Key Concept 1: Sedimentary rock is formed over millions of years from the
deposition of sediment in layers.
Sediment layers on top apply pressure to those on the bottom, compacting them
into sedimentary rocks. Sedimentary rock represents one of three rock groups
formed on Earth. Igneous rocks form from magma that has cooled, and
metamorphic rocks form from heat and pressure deep within Earth changing
existing rocks. In fifth grade, students focus only on sedimentary rock formation
because fossil fuels are formed within these rock layers. Sediments are loose
particles of inorganic material, such as pebbles or sand. These sediments wash
down slopes into bodies of water, such as rivers, lakes, or oceans, where they
settle to the bottom.
Layer after layer of sediment is deposited, after which two processes can occur
that form sedimentary rocks:
(1) Compaction, where the particles are squeezed together by the weight of the
layers of sediment above them, and
(2) Cementation, where the minerals in water fall out of suspension around the
particles and act as glue, sticking the sediments together. Sandstone and shale
are examples of sedimentary rock.
Key Concept 2: Fossil fuels are formed over millions of years from the
deposition of organic materials in layers.
Sometimes organic matter (remains of dead plants or animals) sinks in water, and
is buried in layers of sediment. The organic material begins to decay from the
pressure and heat generated from the layers above them, and can become fossil
fuels. Coal, oil, and natural gas formed millions of years ago, and are burned
today as energy sources for electric power plants, furnaces, stoves, cars, etc.
Two types of ancient sedimentary rock formations are the sources of all fossil
fuels.
Coal Formation - First, in ancient swamps, prehistoric plants and trees began to
die 300 million years ago during the Carboniferous period, long before dinosaurs
appeared. In the swamps, plant remains were covered with soil, layer upon layer.
A thousand years of heat and pressure slowly turned these compacted, decayed
plant layers (organic material rich in carbon) into peat. It took hundreds of
millions of years more for heat and pressure deep within Earth to turn peat into
coal.
Oil and Gas Formation - A second type of sedimentary rock formation occurred
at the bottom of ancient oceans and seas when tiny marine animals called
“plankton” died, sank, and were covered with layers of sand. Millions of years of
heat and pressure turned these organic marine remains into oil and natural gas
trapped in layers of sedimentary rock.
Nonrenewable Resources - Because the remains of ancient plants and animals
are sources for these fuels, they are called fossil fuels. Nature is not replenishing
this supply of fuel, so coal, oil, and natural gas are considered nonrenewable
resources. The pollution produced from burning fossil fuels has led to an
increased interest in alternative energy sources, which is studied in more detail in
a subsequent scope.
Key Concept 3: Fossil fuels are formed in sedimentary rock layers when
materials are compressed and heated deep under the Earth’s surface.
Geologists who look for sources of fossil fuels study sedimentary rock layers in
cross sections exposed in uplifted parts of Earth’s crust or from drilled cores. From
these geologic columns, we have learned that rock layers tell a story about the
events that led to fossil fuel formation.
Students simulate sedimentary geologic columns by constructing models of coal,
sandstone, and shale formed in the area of an ancient swamp using compacted
layers of cereal, coffee, and other materials that represent layers compressed and
heated deep under Earth’s surface. Making models is part of the related Scientific
Investigation and Reasoning TEKS 5.3C in which students draw or develop a
model that represents how something works or looks that cannot be seen.