Download 3.1 Rock Types

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)
Because igneous rocks form from cooled magma and lava, they are generally found near
volcanoes, either above ground (extrusive igneous rocks) or below ground (intrusive igneous rocks)
Since igneous rocks cool at different speeds, they look different. Some things to look for that can help
you identify an igneous rock are bubbles, a glassy appearance, or a grainy texture (big or small
grains of minerals).
Fig 1: Course-Grain Texture
Fig 3: Bubbles in rock
Fig 2: Glassy Texture
SEDIMENTARY ROCKS
Sedimentary rocks begin to form when weathering breaks down existing rocks into small pieces,
called sediments. 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.
Weathering
Sediment
Erosion
Deposition
Compaction
Cementation
Sedimentary
Rock
Sedimentary rocks are classified into 2 main groups based on how they are formed (Figure 3-2),
clastic sedimentary rock or chemical sedimentary rock. Clastic sedimentary rocks are made of
pieces of other rocks and minerals. Sandstone, siltstone, and shale are all examples of clastic
sedimentary rock (Fig 3-2). Chemical sedimentary rocks are made when dissolved minerals in water
precipitate (become solid). Chalk, flint, and rock salt are all examples of chemical sedimentary
rocks.
Sedimentary rocks are often found at the bottom of lakes or rivers, where sediments have been
deposited. Some things to look for in order to identify sedimentary rocks are flat layers (Fig 1) or a
mix of particle sizes (Fig 2).
Fig 1: Flat layers found in a common
sedimentary rock
Fig 2: A mix of particle sizes,
identifying a sedimentary rock
Fig 2: Breccia Sedimentary
Rock
METAMORPHIC ROCKS
Metamorphic rocks form when existing rocks are changed by heat and pressure. Rocks formed by
metamorphism (which means “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 in the rock’s chemical composition, often
leading to a completely new rock being formed. An example of a metamorphic rock formed by
contact metamorphism is marble. Regional metamorphism occurs during events such as the
formation of mountains.
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, therefore metamorphic rocks are often found underground.
Fig 1: Contact Metamorphism
(Marble, Quartzite, Hornfels)
Some hints that might tell you that you are looking at a metamorphic rock are bending layers (Fig 2)
or squished grains (Fig 3)
Fig 2: Bending layers formed by
Regional Metamorphism
Fig 3: Squished layers