chapter 6

time it takes for a magma to cool

-depends on how fast it can transfer heat to its surroundings. -the wall rock surrounding an intrusion acts as insulation, so magma in an intrusive environment cools relatively slowly. -lava that erupts at the ground surface cools relatively quickly.

pyroclastic flow

An explosive eruption may also produce a fast-moving pyroclastic flow, a scalding avalanche of ash and other debris that races down the surface of the volcano, destroying everything in its path.

Dikes and sills

-A dike is a tabular intrusion that cuts across pre-existing layering (bedding or foliation) -Dikes may radiate out from a volcano, or they may develop over broad regions of crust during the process of rifting. In places where tabular intrusions lie within rock that does not have layering, we generally refer to a nearly vertical, wall-like example as a dike and a nearly horizontal, tabletop-shaped example as a sill. -Sill: is a tabular intrusion that injects between layers

Plutons

Plutons are irregular or blob-shaped intrusions that range in size from tens of meters across to tens of kilometers across

What Controls the Speed at Which Molten Rock Flows?

Resistance to low, or viscosity, of a liquid afects the speed at which the liquid moves. The viscosity of molten rock depends primarily on temperature, volatile content, and silica content. Specifically, hotter melt tends to be less viscous than cooler melt because thermal energy breaks bonds and allows atoms or molecules to move more easily. A melt containing more volatiles has lower viscosity than does a dry (volatile-free) melt because volatiles also tend to break apart silicate molecules. Finally, mafic melt is less viscous than felsic melt because relatively more silicon-oxygen tetrahedra occur in felsic melt, and the tetrahedra tend to link together to create long chains that can't move past one another easily. => very hot mafic lava has relatively low viscosity and can flow in thin sheets over wide regions, but cool felsic lava has relatively high viscosity and clumps up at the volcanic vent to form a bulbous mound

Igneous Rocks at Rifts

Rifts occur at places where continental lithosphere under- goes horizontal stretching and, therefore, vertical thinning. As this process takes place, pressure on the asthenosphere decreases and decompression melting takes place, producing basaltic magma. Some of this magma intrudes the crust to form sills or dikes, and some makes it to the Earth's sur- face and erupts as basaltic lava. his magma transfers enough heat to the continental crust to cause partial melting, which in turn, yields felsic magmas that erupt mostly as rhyolitic ash. As a result, extrusive rocks in a rift generally include both basaltic lows and sheets of rhyolitic tuff.

extrusive igneous rock

Rock that forms either by the freezing of lava above ground, after it spills out (extrudes) onto the surface of the Earth and comes into contact with air or water, or by the cementing or welding together of pyroclastic debris

glassy igneous rock

Rocks made of a solid mass of glass or of glass surrounding isolated small crystals are glassy igneous rocks. Glassy rocks typically fracture conchoidally.

factors controlling cooling lava

Same factors that control cooling of magma also control cooling of lava. Specifically, a thin lava flow cools faster than a thick one because the proportion of surface area to volume for a thin low exceeds that for a thick one. Tiny droplets of lava sprayed into the air in an explosive eruption freeze much faster than does a coherent lava flow, for the same reason. Lava immersed in water cools faster than lava immersed in air because water carries heat away from rock faster than air can.

wet melts vs sorry melts

"dry" melts, which contain no volatiles. In fact, wet melts may include up to 15% dissolved volatiles such as water (H2O), carbon dioxide (CO2), nitrogen (N2), hydrogen (H2), and sulfur dioxide (SO2). These volatiles come out of the Earth at volcanoes in the form of gas. Note that because they can release volatiles, magma and lava provide not only the molecules that make up rocks, but also the molecules that comprise the Earth's water and air.

pegmatite

- an exception to the standard cooling rate-grain size relationship. A very coarse-grained igneous rock called pegmatite doesn't form by slow cooling. Pegmatite, which can contain crystals up to tens of centimeters across, typically occurs in dikes. Because dikes generally cool relatively quickly, the coarseness of this rock may seem surprising. Researchers have shown that pegmatite becomes coarse because it forms from water-rich melts in which atoms can difuse so rapidly that large crystals can grow very quickly

what is inside molten rock

-hot liquid consisting of many chemicals -Common oxides in magma or lava include silica (SiO2), alumi- num oxide (Al 2O3), iron oxide (FeO or Fe2O3), calcium oxide (CaO), magnesium oxide (MgO), sodium oxide (Na 2O), and potassium oxide (K 2O). In molten rock, oxide molecules do not lie in an orderly crystalline structure, but bond together instead in clusters or short chains that can move with respect to one another.

