Download Science Notes December, 2012 SOL 5.7 Rock Cycle, Weathering

Science Notes
December, 2012
SOL 5.7
Rock Cycle, Weathering, Erosion, and Human Impact
The rock cycle is the ongoing process by which rocks can change from one
type to another. The three basic types of rocks are igneous, sedimentary, and
metamorphic. These rock types are classified by how they are formed. Igneous
rock forms when magma (liquid rock) cools on the surface of the earth or deep
within the earth. Magma that reaches the surface of the earth is called lava.
Cooled and hardened lava is called igneous rock. Weathered and eroded pieces of
rock are called sediments. These pieces eventually pile up on the ground or under
water. These piles, or layers, soon become buried under more sediment. After long
period of times, these layers become cemented and compacted, together to form
sedimentary rock. As sedimentary rock is covered by more and more layers of
sediment, it is pushed deeper and deeper into the earth and begins to heat up. This
heat is caused by pressure and friction inside the Earth. To illustrate, push your
hands together and rub them vigorously. These rocks work in the same manner.
After many years of heat and pressure, the sedimentary rock changes into
metamorphic rock. So, how can these rocks change from one to another?
Eventually, great pressures inside the Earth push metamorphic rock deep inside
the Earth, or, up to the Earth’s surface. Metamorphic rock that is pushed deep
into the Earth changes into magma, which will eventually erupt out of a volcano to
form igneous rock. The metamorphic rock that is pushed up to the Earth’s surface
during earthquakes is weathered (broken down) and eroded (worn away and
removed) into sediments that will form sedimentary rock. THE ROCK CYCLE
NEVER ENDS!
As we have read and studied, the Earth is constantly changing. Some of
these changes are the results of weathering and erosion. Weathering occurs when
rocks and other materials on the Earth’s surface are constantly being broken down.
Some causes of weathering are:
 Climate – temperatures and the amount of rainfall affect the rate at which
rocks is weathered.
 Water – when frozen, ice expands causing rocks to break; water, mixed with
other chemicals, causes rocks to break and wear away.
 Vegetation – roots from plants and trees can break rocks.
 Mineral composition – different materials weather at different rates.
 Surface area – the greater surface area of rock, the more it is exposed to
weathering.
 Soil composition – different bacteria and decaying organisms in the soil can
cause rocks to break apart.
The products of weathering include clay, sand, and rock fragments. These
products are soon moved by water and wind.
Erosion is the wearing away and removing of these rock materials. Erosion can be
caused by wind, ice, running water, and waves which can result from:
 Hurricanes
 Tornadoes
 Thunderstorms (high winds; downpours)
 Floods
 Snow/Ice storms
 Glaciers
The actions of people can have a huge impact on the Earth’s surface; more
often than not these impacts are negative. Some examples of these impacts
are the killing of vegetation that would normally hold rock particles in place,
cutting down trees, shrubs, and grass that serve as natural wind barriers to
prevent erosion, and moving land formations that expose rocks to erosion. Some
of these activities include:
 Logging
 Construction
 Farming
 Mining
Logging companies cut down trees for the valuable wood that provides
furniture, paper, and homes. Often these companies do not replenish these
trees by planting new ones. Construction companies cut down trees and clear
land to build homes, roads, malls, and office complexes. Also, people build
houses in areas that are not suitable for buildings. Putting homes in low-lying
areas (flood plains) and on coastal beaches invite disaster. Often, farmers allow
their livestock to overgraze areas which affect the vegetation and the
protective top layer of the Earth’s surface. Mining companies strip the top
layer of the Earth’s surface which exposes the rocks to weathering and
erosion.
Fortunately, there are measures that humans can take to help alleviate
weathering and erosion. Some of these preventive measures include:
 Replanting trees when logging.
Farmers should not repeatedly farm the same fields. They could also
properly turn the soil to ensure to keep it healthy.
 Construction companies should take every precaution to build houses and
roads in the proper locations to ensure weathering and erosion are
prevented. They should also investigate better ways of constructing
their buildings and landscaping the areas around their building that can
better prevent erosion.
 Mining companies should refrain from the continuous striping of valuable
top soil which exposes rocks to weathering and erosion.
Other measures that can be taken are:
 Building retaining walls where needed.
 Planting trees, grasses, and shrubs in areas to prevent and/or alleviate
erosion
 Building levees to prevent flooding.
 Digging ditches in and around low-lying areas.

