Question

How does crystal size compare in extrusive vs. intrusive rock? Explain the cause of this difference.

Answer 1

When magma takes a long time to cool, it has time to develop big crystals, but when magma cools quickly, it does not have time to develop big crystals, and therefore form small crystals. If intrusive rocks cool underground, and the temperature is higher underground, will it take a shorter, or longer time to cool @carsonsi ?

Answer 2

For a hint, look into the cooling time of batholiths or plutons and why they cook at that rate.

Answer 3

Igneous rocks which form by the crystallization of magma at a depth within the Earth are called intrusive rocks. Intrusive rocks are characterized by large crystal sizes, i.e., their visual appearance shows individual crystals interlocked together to form the rock mass. The cooling of magma deep in the Earth is typically much slower than the cooling process at the surface, so larger crystals can grow. Rocks with visible crystals of roughly the same size are said to have a phaneritic texture

Answer 4

What is an Igneous Rock?

An Igneous rock is the product of cooled solidified magma/lava. As magma/lava cools it goes through the process of crystallization which turns it into an igneous rock. The amount of time it takes for an igneous rock to form depends on its location and its composition. There are two types of igneous rocks; Extrusive and Intrusive.

What is the difference between an Extrusive and Intrusive igneous rock?

The difference between an Extrusive and Intrusive igneous rock is the way in which they cool. An Extrusive igneous rock cools very fast on the surface and is created by lava. Since the cooling process is very fast extrusive igneous rocks have very small crystals (fine grained). On the other hand an Intrusive igneous rock cools very slowly beneath the surface and is created by magma. Since the cooling process is very slow intrusive igneous rocks have very large crystals (coarse grained). In some instances there is also a third type of igneous rock. Technically it is an extrusive rock, but it resembles glass. This glass type of igneous rock forms when magma/lava is instantly cooled.

What are the properties used to identify Igneous rocks?

Properties used to identify igneous rocks include mineral composition, texture, and color. There are four different mineral compositions; Felsic, Intermediate, Mafic, and Ultramafic. The texture of an igneous rock is determined by its grain size. There are eight terms relating to the type of texture an igneous rock has. They are pegmatitic, phaneritic, porphyritic, aphanitic, glassy, vesicular, frothy, and pyroclastic.



Why does the type of mineral composition matter when identifying an igneous rock?

By looking at what type of mineral composition an igneous rock has you can determine what type of magma made it and what environment it was made in.



What is the difference in composition between Felsic, Intermediate, Mafic, and Ultramafic igneous rocks?

Igneous rocks contain the following common minerals: Plagioclase feldspar, Olivine, Potassium feldspar, Pyroxene, Quartz, Amphibole, Biotite, and Muscovite. The percentage of these minerals present in an igneous rock determines whether it is felsic, intermediate, mafic, or ultramafic. Felsic rocks are those that are light in color and are mostly made up of feldspars and silicates. Mafic rocks are darker colored and are mostly made up of magnesium and iron. Intermediate rocks fall somewhere in between being Felsic and Mafic. Ultramafic rocks are very dark colored. For more detailed descriptions of each type click on its name to be redirected to the glossary.



What determines what type of texture an igneous rock has?

Texture is determined by the grain size of igneous rock which is determined by the rate of cooling. For example a smooth igneous rock was cooled very fast because it is has small crystals and is therefore an extrusive rock. The texture of this rock would be called fine grained or aphanitic. In the reverse situation an igneous rock that cooled very slowly and formed large grains would be described as having a coarse grained or phaneritic texture. To see the meaning of each type of texture click on its name to be redirected to the glossary (Pegmatitic, Phaneritic, Porphyritic, Aphanitic, Glassy, Vesicular, Frothy, Pyroclastic).



Where are igneous rocks found?

The most common igneous rock Basalt (mafic) makes up most of the oceanic plates and is founds at divergent plate boundaries. In fact most igneous rock activity occurs at divergent plate boundaries. However, intermediate to felsic igneous rocks are most commonly found along continental margins.

Answer 5

the major visible difference between the two are crystal size, intrusive rocks have a larger crystal/grain texture due to the slow cooling of magma below the earth surface which encourages the growth of larger crystals, while extrusive rocks, because of the rapid cooling at/above the earth's surface does the opposite.

Answer 6

Igneous rocks

The term igneous comes from the Latin ignis, meaning "fire". Igneous is used to describe rocks that crystallize out of hot molten material in the Earth called magma. When magma pushes up through Earth's crust to the surface, it is called lava. Both magma and lava cool and harden to form igneous rocks.

Intrusive vs. Extrusive
Igneous rocks can be classified into two main categories: intrusive and extrusive. A trick to help kids remember intrusive and extrusive is to think of intrusive -- inside and extrusive -- exit.

Answer 7

Intrusive rocks come from magma. They cool slowly deep in Earth's crust. When magma cools underground, the crust acts like a blanket, insulating it, keeping it warm longer. Because the magma cools slowly, crystals of different minerals have time to grow. The molecules in the magma have time to arrange themselves into crystal formations before the magma hardens. Intrusive rocks have large crystals that can be seen with the naked eye. A common example of an intrusive igneous rock is granite.