how the space for plutons develops

-remains a subject of research. -Some plutons may have originated as diapirs, meaning that they rose upward through the crust as buoyant, light-bulb-shaped blobs of magma that pierced overlying rock and pushed it aside as they rose. Pluton intrusion may also involve stoping, a process during which magma assimilates wall rock, and blocks of wall rock break off and sink into the magma. -plutons form by intrusion of several superimposed dikes or sills, which coalesce to become a single, massive body. If high temperatures are sustained for a long time, diffusion can take place, and the rock may gradually recrystallize and its composition may evolve. The overall space for plutons may form because of crustal stretching or because of uplift and erosion of overlying rock.

intrusive igneous rock

more igneous rock forms from magma that pushed its way, or intruded, into pre-existing rock, or wall rock, and solidified out of view underground. We refer to such rock as intrusive igneous rock and to a body of such rock as an igneous intrusion. In the intrusive realm, magma may accumulate in an irregularly shaped zone called a magma chamber, in a chimney-like column, along planar cracks, or in thin sheets between pre- existing layers

Plate Tectonic Context of Igneous Activity

most igneous activity occurs along the volcanic arcs of convergent boundaries, along the mid-ocean ridge axes of divergent boundaries, or within continental rifts (where continents undergo stretching and pulling apart). Igneous activity happens at isolated hot spots, some of which occur on plate boundaries and some of which occur in plate interiors.

Lapilli

rock fragments ejected from a volcano.

Transforming melt into rock

-But melts don't last forever. Rather, they eventually solidify, or freeze. -Cooling happens when magma rises toward the Earth's surface because the temperature of the crust decreases. If magma becomes trapped underground as an intrusion, it slowly loses heat to the surrounding wall rock, drops below its freezing temperature, and solidifies. If magma reaches the Earth's surface and extrudes as lava on the ground surface, it cools because it comes in contact with much cooler air or water. In some cases, magmas freeze, in part, due to loss of volatiles. The subtraction of volatiles from magma can cause it to freeze. Volatiles bubble out of magma as the magma rises and the pressure acting on it decreases.

How Do You Describe an Igneous Rock?: Characterizing Color and Texture

-Its durability comes from the minerals it contains—most architectural "granite" contains feldspar and other minerals with high numbers on the Mohs hardness scale—and from the way its grains interlock. -the color relects the rock 's composition, but it isn't always so simple, because color may also be influenced by grain size and by the presence of trace amounts of impurities

Why do melts of so many compositions form in the Earth?

-Source-rock composition. -Partial melting -Assimilation -Magma mixing

hree factors can control the cooling time of such magma:

-The depth of intrusion -The shape and size of a magma body -The presence of circulating groundwater:

why does magma rise

-because magma is less dense than surrounding rock, a buoyancy force acts on it to drive it upward. -the weight of rock pro- duces pressure at depth, and this pressure squeezes magma upward.

Plate Tectonic Context of Igneous Activity: Products of subduction

A chain of volcanoes, called a volcanic arc (or, commonly, just an arc), develops along all convergent boundaries. You will ind the volcanic arc along the edge of the overriding plate. Igneous activity at volcanic arcs produces both extrusive rocks and intrusive rocks—in the crust beneath each volcano, dikes, sills, and plutons have developed or are developing.

Classifying Igneous Rocks

A rock 's texture provides clues to the rate at which it cooled, as we've seen, and therefore to the environment in which it formed. A rock 's composition tells us about the original source of the magma and about the way in which the magma evolved before finally solidifying. some key igneous rock types: Crystalline Igneous Rocks, Glassy Igneous Rocks, Fragmental Igneous Rocks

assimilation

As magma sits in a magma chamber before completely solidifying, it may incorporate chemicals dissolved from the wall rock of the chamber or from blocks that detach from the wall and sink into the magma

Geologists distinguish subcategories of crystalline igneous rocks by the size of the crystals

Coarse-grained (phaneritic) rocks have crystals large enough to be identiied with the naked eye. Fine-grained (aphanitic) rocks have crystals too small to be identiied with the naked eye. Porphyritic rocks have larger crystals surrounded by a mass of ine crystals. In a porphyritic rock, the larger crystals are phenocrysts, while the mass of iner crystals is called groundmass.

