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The Facts About Rocks
Type of
Rock
How is it
made?
What is it
made of?
Where is it
made?
Some
examples
of this type
of rock
are:
One of
these
rocks
looks like:
(Draw a
picture)
A place
you could
find this
rock:
(Draw a
picture)
IGNEOUS ROCKS
Igneous rocks form when magma or lava (hot liquid rock) cools and hardens. Depending on where the magma or
lava cools and hardens, different types of igneous rocks are formed. There are two main types of igneous rocks,
intrusive and extrusive.
1. Intrusive igneous rocks are rocks that form when magma hardens below earth’s surface (Fig 1). We call
them intrusive, because they intrude (go into) the rocks that are already there. Since magma is less dense
than the surrounding rock, it slowly makes its way through other rocks to the earth’s surface. As magma
rises, it cools, allowing the elements to combine and form minerals. Over time, the minerals grow in size
forming a solid mass of interlocking crystals. Granite is an example of a common intrusive igneous rock
(Fig 3).
2. Extrusive igneous rocks are formed when lava (after magma gets above earth’s surface, it is called lava)
hardens above earth’s surface (Fig 2). As lava escapes onto the surface of the earth, it quickly cools down
to form igneous rocks. These rocks are considered extrusive igneous rocks because they are extruded
(forced out) to earth’s surface. Obsidian is an example of an extrusive igneous rock (Fig 5).
Fig 1: Intrusive Igneous Rock
Fig 2: Extrusive
Igneous
Rock (white arrow)
Since igneous rocks cool at different speeds, they look different. Magma that cools slowly will form large crystals.
These rocks are considered to have a course-grain texture (Fig 3). Magma or lava that cools quickly will not have as
much time to form crystals, so the crystals will be smaller. These rocks have a fine-grained texture (Fig 4).
Sometimes, when lava cools very fast, there is not enough time for crystals to form. These rocks are smooth, like
glass. These rocks are considered to have a glassy texture (Fig 5). Sometimes, rocks deep in earth’s surface take
thousands of years to cool and harden. As this happens, the minerals cool at different rates. These rocks have a
porphyritic texture (Fig 6).
GO: two types; how each formed; drawing of crystals size for each texture
Fig 3: Course-Grain Texture
Fig 4: Fine-Grain Texture
SEDIMENTARY ROCKS
Sedimentary rocks begin to form when existing rocks are broken down into small pieces, called sediments
(weathering). Once the rock is broken down into sediments, typically the sediments are carried away by water,
wind, ice or gravity (erosion). Eventually, whatever is carrying the sediment loses energy and the sediment is
dropped (deposition). After sediments have been deposited, they are turned into rock. Over time, the sediment is
pressed together (compaction) and then gets “glued” together by other minerals (cementation) The processes of
weathering, erosion, deposition, compaction, and cementation lead to the formation of sedimentary rocks.
Sedimentary
Depositio
Compaction
Cementation
y Rock
n
Sedimentary rocks are classified into 2 main groups based on how they are formed (Figure 3-2) . Clastic
sedimentary rocks are made of pieces of other rocks and minerals. Clastic sedimentary rocks can be classified
according to the size of the sediments in the rocks (texture). When rounded, gravel-size sediment or larger make
up most of the rock, it is considered conglomerate (Fig 1). If the sediment is angular, the rock is called breccia (Fig
2). Sandstone, siltstone, and shale are all made of very small sediment (Fig 3). Chemical sedimentary rocks are
made when dissolved minerals in water precipitate (become solid). Chemical sedimentary rocks are classified
based on their composition and texture. Refer to figure 3-2 for examples of chemical sedimentary rocks.
Weathering
Sediment
Erosion
Fig 1: Conglomerate
Sedimentary Rock
Fig 3: Shale
Fig 2: Breccia Sedimentary Rock
METAMORPHIC ROCKS
Metamorphic rocks form when existing rocks are changed by heat and pressure. Rocks formed by metamorphism
(to change) look very different from the original rock. Most metamorphism occurs as either regional or contact
metamorphism. Contact metamorphism occurs when magma intrudes (forces its way into) rock (Fig 1). The main
cause of contact metamorphism is heat (Fig 5). Contact metamorphism causes small changes. An example of a
metamorphic rock formed by contact metamorphism is marble (Fig 4). Regional metamorphism occurs during
events such as the formation of mountains. Large areas of rock are subjected to extreme heat and pressure.
Regional metamorphism results in large changes to the rocks (Fig 2; pg 82 in book).
Heat is the most important thing for metamorphism. The heat causes chemical reactions in the original rock.
These chemical reactions cause existing minerals to re-crystallize, and sometimes causes new minerals to form.
The heat comes from either magma or the increasing heat with depth inside the earth. Pressure (stress) also
causes metamorphism. Like temperature, pressure increases with depth. Pressure on rocks causes the space
between mineral grains to close. This creates a more compact rock.
Fig 1: Contact Metamorphism
(Marble, Quartzite, Hornfels)
Fig 2: Regional Metamorphism
Metamorphic rock can be classified by its texture. Metamorphic rocks can be foliated or non-foliated. Foliated
metamorphic rocks have a banded (striped) texture (Fig 3). Non-foliated metamorphic rocks are usually made up
of only one mineral, so they do not have a banded texture (Fig 4).
Fig 3: Foliated Metamorphic Rock
Fig 4: Non-foliated Metamorphic
Rock
Fig 5: Causes of Metamorphism