Pahoehoe Lava Field, Hawaii.
Extrusive igneous rocks come from lava. Lava, at the surface, is exposed to air and water which causes the molten rock to cool rapidly. Solidifying rocks at the surface cool too quickly for large crystals to form. Molecules in the lava do not have time to arrange themselves to form large crystals. Extrusive rocks have crystals that are too small to see without magnification. A common example of an extrusive igneous rock is basalt. Some extrusive rocks, such as obsidian and pumice, cool so rapidly that they completely lack crystal structure and are considered a volcanic glass. Pumice is just like obsidian except it is tiny shards of glass.

Texture
Within the two main categories of intrusive and extrusive, rock can be classified even further using texture and chemical composition. The word "texture" has nothing to do with how the rock feels. Texture, in geology, is used to describe how the rock looks. The most noticeable textural feature of igneous rocks is grain size. Grain size refers to the size of the individual mineral crystals. As mentioned above, intrusive igneous rocks, such as granite have large, individual crystals visible to the naked eye. The textural term used to describe a rock with large crystals is coarse-grained. In contrast, fine-grained rocks, such as basalt, are igneous rocks that have crystals too fine to see with the naked eye. Under magnification they are still very small but easily identifiable with a few optic tests. Quickly-cooled lavas can contain trapped bubbles of gas, which are called vesicles. The resulting texture is described as vesicular.
Chemical Composition
Chemical composition of igneous rock can often be estimated just from looking at the rock. Geologists look at the proportions of light-colored and dark-colored minerals in an igneous rock to estimate the chemical makeup of rock. Light-colored or felsic, minerals have more silica in them. Silica is one of the most abundant elements on Earth and is the chief component of quartz. Felsic minerals are most often colorless, white, gray or pink but can be any number of colors. The dark, or mafic, minerals are richer in iron and magnesium. Mafic minerals are chiefly black, brown, dark gray and sometimes green.

The mineral proportions of the rock are what allows geologists to classify rocks chemically. Depending on the proportion of light minerals to dark minerals, igneous rocks can be broken into four main types: felsic, intermediate, mafic and ultramafic. The following list gives more information about igneous chemical categories.

Felsic rocks are high in silica (65% +). They are usually light-colored. Some examples are: Rhyolite (extrusive) and granite (intrusive).
Intermediate rocks have lower silica content (55-65%). They are darker than felsic rocks but lighter than mafic rocks. Some examples are: Andesite/dacite (extrusive) and diorite/granodiorite (intrusive).
Mafic rocks have low silica content (45-55%). They are usually dark-colored and contain iron and magnesium. Some examples are: Basalt (extrusive) and gabbro (intrusive). Basalt is the rock that is produced at spreading ridges and makes up the sea floor.
Ultramafic rocks have extremely low silica content (less than 45%) and contain large amounts of iron and magnesium. They are usually dark-colored, but high olivine content can lend green shades to the rock. Other rare colors can be found. An example of ultramafic rock is Peridotite (intrusive).

Answer 8

When magma cools, crystals form because the solution is super-saturated with respect to some minerals. If the magma cools quickly, the crystals do not have much time to form, so they are very small. If the magma cools slowly, then the crystals have enough time to grow and become large. Some granites contain minerals which are up to one meter (3 ft) across!

Granite with large
K-feldspar crystals Diorite with large
Ca-feldspar crystals

The size of crystals in an igneous rock is an important indicator of the conditions where the rock formed. An igneous rock with large crystals probably indicates that the rock formed deep within the Earth, since it is typically warmer deep inside the Earth than near the surface. These are called intrusive rocks, and they have a phaneritic texture (from the Greek "phanerous" meaning visible). Similarly, a rock with small crystals probably formed at or near the surface and cooled quickly. These are called extrusive rocks and have an aphanitic texture (from the Greek "a-" meaning not, and "phanerous"). And some magma cools so quickly that no crystals form; we say that these have a hyaline texture (from the Greek "hyalis" meaning glass).

Sometimes, a rock will contain both aphanitic and phaneritic crystals in it. This means that something truly odd happened to the magma before it was erupted. Since we know that large crystals need time to grow, the magma must have spent some time deep underground. But the smaller crystals mean that the rest of the cooling happened very quickly. If a rock has both crystal types, it means that the mamga spent some time in a magma chamber, where the large crystals grew, then was violently erupted onto the surface, where the small crystals were formed. A good example of this is the Colbert Rhyolite in the Arbuckle Mountains of Oklahoma.

Answer 9

@coolman500 Rather than copy and paste a whole page, which is a copyright violaion, just post a link:
http://www.arch.dcr.state.nc.us/ncarch/GeoArcheo/IgneousA.htm

Answer 10

Guys im sorry i had already went back into my lesson and figured it out. Sorry i wasted your time

Answer 11

That's fine @carsonsi