Magma mixing

Diferent magmas formed in diferent locations from different source rock may enter the same magma chamber. mixing a felsic magma with a mafic magma in equal proportions produces an intermediate magma.

Fragmental Igneous Rocks

Explosive eruptions may spray out a fine mist of lava, pulverize recently solidified pumice that froze in the vent of the volcano, or blast apart pre-existing volcanic rocks that formed the volcano and send them flying, too. Volcanic fragments come in a great range of sizes—dust-sized specks or lakes of glass or pulverized rock comprise ash; pea- to golf ball-sized pellets are called lapilli; and still larger chunks can be called bombs (if streamlined) or blocks. Accumulations of pyroclastic debris are called pyroclastic deposits. When the material in these deposits consolidates into a solid mass, due either to welding together of still-hot clasts or to later cementation by minerals precipitating from ground-water, it becomes a pyroclastic rock. several types of pyroclastic rocks based on grain size; -Tuff = fine-grained pyroclastic rock composed of volcanic ash or of ash mixed with lapilli composed of pumice. Can form from debris that settle out like snow, accumulating in beds that blanket the landscape. May also form from debris that rushes down volcano side. Ash in the interior of a pyroclastic low may be so hot that it welds together, when the low stops moving, to produce welded tuff. -Volcanic agglomerate consists of accumulations of lapilli or bombs. -Volcanic breccia= angular fragments of pyroclastic debris that have been cemented together.

Glassy Igneous Rocks

Glassy texture develops most commonly in felsic igneous rocks because the high concentra- tion of silica inhibits difusion and, therefore, the growth of crystals. But basaltic and intermediate lavas can form glass if they cool rapidly enough. Rapidly cooling lava freezes while it still contains gas bubbles; these bubbles remain as open holes known as vesicles. Different kinds of glassy rocks; Obsidian =vesicle free felsic glass. Breaks conchoidally. Tachylite= rare vesicle-free maic glass. Pumice= felsic volcanic rock contains abundant vesicles. Form by quick cooling of frothy lava. Scoria= mafic volcanic rock with many vesicles.

The shape and size of a magma body

Heat escapes from magma at an intrusion's surface, so the greater the surface area for a given volume of intrusion, the faster it cools. So a body of magma with the shape of a pancake cools faster than one with the shape of a melon. And because the ratio of surface area to volume increases as size decreases, a body of magma the size of a car cools faster than one the size of a ship

relationship between cooling rate and texture

If a melt cools very rapidly, it can become solid before seeds have grown into crystals, so the resulting rock has a glassy texture. If the melt cools rapidly, but not quickly enough to form glass, it will develop a fine-grained (aphanitic) texture. If a melt cools slowly, successful crystals have time to grow large before the rock solidifies completely. As the large crystals are growing, new seeds and small crystals do form, but because the melt remains hot, they tend to dissolve into the melt again before they can grow large. -lava flows, dikes, and sills tend to be composed of fine-grained igneous rock. In contrast, plutons tend to be composed of coarse-grained rock. Plutons that intrude into hot wall rock at great depth cool particularly slowly and thus tend to have larger crystals than plutons that intrude into cool wall rock at shallow depth. Porphyritic rocks form when a melt cools in two stages. First, the melt cools at depth slowly enough that phenocrysts form. Then the melt erupts, and the remainder cools quickly, so groundmass forms around the phenocrysts

xenolith

If a stoped block does not melt entirely, but rather becomes surrounded by new igneous rock, it becomes a xenolith

why do granitic plutons, as well as felsic and andesitic lavas, form beneath continental arcs

In continental volcanic arcs, not all the mantle-derived basaltic magma rises directly to the surface. Some gets trapped at the base of the continental crust and some in magma chambers deep in the crust. When this happens, fractional crystallization may occur, so part of the magma evolves into a felsic melt. Furthermore, heat can transfer from the magma into the continental crust and cause partial melting of this crust. Because much of the continental crust is maic to intermediate in composition to start with, the resulting magmas are intermediate to felsic in composition. (Remember that partial melting always produces magma that is richer in silica than was the source rock.) his magma rises, leaving the basalt behind, and either cools higher in the crust to form plutons or rises to the surface and erupts.

laccolith

In some places, the magma injecting between layers gets blocked and can't spread laterally very far, so the magma accumulates to form a blister-shaped intrusion known as a laccolith, which pushes overlying strata upward into a dome.

batholith

Intrusion of numerous plutons in a region yields a vast composite igneous body, called a batholith