Science Notes
December, 2012
SOL 5.7
Scientists are able to study the Earth through fossil evidence. Fossils are
the remains of plants and animals preserved in rocks. Fossils provide scientists with
evidence about life on Earth, past and present. Fossils can also tell scientists how
the Earth’s surface has changed over time, the age of the Earth, and how plants
and animals lived long ago in their environments.
Fossils are found mostly in sedimentary rocks. Sedimentary rocks are
formed when small pieces of rock, called sediments, are dropped by water, wind, or
ice and build up in layers. These layers eventually harden and turn into rock. In
addition to pieces of rock, sediments also contain materials that were once living
organisms. Many fossils form when plants and animals die and are quickly buried by
clay, sand and other sediments.
The Earth is made up of four layers; crust, mantle, outer core, and inner core. Each
of these layers is unique and possesses their own distinctive characteristics.
The crust is the outer layer of the Earth. It is the coolest and the thinnest
compared to the other layers. It is approximately 3 to 25 miles thick and is mainly
composed of lakes, oceans, rivers, plants, and soil. The crust is the layer on which
we live.
The mantle is located beneath the crust. Even though no one has been able to dig
through the crust to the mantle, scientists think they know what it is like. The
mantle is approximately 1800 miles thick, which makes it the largest layer. The
temperatures are very hot, so hot, that the rocks that are found there flow
(imagine watching your food on the conveyor belt in the checkout line at Walmart!). There are also pockets of magma that sometimes erupt through the Earth’s
crust. When this magma reaches the surface, it is called lava.
Under the mantle is the core. The core is divided into two parts. The first part,
right under the mantle is the outer core. This layer is about 1400 miles thick. This
layer is composed of hot liquid nickel and iron. The temperatures are so hot that
these materials are melted into a thick liquid state like pudding.
The other part of the core is found at the very center of the Earth. It is called
the inner core. The inner core, which is about 800 miles thick, is a solid ball of hot
iron and nickel. This layer is the smallest and the hottest of all the layers.
Temperatures and pressure increase from the crust to the inner core of the Earth.
These two factors can cause the following events to happen:

Magma from the mantle can erupt from a volcano

Flowing rocks of the mantle can cause the crust to move and earthquakes to
occur.

Flowing rocks of the mantle can cause continents to move across the surface
of the Earth.
The extreme pressure and heat from the Earth can cause materials in the Earth to
move or shift. This heat and pressure cause large continent-sized blocks called
plates to move slowly about the Earth’s surface. They are not connected to one
another, but move freely about. These plates can bump, push, and scrape past the
other plates that are around them. The edges, or boundaries, of plates are called
faults. Most volcanoes and earthquakes occur on these faults.
Plates move in three main ways to form boundaries:
 Convergent Boundaries – Occur when plates push together; cause mountain
ranges to rise up from the Earth’s surface (example - Appalachian
Mountains); however, deep in the Earth’s oceans, these boundaries force
plates downward instead of upward and deep trenches are formed.
 Divergent Boundaries – Occur when plates move apart from each other.
These boundaries occur mostly on the ocean floors of the Earth. At these
boundaries, magma rises up between the two separating plates forming
volcanoes and mountain ranges deep under water called mid-ocean ridges.
Most of Earth’s new crust comes from the magma that erupts from these
divergent boundaries and the volcanoes they create.
 Transform
Boundaries
–
Occur when plates slide past each other
horizontally. These types of boundaries grind against each other causing
earthquakes. One example of this boundary is located on the west coast of
the U.S, which causes earthquakes in California (San Andreas Fault).
The process of plate movement that moves the Earth’s continents is called
continental drift. These movements, past and present, are responsible for the
amazing geological features of our Earth’s ever-changing surface.