralogy, and color index.
  4. Infer the origin of common igneous rocks** First, lets review rock-forming processes for igneous rocks. Rocks are the solid materials that comprise most of Earth, our Moon, and other rocky planets of our solar system. Nearly all rocks are solid aggregates of mineral grains (particles), either mineral crystals or clasts (broken pieces) of mineral crystals and rocks (e.g., pebbles, gravel, sand, and silt), with a few notable examples like obsidian, which is made of volcanic glass. Rock forming materials come from three main processes:
  5. The Earth’s mantle as molten rock called magma while underground and lava when it erupts to the surface.
  6. Space in the form of meteorites.
  7. The fragmentation and chemical decay of mineral crystals and other rocks. Environmental changes and processes affect these materials and existing rocks in ways that produce three main rock groups: Igneous rocks, Sedimentary rock, and Metamorphic rocks. Today we will talk about igneous rocks……… Igneous rocks form when molten rock (magma or lava) cool to a solid form, either as glass or masses of tightly intergrown mineral crystals. If the rock had a log time to grow in a slowly cooling magma the crystals would be large, however, if the rock cooled rapidly the crystals would be very small. Because when molten rocks cool they form a mass of intergrown crystals and/or glass, all igneous rocks and fragments of igneous rocks have crystalline or glassy texture. Igneous processes and rocks Under the Earth lies magma , which consists of molten rock, dissolved gases (H20, C02, etc.), and tiny crystals that may grow in size or abundance as the magma cools. Magma is under great pressure and is less dense than the rocks that confine it, which causes it to rise through Earth’s cooler crust through any fractures or zone of weakness in the form of an intrusion.

An Intrusive igneous rock is coarse-grained rock comprised of visible mineral crystals and have different sizes and shapes.  Batholiths – massive intrusions covering regions of 100 km^2 or more that have no visible bottom and form when small bodies of lava amalgamate into one large body.  Sills – sheet-like intrusions that force their way between layers of bedrock.  Laccoliths – blister-like sills that inject magma between two layers of sedimentary rock.  Pipes – Vertical tubes or pipe-like intrusions that feed volcanoes.  Dikes – sheet-like intrusions that cut across layers of bedrock and occur in the following geometries. o Sheet dikes – nearly planar dikes that often occur in parallel pairs or groups. o Ring dikes – curved dikes that form circular patterns when viewed from about typically underneath volcanoes. o Radial dike – develop on pipes that feed volcanoes. As magma rises to the earth’s surface the decrease in pressure causes the dissolved gases to separate from the magma creating lava. An Extrusive igneous rock forms when this lava is ejected on to earth’s surface and cools rapidly creating very fined- grained/glassy rocks. Textures of Igneous Rocks Texture is a description of its constituent parts and their sizes, shapes, arrangement, and helps to classify and inter the origin of igneous rocks. As earlier mentioned, the size of mineral crystals generally indicates the rate at which the lava or magma cooled and the availability of the chemicals required to form the crystals.  Large crystals require a long time to grow, indicative of slow cooling and ample atoms of the chemical required to form the crystal.  Tiny crystals indicate a rapid cooling of magma  Volcanic Glass can indicate that a magma was quenched (cooled immediately), and also that there was poor nucleation o Nucleation is the initial formation of a microscopic crystal, to which other atoms progressively bond. Basically, atoms are mobile in a fluid magma. If it has low viscosity and slow cooling, the crystals have plenty of time to grow. If the magma is very viscous, the atoms cannot easily move to the nucleation sites causing a lack of large crystals. Several terms are used to describe a rock based on crystal size:  Aphanitic texture – crystals are too small to see without a hand lens (<1mm).  Phaneritic texture – visible crystals, coarse grained (1-10mm).  Pegmatitic texture – very coarse grained with large crystals (>10mm).  Porphyritic texture – Igneous rock with two distinct crystal sizes

It is important to note that the color index in only an approximation of the mineral composition and that the specific minerals should be identified as sometimes a mafic, light colored mineral may appear to be a dark mineral and vice versa. Steps to classify an igneous rock Step 1 and 2: Identify the rock’s color index, then if possible, identify the minerals that make up the rock and their abundance percentage. o If rock is fine-grained or you cannot identify the minerals, use the color index to estimate the mineralogy. o If the rock is coarse-grained, estimate the color index and abundance percentages of the minerals. Step 3: Identify the rock’s texture (use Figure 5.2). Step 4: Classify the rock using the flowchart (Figure 5.2 and 5.3). o Use textural terms as adjectives. For example, if a rhyolite contains phenocrysts it must be called porphyritic rhyolite. o Use the textural information to infer the origin of a volcanic rock. For example, vesicles imply that the rock formed by cooling of a gas-rich lava. Bowen’s Series of Mineral Crystallization and Reaction in Magma American Geologist, Norman L. Bowen made the observation in the early 1900’s that a single dike, sill, or batholiths can usually contain more than just one kind of igneous rock even when originating in a single homogenous body of magma as it cools. In order to understand this he carried out experiments to study how magmas

might evolve in ways that could explain this differentiation. First, he placed pieces of periodite into bombs (strong pressurized ovens used to melt the rocks at high temperatures around 1200-1400C). Once melted the molten rock was cooled at a given temperature for a length of time in order for crystals to form and then quenched. The minerals were then identified in each rock cooled at different temperatures, and the following was shown: As magma cools, different silicate minerals crystallize in predictable series. The information is summarized in the Bowen Reaction Series diagram with one series being the continuous crystallization of plagioclase feldspar (on the right in the figure) and the other series being the discontinuous crystallization of various mafic silicate minerals (on the left in the figure). As a mineral on the discontinuous series remains in the magma and continues to cool, then it will react with the magma at a lower temperature and a different mineral will form. However, crystals that form continuously, and/or settle out of the magma as it cools, no longer react with the remaining magma, which causes the magma to loose chemical elements changing the magmas composition for the next crystal to form. It is this process that allows intermediate and felsic magmas/rocks to be differentiated from what stated out as a mafic magma. Bowen’s Reaction Series shows the relationship between temperature, the composition of magmas, and the mineralogy and names of igneous rocks. By using this information geologists can interpret the origin of igneous rocks. For example, there are many examples of mafic magmas differentiating into felsic rocks. However, there is no example of a complete differentiation of an ultramafic magma into the four main groups of rocks. By this we know that there must exist other significant factors in changing the composition and texture of a magma. Some of the other important processes are: o Assimilation – incorporation of melted host bedrock into the magma introduces new chemical elements. o Partial melting – situation where a rock can be partly molten partly solid due to the different melting points of individual minerals in that rock. Bowen’s Reaction Series can be used to predict the sequence of melting for a rock as it undergoes heating.