Large Igneous Provinces

Mafic LIPs (large igneous province (LIP) may form when the bulbous head of a mantle plume first reaches the base of the lithosphere. More partial melting can occur in a plume head than in asthenosphere rising from shallow depths beneath normal mid ocean ridges or rifts, because the rock in the plume head came from greater depth in the mantle, so its hotter. Unusually large quantity of unusually hot basaltic magma forms in the plume head. The particularly hot basaltic lava that erupts at such localities has such low viscosity that it can flow tens to hundreds of km across landscape = flood basalts

Whether an eruption produces mostly sheets of lava, mounds of lava, or clouds of pyroclastic debris depends largely on a lava's viscosity and volatile content.

Mafic lavas tend to have low viscosity and can spread in broad, thin flows; gas-poor felsic lavas tend to form bulbous lows; and volatile-rich felsic lavas tend to erupt explosively, producing pyroclastic debris

melting as a result of the addition of volatiles

Magma also forms at locations where chemicals called volatiles have the opportunity to mix with hot mantle rock. (volatiles are substances, such as water (H2O) and carbon dioxide (CO2), that evaporate easily and can exist in gaseous forms at the Earth's surface). When volatiles mix with hot, dry rock, they help break chemical bonds and the rock begins to melt. => Adding volatiles decreases rocks melting temp.

The depth of intrusion:

Magma intruded deep in the crust, where hot wall rock surrounds it, cools more slowly than does magma intruded into cool wall rock near the ground surface.

Comparing Extrusive and Intrusive Environments: Intrusive Igneous Settings

Magma that doesn't make it to the surface freezes solid underground. The boundary between this new intrusive igneous rock and the wall rock is called an intrusive contact.

The Formation of Igneous Rocks at Mid-Ocean Ridges

Magmas form at mid-ocean ridges because of decompression melting. Seafloor spreading => oceanic lithosphere moves away from the ridge, hot asthenosphere rises beneath the ridge axis => weight of the overlying rock, and therefore the pressure on the asthenosphere, progressively decreases, which leads to partial melting and the generation of basaltic magma. Magma rise into crust and collects in a shallow magma chamber, forming a mush of liquid and crystals. Slow cooling along the margins of the magma chamber produces gabbro. two-thirds of volume of melt intrudes and freezes within vertical cracks that propagate as newly formed crust splits apart; this magma forms basalt dikes. The remaining third reaches the seafloor and extrudes as lava on its surface, commonly in the form of pillow basalt

Changes in Molten Rock during Cooling: Fractional Crystallization

Molten rock can freeze too, but during the freezing of molten rock, many diferent minerals form because the melt contains a variety of chemicals. When a mafic magma cools to the freezing temperature, mafic minerals (meaning iron- and magnesium-rich minerals), such as olivine and pyroxene, crystallize first. These solid crystals are denser than the remaining liquid magma, so they may start to sink. Some of the crystals react chemically with the remaining magma, but some become isolated from the magma. It extracts iron and magnesium from the magma so that the remaining magma becomes more felsic. If a magma freezes completely before much fractional crystallization has occurred, the magma becomes maic igneous rock. As the process of fractional crystallization continues, more and more iron and magnesium separate from the magma, so the magma evolves to become progressively more felsic.

Products of Hot Spots

Most oceanic hot-spot volcanoes erupt in the interior of an oceanic plate, away from convergent or divergent boundaries. But some, such as Iceland, sit astride a divergent boundaries. Eruptions from such continental hot-spot volcanoes produced the colorful sulfur- and iron-stained layers of volcanic ash that form the "yellow stone" of Yellowstone National Park in northwestern Wyoming and adjacent states. According to the plume hypothesis, the plume itself does not consist of magma—it's composed of solid rock, but rock that is hot and soft enough to low plastically at rates of a few centimeters a year. When the hot rock of a plume reaches the base of the lithosphere, decompression causes partial melting, a process that generates maic magma. At oceanic hot spots, much of the maic magma erupts as basalt. At continental hot spots, part of the maic magma erupts as basalt, but some trans- fers heat to the continental crust, which itself partially melts, as happens beneath rifts, producing felsic magmas that erupt as rhyolite.

How does subduction trigger melting?

