Activity 3a: Geological Processes – A Review of the Rock Cycle
Unit C: Earth’s History
Learning Target:
a. Review: I can identify and describe how various rock types (igneous vs sedimentary vs metamorphic) form. (DOK 1)
Activity 3A: Reading - Geologic Processes – A Review of the Rock Cycle
Earth is 4.6 billion years old. By comparison, the oldest people in the world live to be about 100. How many
hundreds does it take to make a billion? Earth is constantly changing, yet most people are unable to observe most of
Earth’s changes because those changes occur over such long periods of time. But the fact is Earth has changed
dramatically over the past four billion years, and it will continue to change. A single rock provides an example of how
slowly geological changes occur on Earth. If you picked up a rock and kept it the rest of your life, you would probably
notice little or no change. Yet rocks can and do change, it often just takes quite a long time. Many factors cause
rocks to change, but what is important to understand is that change does occur.
Deep within the Earth, rocks can encounter temperatures high enough to make them melt. When one
tectonic plate sinks down deep beneath another (subduction) the rocky crust can melt. After the hot melted rock has
risen toward the cooler surface, it can cool and crystallize into a solid, a new rock is formed. This new rock is called
igneous rock. Melted rock beneath the surface of the earth is called magma. Melted rock that reaches the surface
through breaks in the crust is called lava. An example of igneous rock is the granite Pike’s Peak is made from. Pike’s
Peak was a batholith not a volcano. Igneous rock that forces its way into existing rock layers is referred to as an
igneous intrusion.
Rocks that are on the surface of the Earth are exposed to rain, wind, ice, and repeated freezing and thawing.
Rain, wind, ice, and temperature are agents that can cause the rocks to weather. Weathering is the breakdown of
rocks into smaller pieces. Once the rock has been weathered into smaller pieces then is can be transported in a
process called erosion. Rock fragments can be eroded or transported by water, ice, wind, or gravity. When the
water and wind slow down, they drop or deposit the sediments (loose Earth materials or fragments). The dropping
of sediments is a process called deposition. As the layers of sediments build up, their combined weight causes the
lowest layers to compact. The tiny spaces between rock fragments fill with natural cementing agents that act like a
kind of glue. This compaction and cementation forms a new type of rock called sedimentary rock. Examples of
sedimentary rocks sandstone and conglomerate can be seen at Garden of the Gods. Some sedimentary rocks form
when minerals dissolved in water accumulate and become solid this is called precipitation. Different environments
produce different types of sedimentary rock. The chart below shows the some common sedimentary rocks and the
environmental conditions that produce them.
Sedimentary Rock
Limestone
Environment
Shallow ocean
Process
Precipitation from dissolved minerals in seawater
Shale
(very small clay sized
particles)
Conglomerate
(pebble to boulder sized
rock pieces)
Sandstone
(sand sized particles)
Lakes or deeper ocean –
quiet slow moving water
Deposited as particles sink to the bottom and are
later compacted
River beds - fast moving
water
Deposited as large pieces of rock are dropped as the
moving water slows down and are later cemented
together
Deposited by wind or water and later compacted
and cemented
Coal
Beach/shallow ocean
Desert
River – medium speed water
Swamp
Dead plant material accumulates and is later
compacted
In some locations on the Earth, tectonic plates are moving toward one another and even colliding. In such areas,
rocks can be affected by the enormous pressures and temperatures associated with plate movement. These
extremes of heat and pressure can transform both igneous and sedimentary rock into a new type of rock. This new
rock is called metamorphic rock. An example of a metamorphic rock is gneiss which forms from squeezing and
heating granite. These tectonic movements can slowly bend or fold the rock layers into new shapes. Sometimes the
movement is too fast for the rock to bend and instead it breaks in various types of faults. In addition to folding and
faulting the rock layers can be pushed upward relative to sea level in a process known as uplift. Uplift results in an
increase in erosion, for example the Colorado River cut the Grand Canyon as a result of the Colorado Plateau being
uplifted. Rock layers can also sink downward relative to sea level in a process called subsidence. Subsidence is often
a result of water dissolving limestone and creating caves that collapse to produce sinkholes.
All three types of rock that make up the crust of the Earth can be melted or weathered, eroded and
deposited or exposed to heat and pressure over and over again after they are formed. Some processes build up
rocks while others break down rock. All these processes taken together are called the rock cycle. The rock cycle
occurs over geologic time not over human time.
Geologic Processes Reading Guide
Read the accompanying article and answer the following in your notebook using complete sentences that express complete
thoughts.
1.
The Earth has changed over time, but it is sometimes difficult for humans to observe this directly.
a. Describe two ways the Earth changes that are difficult for humans to observe.
b. Describe two ways the Earth changes that are easy for humans to observe.
2.
How do igneous rocks form?
3.
What is an igneous intrusion?
4.
What is the difference between weathering and erosion?
5.
An older mountain range known as the ancestral Rockies weathered and the material ended up forming a sedimentary
sandstone in Garden of the Gods. Write a paragraph or draw a flowchart to describe the stages in the formation of this
sandstone.
6.
Explain the phrase “sedimentary rocks are recycled rocks”.
7.
What are three environments that produce sandstone?
8.
Why would conglomerates need faster moving water than shale when forming?
9.
Why might gneiss be common in the Himalayan Mountains?
10. Label the diagram of the rock cycle using the following terms.
Boxes and ovals:
Magma has been filled in to help you start.
Sediments
Igneous Rock
Metamorphic Rock
Sedimentary Rock
Process arrows:
weathering/erosion (3 times)
heat and pressure (2 times)
cooling and crystallization (1 time)
melting (1 time)
compaction and cementation (1 time)
Magma