Transcript Classifying Rocks

Classifying Rocks
Geologists observe:
1. Mineral composition
2. Color
3. Texture
a. Grain size
b. Grain shape
c. Grain pattern
Igneous Rocks
• Classified according to their origin, texture,
and mineral composition.
ORIGIN:
Extrusive rock – igneous rock formed from lava
that erupted onto Earth’s surface
Intrusive rock – igneous rock formed when
magma hardens beneath Earth’s surface
Igneous Rocks (continued)
Texture depends on the size and shape of it’s
mineral crystals.
Rapidly cooling lava forms fine grained igneous
rocks with small crystals (extrusive rocks)
Slowly cooling magma forms coarse grained
rocks with large crystals (intrusive rocks)
 Mineral composition – low silica = dark color, no
quartz / high silica = light colored rocks, has
quartz inside
Examples of Igneous Rocks
Extrusive rock – igneous rock formed from lava
that erupted onto earth’s surface – Basalt
Intrusive rock – igneous rock formed when
magma hardens beneath earth’s surface Granite
Sedimentary Rocks
(from sediment to rock)
Sediment: small, solid pieces of material that come
from rock or living things
Erosion: running water, wind, or ice loosen and
carry away fragments of rock
Deposition: sediment settles out of the water or
wind carrying it
Compaction: process that presses sediments
together
Cementation: dissolved minerals crystallize and glue
particles of sediment together
Types of Sedimentary Rocks
• Clastic Rocks: sedimentary rock that forms when
rock fragments are squeezed together (Shale,
Sandstone, & Conglomerate)
• Organic Rocks: remains of plants and animals
are deposited in thick layers (Coal & Limestone)
• Chemical Rocks: when minerals that are
dissolved in a solution crystallize (rock salt)
Metamorphic Rocks
Formed by heat and pressure
Foliated Rocks: metamorphic rocks that have
their grains arranged in parallel layers or
bands – Slate & Gneiss
Nonfoliated Rocks: mineral grains are arranged
randomly, they do not split into layers –
Marble & Quartzite
Dating Rocks
Relative age – the age compared to the ages of
other rocks
Absolute age – the number of years since the
rock was formed
Law of Superposition – the oldest rock layer is
found at the bottom
Unconformity: location where new rock meets
older rock beneath the surface – shows a gap
in the geological record
Radioactive Decay
Radioactive Decay: the atoms of one element
break down to form atoms of another element
* Occur naturally in igneous rocks
Half-life in a radioactive element: the time it
takes for half of the radioactive elements to
decay
Scientists use radioactive dating to determine
the absolute ages of rocks
The Rock Cycle