13.12: Detailed Figure Descriptions

Figure 13.1: Geologic Province Map

A map of California highlights the area that corresponds to the Mojave and Colorado Desert Geomorphic Provinces. The larger Mojave Desert Province is northeast of the is north of the Colorado Desert Province; they are separated by northwest trending San Andreas Fault. Together, these geomorphic provinces define a triangular region in the southeastern edge of the state. The western boundary of this larger geomorphic province is formed by the states of Nevada and Arizona. The northern boundary is the east-northeast trending Garlock fault, which separates the Mojave and Colorado Desert Provinces to the south from the (from east to west) Basin and Range, Sierra Nevada and Great Valley Province. Along their southwestern boundary, these provinces are bounded by the Transverse Range Province and the Peninsular Ranges Province. The southern boundary is truncated by the Mexican border.

Figure 13.1.1 Map of the Mojave and Colorado Desert Provinces from space

Map of the Mojave and Colorado Desert Provinces from space. The map shows the intersection of the east-northeast trending Garlock fault with the northwest trending San Andreas fault. These faults meet at a point that defines the western limit of the combined Mojave and Colorado Desert Provinces. To the north of the Garlock Fault is the southern edge of the Basin and Range, Sierra Nevada, and Great Valley Provinces. To the southwest of the San Andreas Fault is the Los Angeles Basin, which lies at the junction of the Transverse Ranges Province to the west, and the Peninsular Ranges Province to the south. The region encompassed by the Mojave and Colorado Desert Provinces is arid and contains numerous mountain ranges separated by valleys, some of which are very light in color, indicating the presence of seasonal lakes. Major highways cross this region, including US 10, which travels east from Los Angeles; US 15 travels northeast through the center of this province; and US 40 travels east-west across this region. Joshua Tree National Park is situated along the southwest edge of this province, north of US 10 where it meets the San Andreas Fault. The Salton Sea west of the San Andreas Fault, in the adjacent Peninsular Ranges Province.

Figure 13.1.2 Excerpt from the Geologic Map of California with the Mojave and Colorado Desert Provinces

Excerpt from the Geologic Map of California with the Mojave and Colorado Desert Provinces highlighted. An excerpt of the larger state map focuses on the combined provinces of the Mojave and Colorado Desert. This map highlights map units according to large ranges of geologic age and lithology. Most of the bedrock geology of this region has been mapped as Quaternary surficial deposits which generally correspond to basin regions. Areas of older rocks are found in uplifted mountains. Isolated regions of pre-Cambrian rocks are located along the boundary between the Mojave and Colorado Desert Provinces, where they form a northwestern linear belt corresponding, and along the eastern edge of the provinces. Smaller occurrences are found throughout the central part of the province. Mesozoic granitic igneous rocks occur throughout the region, as do Cenozoic rocks of volcanic origin. Paleozoic sedimentary and metamorphic rocks are scattered throughout the northeastern portions of the province, as in the central western region. Major fault systems are oriented northwest as well as east west. The northwest trending faults occur in the western part of the province and truncate parallel east-west trending faults in the northern central part of the province.

Figure 13.2.4 Simplified geologic history of the Mojave region in three panels

Three panels describing the simplified geologic and tectonic history of the Mojave region. Panel A depicts geologic events prior to 250 million years ago when the Mojave region was a passive continental margin. At this time, the ancient Pacific Ocean Basin was situated to the west. A blanket of passive margin marine sediments covered the shallow shelf platform and continental margin on top of very ancient igneous, sedimentary, and metamorphic rocks. Panel B depicts the period of plate convergence along the continental margin roughly 250-60 million years ago. In the region that would become the Sierra Nevada and the Mojave Provinces, a volcanic arc system developed to the west of the active east-directed subduction zone. Panel C depicts the period of extension and faulting of the Great Basin region (including the Mojave Province) that began roughly 30 million years ago. Alluvium-filled basins in down dropped grabens are punctuated by uplifted basement rocks. Younger strike-slip style fault systems (like the Eastern California Shear Zone and the San Andreas and Garlock Fault System) are still active in the region today.

Figure 13.3.1 Major global climatic regions in relation to atmospheric convection cells and the Earth's continents

A map of the world with continents and oceans. The continents are color-coded by climate type. The climate types are divided into categories and then sub-categories. Tropical climates are divided into tropical wet and tropical wet and dry.

Tropical wet climate areas are found in much of central America, the western and northern Amazon Basin in South America, equatorial Africa, coastal India, coastal southeast Asia, the Indonesian Archipelago and equatorial Pacific Islands. Tropical wet and dry includes southern Mexico; northern South America; the central, southern, and western Amazon Basin in South America; south of equatorial Africa except along the west coast; inland southeast Asia, northern Australia, the Philippines, and southernmost Japan.