Most of the melt at convergent boundaries comes from flux melting of the asthenosphere in the region just above the downgoing plate. This melting happens because some of the minerals in oceanic-crust rocks contain volatile compounds (mostly water). At shallow depths, the volatiles chemically bond to the minerals in the crust. But when subduction brings oceanic crust down into the hot asthenosphere, the "wet" crustal rocks start to warm, and at a depth of about 150 km, they become so hot that the volatiles separate from the minerals and difuse upward into the overlying asthenosphere. The addition of volatiles triggers partial melting of the hot ultramafic rock in the asthenosphere, a process that yields maic magma. Some of this magma rises to form basaltic sills and dikes in the crust, and some makes it all the way to the surface to be extruded as basaltic lava.

If intrusive igneous rocks form deep beneath the Earth's surface, why can we see them exposed today?

Over long periods of geologic time, mountain building slowly uplifts belts of crust. Erosion by water, wind, and ice can gradually strip away the thick, overlying rock and expose the intrusive rock that has formed below. Some intrusive rocks exposed in mountain cliffs today solidified kilometers to tens of kilometers below the surface.

Comparing Extrusive and Intrusive Environments: Extrusive Igneous Settings

Some volcanoes erupt streams of low viscosity lava that cover broad swaths of the countryside with relatively thin lava flows. Others erupt viscous masses of lava that pile into mounds of angular blocks. And still others erupt in cataclysmic explosions, which forcefully eject turbulent clouds of pyroclastic debris that can rise several km into the sky.

decompression melting

Specifically, if the pressure affecting hot mantle rock decreases while the rock 's temperature remains nearly unchanged, the rock may melt. his kind of melting, called decompression melting, takes place in locations where hot mantle rock rises to shallower depths in the Earth. (mantle plumes, beneath rifts, beneath mid ocean ridges)

Ever since the heat-producing catastrophes of its early days, the Earth has radiated heat into space and, therefore, has slowly cooled => igneous rock formed as the first rock. If no heat had been added to the Earth after the era of intense bombardment, the Earth might have become too cold by now for igneous activity to take place today.

Such cooling hasn't happened because of the presence of radioactive elements in the Earth (primarily in the crust). Decay of a single radioactive atom produces only a tiny amount of heat, but the cumulative heat of radioactive decay in the Earth has been sufficient to slow the cooling of this planet overall. Therefore, the Earth remains very hot today, with temperatures at the base of the lithosphere reaching almost 1,300°C, and temperatures at the planet's center exceeding 4,700°C.

We distinguish among diferent types of intrusions by their shape

Tabular intrusions, or sheet intrusions, are roughly planar and of fairly uniform thickness. Smaller ones may be only centimeters thick or meters long, but the largest reach sizes of meters to hundreds of meters wide and tens to hundreds of kilometers long.

sure-rock composition

The composition of a melt reflects the composition of the solid from which it was derived. Not all melts form from the same source rock, so not all melts have the same composition.

Melting Due to a Decrease in Pressure

The graph of a geotherm emphasizes that temperature always increases with depth, but that the rate of increase varies with depth. By reading the graph, we see that temperatures comparable to those of lava exist in the upper mantle. Because pressure prevents melting, a decrease in pressure can permit melting.

importance of molten rock movement

The rise of molten rock serves an important role in the Earth System in that it transfers material from deeper parts of the Earth upward and provides the raw material from which new rocks, as well as the atmosphere and oceans, form.

Partial melting

Under the temperature and pressure conditions that occur in the Earth, only 2% to 30% of a source rock can melt to produce magma at a given location. The temperatures at sites of magma production simply never get high enough to melt the entire source rock, and magma tends to migrate away from the site of melting before all of the original rock has melted. Magmas formed by partial melting are more felsic than the source rock from which they were derived because more silica enters the liquid, as melting begins, than remains behind in the still-solid source. For example, partial melting of an ultramafic rock produces a mafic magma.

The presence of circulating groundwater:

Water passing through wall rock carries away heat, much like the coolant that lows around an automobile engine. Consequently, magma that interacts with circulating groundwater cools faster than does magma that intrudes dry rock

geotherm

We can portray how temperature changes with increasing depth in the Earth by drawing a geotherm, a curving line on a graph that plots temperature on the horizontal axis and pressure on the vertical axis.

