Geologic Time Scale, Earth’s Structure, and Rock Properties
The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth’s history.
The 4.6 billion-year geologic time scale is subdivided into different time periods of varying lengths. All of Earth history is divided into two great expanses of time.
The Precambrian began when Earth first formed 4.6 billion years ago and ended about 570 million years ago. The Phanerozoic Eon began 570 million years ago and continues today. Geological maps provide detailed geologic information.
- It’s important because it helps civil engineers to make proper plans.
- The line of intersection on a map is called a geologic map.
- There are two types of geologic maps: areal and surficial maps.
- Surficial maps show the unconsolidated materials.
Breccia is a rock composed of broken fragments of minerals. The biosphere is one of the four layers that surround the Earth along with the lithosphere (rock), hydrosphere (water), and atmosphere (air), and it is the sum of all the ecosystems. The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth’s gravity.
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The five main layers are:
- Exosphere: 700 to 10,000 km
- Thermosphere: 80 to 700 km
- Mesosphere: 50 to 80 km
- Stratosphere
- Troposphere: 0 to 12 km
Mesosphere: This layer is known to have a relatively high concentration of iron and other metal atoms.
Exosphere: Chiefly of ionized hydrogen.
Thermosphere: Molecules of oxygen and nitrogen are bombarded by radiation and energetic particles from the Sun, causing the molecules to split into their component atoms.
A tectonic plate (also called a lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
Plate size can vary greatly, from a few hundred to thousands of kilometers across the earth. The Pacific and Antarctic Plates are among the largest.
Tectonic Plates and Plate Boundaries
The Earth’s outer shell, the lithosphere, consisting of the crust and uppermost mantle, is divided into a patchwork of large tectonic plates that move slowly relative to each other. There are 7-8 major plates and many minor plates. Varying between 0 to 100mm per year, the movement of a plate is driven by convection in the underlying hot and viscous mantle.
Continental drift refers to large-scale horizontal movements of continents relative to one another and to the ocean basins during one or more episodes of geologic time.
The composition of granite is typically 70-77% silica, 11-13% alumina, 3-5% potassium oxide, 3-5% soda, 1% lime, 2-3% total iron, and less than 1% magnesia.
Types of Plate Boundaries: Divergent, Convergent, Transform.
Faults of the rock: A fracture in the rocks along which some definite displacement has taken place.
The movement in a fault may be slow, sudden, or of a recurring type that is highly variable, ranging from a few cm to several km.
Convergent Plate Boundaries: Where plates move toward each other, wet crust is partially melted to form silicic (Silica-rich, iron-poor, i.e., granitic) magma. Stress: Compression, Earthquakes, Volcanism, Rocks, Features.
Divergent Plate Boundary: Stress: Tensional > extensional strain. Volcanism: non-explosive, fissure (gap) eruptions, basalt floods. Earthquakes: Shallow, weak.
Rocks: Basalt. Features: Ridge, rift, fissures.
Aquifer: A geological formation that stores water in the subsurface. Consists of porous material, such as sand, gravel, and fractured rock. Capable of yielding water as a water supply.
Earth’s structure leads to intense geologic activity:
- Inner core: Solid iron
- Outer core: Liquid iron, convecting (magnetic field)
- Mantle (Asthenosphere): Solid iron-magnesium silicate, plastic, convecting
- Crust (Lithosphere): Rigid, thin O, Si, Al, Fe, Ca, Na, K, Mg
Consolidation is a process by which soils decrease in volume, which involves a decrease in water content of saturated soil without replacement of water by air.
Rock or stone is a natural substance, a solid aggregate of one or more minerals or mineraloids. For example, granite, a common rock, is a combination of the minerals quartz (SiO2), feldspar (alum-inosilicate), and biotite (K(Mg,Fe2+3)(Al,Fe3+)Si3O10(OH,F)2.).
Rock is generally non-homogeneous & anisotropic. Rock properties tend to vary widely, often over short distances. Few common similarities exist between rock and concrete.
Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength.
The Rock Cycle: Many natural phenomena occur in cycles. The rock cycle explains the relationship between the three major groupings of rocks.
Igneous rocks are formed deep within the Earth. They come up to the surface underground (in the form called magma).
Sedimentary rocks are formed in layers as sediments, usually on the bottom of the ocean.
Metamorphic rocks begin as either igneous rocks or sedimentary rocks.
Color: some rocks are dark, others are light.
Crystals: some rocks have crystals and others do not. If they do have crystals:
- Arrangement: are the crystals random or in layers?
- Similarity: are the crystals uniform or different?
Particles: some rocks contain sand or gravel particles.
Holes: many volcanic rocks have holes in them.
Layers: sedimentary rocks have layers as do some metamorphic rocks.
Reflectivity: some rocks reflect light like a mirror.
Luster: some rocks are shiny, others are dull.
Hardness.
Magnetic properties: some rocks attract a magnet, others do not.
The testing of construction materials:
- Physical
- Chemical
- Verifying quantity
- Checking for damage
The most common construction materials:
Soil, Aggregates, Asphalt, Concrete.
Rock mass properties dictate overall types of a large mass of rock as related to engineering construction. Examples: The abutments and foundation of dams, rock slope stability of a highway cut. Petrographic features of a rock are intrinsic properties, which control the mechanical behavior of the rock mass at the fundamental level.
Portland Cement Concrete: Everyone is familiar with Portland cement concrete. Benefit: The use of IRC materials in Portland cement concrete has significant environmental benefits.
Soil liquefaction describes a phenomenon whereby a saturated or partially saturated soil substantially loses strength.
Folds: The undulation and wavy patterns of the rocks because of the compression being generated in the geological formation.
Construction materials are tested to meet the quality control and assurance requirements that are within the construction plans, specifications, and building codes.