Dry climates include semiarid and arid conditions. Semiarid areas are western, non-coastal North America from central Mexico to southern Canada, except for the highest mountains in the Rockies, the highlands of Venezuela; the Chilean highlands; onshore areas of Morocco and Tunisia; areas of Africa on the northern and southern borders the equatorial regions, much of Madagascar: the highland regions of southeast Asia, Australia between the coastal areas and the central area; Mongolia, inland China, much of the Middle East and Turkey. Arid regions include the Mojave and Sonoran deserts of the US and Mexico, coastal south America from Ecuador, Peru, and Chile; north Africa and the western coast of southern Africa, Ethiopia: the Saudi Arabian peninsula, inland Iran; western Pakistan; central Australia; inland China north of the Himalayas.

Moderate climates include Mediterranean, humid subtropical, and marine west coast types. Mediterranean climates are coastal southern California and northernmost Mexico, Spain and the Mediterranean, eastern South Africa, southern coastal Australia. Humid subtropical includes the southeast US, as well as southern Brazil, Uruguay, mainland China, northern India, and central Japan. Marine west coast conditions are found on the west coasts of central and northern California, Canada, and coastal southern Alaska and the Aleutian Islands, and coastal southern Chile, as well as the west coasts of the British Isles and northern Europe, western Scandinavia, southeast coastal Australia and New Zealand, parts of the eastern Philippines.

Continental climates are humid continental and subarctic conditions. Humid continental climate examples include north and central US and southern Canada, as well as central, non-coastal Scandinavia, northern Europe, and northeast China. Subarctic climates are founnd in most of Canada and Alaska, northern Scandinavia, northern Russia and Siberia, and coastal Greenland.

Polar climates include tundra, ice cap, and highlands conditions. Tundra areas include northernmost Alaska and Canada, and Greenland coastal areas, as well as northernmost Siberia. Icecap regions are Antarctica and inland Greenland. Highlands areas include the highest Rocky Mountains, the highest of the Andes Mountains, and the Himalayas.

Non-permanent ice is the last category, and it includes near coastal Antarctica, and the around the islands of northern Canada and offshore of Siberia.

Figure 13.3.3 Diagram of a rain shadow and its effects

The rain shadow effect is illustrated by a cross-section of a mountain with the sea on one side and an area labeled “rain shadow” on the other. The side facing the sea is the windward side and arrows labeled prevailing wind point towards the mountains. When the arrows reach the mountains, they move upwards along the mountain face. Near the top of the mountains, clouds form and rain falls on the side of the mountains facing the sea.

The side of the mountains facing away from the sea is the leeward side. Here the arrows portraying air movement move down the mountain. There is no rainfall on this leeward side.

Inset Box Figure 13.3.3 Atmospheric Circulation and Deserts

The pattern of global circulation of the atmosphere is based on where on the globe cooler air sinks and where warmer air rises, and on the interaction of this air movement as the Earth rotates beneath. Earth’s surface is divided into three major areas - warmer areas near the equator called the tropics, colder areas near the poles called the polar regions, and areas in between the tropics and the poles called the temperate zones. The atmosphere above, can similarly be divided into zones or cells based on, but not identical to, these same areas. The atmospheric cells are the Hadley cells associated with the tropics, the polar cells associated with the polar regions, and the mid-latitude cells (also called the Ferrel cells) associated with the temperate zones.

The cells are three dimensional motion of the atmosphere which is determined by whether the air in a cell is warm, cold, or interacting between warm and cold areas.

Starting with the tropics, this is where the Earth’s surface receives the maximum amount of solar heating because the rays of the sun, no matter what season of the year, strike the surface at the highest angle (angle of incidence of the sun’s rays). Because this is the area of maximum solar heating, the atmosphere above is an area of warm air and warm air rises. As the air rises, and as the globe spins beneath the atmosphere a wind pattern is set up to the north and south of the equator, near Earth’s surface.

These winds are easterly - they come from the east and move towards the west. However, they do not move straight east to west; the Earth’s rotation sets up a pattern of northeast winds, north of the equator and southeast winds, south of the equator as indicated by the arrows in the diagram. This is the most consistent wind pattern on Earth. They are the trade winds, because in the days of sailing ships, they were the winds used to power the trade fleets sailing the oceans between the continents. These winds meet or converge, near the equator, and establish the meteorological equivalent of the equator - the intertropical convergence zone, sometimes abbreviated ITCZ.

The ITCZ is the boundary between the atmospheric Hadley cells and as these winds of relatively warm air converge, they rise and cannot fall back to Earth because the air below them is also warm. This warm air continues to move in the upper atmosphere, now away from the ITCZ, until the air beneath is no longer the warm rising air of the tropics. Just beyond the tropics at approximately 30° north and south of the equator, this air, now cooler because of its time in the upper atmosphere, and because warm air is no longer rising beneath it, can finally sink to the surface of Earth.