melting as a result of heat transfer

When very hot magma from the mantle rises up into the crust, it brings substantial amounts of heat with it. his heat can be conducted into the wall rock surrounding an intrusion and can raise the temperature of the wall rock. In some cases, the added heat may be sufficient to cause the wall rock to begin melting. This process = heat-transfer melting because it results from the movement of thermal energy from a hotter material to a cooler one. Can happen in rifts, along convergent boundaries, and at hot spots, for at all these locations, magma from the mantle can carry heat up into rocks that have a lower melting temperature

four major compositional types of molten rock

proportion of silica relative to the combination of magnesium oxide and iron oxide that a melt contains; Mafic melts contain a relatively high proportion of magnesium oxide and iron oxide relative to silica— the "ma-" in the word stands for magnesium, and the "-ic" comes from the Latin word for iron. Ultramaic melts have an even higher proportion of magnesium oxide and iron oxide relative to silica. Felsic melts have a relatively high proportion of silica relative to magnesium oxide and iron oxide. Intermediate melts are so named because their composition lies partway between those of maic and felsic melts.

Crystalline igneous rocks

consist of mineral crystals that intergrow when the melt solidifies, so that they it together like pieces of a jigsaw puzzle. The interlocking of crystals in these rocks happens because the rock does not solidify instantly. Rather, diferent crystals grow at different rates and at different times and, as the crystals grow, they interfere with each other. To classify crystalline igneous rocks, we can make a simple chart with the different com- positional groups (ultramafic, mafic, intermediate, and felsic). Note that the basalt, andesite, and rhyolite could have come from the same magmas as the gabbro, diorite, and granite, respectively. But the three fine-grained rocks probably cooled quickly in lava flows or near-surface dikes and sills, while the three coarse-grained rocks probably cooled more slowly in plutons. maic rocks tend to be black or dark gray, intermediate rocks tend to be lighter gray or greenish gray, and felsic rocks tend to be tan to pink or maroon. Ultramaic rocks are relatively rare at the Earth's surfac

ash

debris that can rise several kilometers into the sky. The debris includes pea- to golf ball-sized fragments, known as lapilli, that fall like hail on or near the volcano, as well as finer, dust-sized material, known as ash, which may be carried by the wind for great distances before it drifts down from the sky like snow

Causes of melting

decrease in pressure, addition of volatiles, heat transfer from rising magma

Bowen´s reaction series

determine the sequence in which silicate minerals crystallize from a melt.

2 main categories of igneous rock based on where it solidifies

extrusive igneous rock

Fragmental igneous rocks

form from pyroclastic debris and consist of igneous chunks, grains, or lakes that are packed together, welded together, or cemented together after they have solidiied

Where does the space for intrusions come from?

igneous dikes form in regions where the crust has stretched horizontally in response rifting or seafloor spreading, so as magma intrudes, vertical cracks open up. In effect, the magma fills space being generated by stretching. Intrusion of sills occurs near the surface of the Earth, so the pressure of the magma simply pushes overlying rock and the Earth's surface upward to make room

lava

melt that has emerged at the Earth's surface

magma

melt that's underground

flux melting

melting due to addition of volatiles. Water plays the most important role in triggering melting. We'll see that flux melting happens in the mantle above subducting oceanic crust

the texture of a nonfragmental igneous rock largely reflects its cooling rate. Why?

mineral crystals grow when atoms difuse (move) through melts and attach to crystal seeds. At high temperatures, seeds constantly form and dissolve because heat causes diffusion to occur rapidly. Only some seeds are "successful" enough to grow into small crystals, and even most of these end up dissolving in the melt again. Relatively few crystals grow large enough to survive until the melt cools to a level at which crystals stop dissolving.

volcaniclastic rock

pyroclastic debris may mix with water after deposition and move down the slope of a volcano before eventually being buried and turned to rock. Geologists use the term volcaniclastic rock to refer to any rock that contains a large proportion of volcanic fragments; this category includes both pyroclastic rocks and rocks formed from water transported volcanic debris.

Where does our planet's internal heat come from

some heat was left over from the Earth's early days (collision and merging of countless planetesimals. Every time a collision occurred, its kinetic energy (energy of motion) transformed into heat energy.) As the Earth grew, gravity pulled matter inward until eventually, the weight of overlying material squeezed the matter inside tightly together. Such compression made the Earth's insides even hotter. Eventually, the Earth became hot enough for iron inside it to melt, and the dense iron sank to the center to form the core. Friction between sinking iron and its surroundings generated still more heat. Later a mars sized object collided with earth => generated heat. Bombardments continued to add heat to the earth. Collisions and diferentiation made the early Earth so hot that it was at least partially molten throughout, and its surface may, at times, have been an ocean of lava.