Next consider the polar regions. This is the coldest air in the atmosphere. The polar regions receive the least amount of solar surface heating of any area on Earth. Two things combine to make this situation. First the angle of incidence of the sun’s rays is lowest in the polar regions. When these areas receive sunlight, they get much less solar energy for the sunlight than does the Earth’s surface in the tropics. Secondly, each polar region has a time of no sunlight (their respective winter seasons) when they get no solar energy. Even with the opposite occurring in their respective summers, it’s not enough to effectively heat Earth’s surface by much. Cold air sinks and moves over the surface of Earth away from the poles as indicated by the arrows moving outwards from the north and south poles.

As the air moves over Earth’s surface and away from the poles, it gets comparatively warmer and by the time it gets just beyond the polar regions, to approximately 60° north and south of the equator, it starts to rise.

Now consider the temperate zones of the mid-latitude or Ferrel cells in between. They are between a colder polar cell from 90° to 60° and a warmer Hadley cell from 0° to 30° of latitude north and south of the equator. At 60° is warmer air rising, and at 30° air is cooler air falling. This sets up a 3-dimensional pattern with the surface winds moving from west to east, or westerly winds, as indicated by the arrows on the figure.

The pattern is cool air (and generally dry air) falling at 30° and 90° north and south of the equator; and warm air (and generally moist air) rising at 0° and 60° north and south of the equator. The warm, dry air at 30° north of the equator, is a major contributor to the formation of both the Mojave and the Colorado deserts of California.

Figure 13.5.2 A portion of the Algodones dune field with crescent and linear dunes indicated

The Algonones dune field is situated between alluvial fans shed from adjacent mountain ranges. The dune field includes both crescent and linear dunes. In the enlargement, a zone of arcuate crescent dunes is adjacent to a zone of complex crescent dunes with a more chaotic shape. Long narrow linear dunes that trend northwest-southeast (transverse to the trend of the other dunes in the image) are in the southwest portion of the dune field.

Figure 13.7.3 A map of the interconnected Pleistocene Lake system of California.

A digital elevation model focuses on the region of the Mojave Desert to the north of the San Gabriel and San Bernadino Mountains and extending to the Colorado River on the east. A network of ancient lakes is developed throughout this region. A central part of this system is the Mojave river, which flows northeastward from the San Gabirle mountains, and connects a number of these lakes including the ancient Lake Manix, Cronese Lakes, and Soda Lake (also known as Lake Mojave). Flow from this system continued northward to Lake Tecopa (near Shoshone, CA), and then into Lake Manley (in the modern Death Valley region). This lake drained into a series of large lakes to the west: Panamint Lake, China Lake and Owens Lake.

Figure 13.9.2 Map showing the relative location of the 1999 Hector Mine.

Map showing the relative location of the 1999 Hector Mine. The epicenters of a range of earthquakes in the Mojave Desert Province are plotted atop a digital elevation model. The region depicted is northeast of the San Andreas Fault and includes Joshua Tree National Park in the southeast, the Garlock Fault in the north, and is centered on Barstow, CA. Earthquake epicenters form distinctive clusters: A linear belt of epicenters includes the 1999 Hector Mine Earthquake; this belt trends north-northwest and is located to the east and slightly south of Barstow. A second cluster forms a linear belt that includes the Landers Earthquake epicenter. This belt is parallel to that of the Hector Mine cluster but located slightly west of it. Another large cluster of epicenters is plotted around Big Bear Mountain. The epicenter of the 1999 Hector Mine Earthquake is southeast of Barstow. The epicenter of the Landers Earthquake is south-southeast of Barstow and slightly southwest of the Hector Mine earthquake. Zones of aftershocks are also shown. These form linear belts that trend north-northwest through the Hector Mine epicenter and a second linear belt through the Landers epicenter is parallel to that of the Hector Mine trend but located to the west of it.

Figure 13.9.3 Map of CISN / SCSN Relocations for Brawley seismic zone events.

Clusters of epicenters in this map occur in two distinct linear belts. One belt is west-southwest of the Salton Sea, where a cluster of epicenters is aligned with the northwest trending Superstition Hills fault. The northern edge of this zone is a broad region of epicenters with a northeast trending sense. The second zone falls along the north-northwest trending Imperial Fault to the south and extends northward to the southeastern shores of the Salton Sea, where it merges with a broad northeast trending zone of epicenters and some events offshore in the Salton Sea itself. In this second zone, a recent cluster of events is associated with the northeast trending 2012 Brawley swarm.